Merge pull request #2452 in HDFFV/hdf5 from ~JAKE.SMITH/hdf5:feature/vfd_splitter_mirror_a to develop

Add Splitter VFD and Mirror VFD.

* commit '664fc6af47e369152727ff66d68d8532feb0e210':
  Add enable-mirror-vfd flag to configure options.
  Tidying of Mirror VFD.
  Add function header comment for `mirror_writer:run_writer()`.
  Add Splitter VFD to library.

45 files changed, 11593 insertions, 568 deletions

diff --git a/CMakeInstallation.cmake b/CMakeInstallation.cmake
index 12d8273..94d3139 100644
--- a/CMakeInstallation.cmake
+++ b/CMakeInstallation.cmake
@@ -554,6 +554,13 @@ The HDF5 data model, file format, API, library, and tools are open and distribut
     )
   endif ()
 
+  cpack_add_component (utilsapplications
+      DISPLAY_NAME "HDF5 Utility Applications"
+      DEPENDS libraries
+      GROUP Applications
+      INSTALL_TYPES Full Developer User
+  )
+
   if (HDF5_BUILD_TOOLS)
     cpack_add_component (toolsapplications
         DISPLAY_NAME "HDF5 Tools Applications"
diff --git a/CMakeLists.txt b/CMakeLists.txt
index b63d579..7d6e7a2 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -137,6 +137,7 @@ set (HDF5_CPP_LIB_CORENAME          "hdf5_cpp")
 set (HDF5_HL_LIB_CORENAME           "hdf5_hl")
 set (HDF5_HL_CPP_LIB_CORENAME       "hdf5_hl_cpp")
 set (HDF5_TOOLS_LIB_CORENAME        "hdf5_tools")
+set (HDF5_UTILS_LIB_CORENAME        "hdf5_utils")
 set (HDF5_F90_LIB_CORENAME          "hdf5_fortran")
 set (HDF5_F90_C_LIB_CORENAME        "hdf5_f90cstub")
 set (HDF5_F90_TEST_LIB_CORENAME     "hdf5_test_fortran")
@@ -156,6 +157,7 @@ set (HDF5_CPP_LIB_NAME          "${HDF5_EXTERNAL_LIB_PREFIX}${HDF5_CPP_LIB_COREN
 set (HDF5_HL_LIB_NAME           "${HDF5_EXTERNAL_LIB_PREFIX}${HDF5_HL_LIB_CORENAME}")
 set (HDF5_HL_CPP_LIB_NAME       "${HDF5_EXTERNAL_LIB_PREFIX}${HDF5_HL_CPP_LIB_CORENAME}")
 set (HDF5_TOOLS_LIB_NAME        "${HDF5_EXTERNAL_LIB_PREFIX}${HDF5_TOOLS_LIB_CORENAME}")
+set (HDF5_UTILS_LIB_NAME        "${HDF5_EXTERNAL_LIB_PREFIX}${HDF5_UTILS_LIB_CORENAME}")
 set (HDF5_F90_LIB_NAME          "${HDF5_EXTERNAL_LIB_PREFIX}${HDF5_F90_LIB_CORENAME}")
 set (HDF5_F90_C_LIB_NAME        "${HDF5_EXTERNAL_LIB_PREFIX}${HDF5_F90_C_LIB_CORENAME}")
 set (HDF5_F90_TEST_LIB_NAME     "${HDF5_EXTERNAL_LIB_PREFIX}${HDF5_F90_TEST_LIB_CORENAME}")
@@ -175,6 +177,7 @@ set (HDF5_CPP_LIB_TARGET          "${HDF5_CPP_LIB_CORENAME}-static")
 set (HDF5_HL_LIB_TARGET           "${HDF5_HL_LIB_CORENAME}-static")
 set (HDF5_HL_CPP_LIB_TARGET       "${HDF5_HL_CPP_LIB_CORENAME}-static")
 set (HDF5_TOOLS_LIB_TARGET        "${HDF5_TOOLS_LIB_CORENAME}-static")
+set (HDF5_UTILS_LIB_TARGET        "${HDF5_UTILS_LIB_CORENAME}-static")
 set (HDF5_F90_LIB_TARGET          "${HDF5_F90_LIB_CORENAME}-static")
 set (HDF5_F90_C_LIB_TARGET        "${HDF5_F90_C_LIB_CORENAME}-static")
 set (HDF5_F90_TEST_LIB_TARGET     "${HDF5_F90_TEST_LIB_CORENAME}-static")
@@ -190,6 +193,7 @@ set (HDF5_CPP_LIBSH_TARGET        "${HDF5_CPP_LIB_CORENAME}-shared")
 set (HDF5_HL_LIBSH_TARGET         "${HDF5_HL_LIB_CORENAME}-shared")
 set (HDF5_HL_CPP_LIBSH_TARGET     "${HDF5_HL_CPP_LIB_CORENAME}-shared")
 set (HDF5_TOOLS_LIBSH_TARGET      "${HDF5_TOOLS_LIB_CORENAME}-shared")
+set (HDF5_UTILS_LIBSH_TARGET      "${HDF5_UTILS_LIB_CORENAME}-shared")
 set (HDF5_F90_LIBSH_TARGET        "${HDF5_F90_LIB_CORENAME}-shared")
 set (HDF5_F90_C_LIBSH_TARGET      "${HDF5_F90_C_LIB_CORENAME}-shared")
 set (HDF5_F90_TEST_LIBSH_TARGET   "${HDF5_F90_TEST_LIB_CORENAME}-shared")
@@ -212,6 +216,7 @@ set (HDF5_HL_TOOLS_DIR       ${HDF5_SOURCE_DIR}/hl/tools)
 set (HDF5_TOOLS_DIR          ${HDF5_SOURCE_DIR}/tools)
 set (HDF5_TOOLS_SRC_DIR      ${HDF5_SOURCE_DIR}/tools/src)
 set (HDF5_PERFORM_SRC_DIR    ${HDF5_SOURCE_DIR}/tools/src/perform)
+set (HDF5_UTILS_DIR          ${HDF5_SOURCE_DIR}/utils)
 set (HDF5_F90_SRC_DIR        ${HDF5_SOURCE_DIR}/fortran)
 set (HDF5_JAVA_JNI_SRC_DIR         ${HDF5_SOURCE_DIR}/java/src/jni)
 set (HDF5_JAVA_HDF5_SRC_DIR        ${HDF5_SOURCE_DIR}/java/src/hdf)
@@ -920,6 +925,16 @@ if (BUILD_TESTING)
 endif ()
 
 #-----------------------------------------------------------------------------
+# Option to build HDF5 Utilities
+#-----------------------------------------------------------------------------
+if (EXISTS "${HDF5_SOURCE_DIR}/utils" AND IS_DIRECTORY "${HDF5_SOURCE_DIR}/utils")
+  option (HDF5_BUILD_UTILS  "Build HDF5 Utils" ON)
+  if (HDF5_BUILD_UTILS)
+    add_subdirectory (utils)
+  endif ()
+endif ()
+
+#-----------------------------------------------------------------------------
 # Option to build HDF5 Tools
 #-----------------------------------------------------------------------------
 if (EXISTS "${HDF5_SOURCE_DIR}/tools" AND IS_DIRECTORY "${HDF5_SOURCE_DIR}/tools")
diff --git a/MANIFEST b/MANIFEST
index b1be241..bd8edd7 100644
--- a/MANIFEST
+++ b/MANIFEST
@@ -699,6 +699,8 @@
 ./src/H5FDint.c
 ./src/H5FDlog.c
 ./src/H5FDlog.h
+./src/H5FDmirror.c
+./src/H5FDmirror.h
 ./src/H5FDmodule.h
 ./src/H5FDmpi.c
 ./src/H5FDmpi.h
@@ -716,6 +718,8 @@
 ./src/H5FDsec2.c
 ./src/H5FDsec2.h
 ./src/H5FDspace.c
+./src/H5FDsplitter.c
+./src/H5FDsplitter.h
 ./src/H5FDstdio.c
 ./src/H5FDstdio.h
 ./src/H5FDtest.c
@@ -1158,6 +1162,7 @@
 ./test/memleak_H5O_dtype_decode_helper_H5Odtype.h5
 ./test/mergemsg.h5
 ./test/mf.c
+./test/mirror_vfd.c
 ./test/mount.c
 ./test/mtime.c
 ./test/multi_file_v16-r.h5
@@ -1218,6 +1223,7 @@
 ./test/testhdf5.c
 ./test/testhdf5.h
 ./test/testlibinfo.sh.in
+./test/test_mirror.sh.in
 ./test/test_usecases.sh.in
 ./test/test_vol_plugin.sh.in
 ./test/testmeta.c
@@ -1261,6 +1267,7 @@
 ./test/unlink.c
 ./test/unregister.c
 ./test/use_append_chunk.c
+./test/use_append_chunk_mirror.c
 ./test/use_append_mchunks.c
 ./test/use_common.c
 ./test/use_disable_mdc_flushes.c
@@ -2817,6 +2824,18 @@
 ./tools/test/perform/sio_standalone.h
 ./tools/test/perform/zip_perf.c
 
+# Utils directory
+./utils/COPYING
+./utils/Makefile.am
+
+# Mirror VFD utilities
+./utils/mirror_vfd/Makefile.am
+./utils/mirror_vfd/mirror_remote.c
+./utils/mirror_vfd/mirror_remote.h
+./utils/mirror_vfd/mirror_server.c
+./utils/mirror_vfd/mirror_server_stop.c
+./utils/mirror_vfd/mirror_writer.c
+
 # high level libraries
 ./hl/COPYING
 ./hl/Makefile.am
@@ -3554,6 +3573,8 @@
 ./tools/test/misc/vds/CMakeLists.txt
 ./tools/test/perform/CMakeLists.txt
 ./tools/test/perform/CMakeTests.cmake
+./utils/CMakeLists.txt
+./utils/mirror_vfd/CMakeLists.txt
 
 # CMake-specific User Scripts
 ./config/cmake/CTestScript.cmake
diff --git a/Makefile.am b/Makefile.am
index d96ffe3..9689b58 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -85,9 +85,9 @@ else
    TOOLS_DIR=
 endif
 
-SUBDIRS = src $(TESTSERIAL_DIR) $(TESTPARALLEL_DIR) bin $(TOOLS_DIR) . \
+SUBDIRS = src $(TESTSERIAL_DIR) $(TESTPARALLEL_DIR) bin utils $(TOOLS_DIR) . \
           $(CXX_DIR) $(FORTRAN_DIR) $(JAVA_DIR) $(HDF5_HL_DIR)
-DIST_SUBDIRS = src test testpar tools . c++ fortran hl examples java
+DIST_SUBDIRS = src test testpar utils tools . c++ fortran hl examples java
 
 # Some files generated during configure that should be cleaned
 DISTCLEANFILES=config/stamp1 config/stamp2
@@ -190,7 +190,7 @@ trace:
 # Run tests with different Virtual File Drivers.
 # Currently, only invoke check-vfd in the test directory.
 check-vfd:
-	for d in src test; do                                 \
+	for d in src utils test; do                                 \
 	  if test $$d != .; then                                       \
 	    (cd $$d && $(MAKE) $(AM_MAKEFLAGS) $@) || exit 1;          \
 	  fi;                                                          \
diff --git a/bin/trace b/bin/trace
index 15baa8a..fd0248e 100755
--- a/bin/trace
+++ b/bin/trace
@@ -43,6 +43,7 @@ $Source = "";
                "H5D_space_status_t"         => "Ds",
                "H5D_vds_view_t"             => "Dv",
                "H5FD_mpio_xfer_t"           => "Dt",
+               "H5FD_splitter_vfd_config_t" => "Dr",
                "herr_t"                     => "e",
                "H5E_direction_t"            => "Ed",
                "H5E_error_t"                => "Ee",
@@ -159,6 +160,7 @@ $Source = "";
                "H5FD_ros3_fapl_t"           => "x",
                "H5FD_hdfs_fapl_t"           => "x",
                "H5FD_file_image_callbacks_t" => "x",
+               "H5FD_mirror_fapl_t"         => "x",
                "H5G_iterate_t"              => "x",
                "H5G_info_t"                 => "x",
                "H5I_free_t"                 => "x",
diff --git a/config/cmake/ConfigureChecks.cmake b/config/cmake/ConfigureChecks.cmake
index 11bf39c..5a401f7 100644
--- a/config/cmake/ConfigureChecks.cmake
+++ b/config/cmake/ConfigureChecks.cmake
@@ -171,6 +171,23 @@ option (HDF5_ENABLE_ROS3_VFD "Build the ROS3 Virtual File Driver" OFF)
     endif ()
 endif ()
 
+# ----------------------------------------------------------------------
+# Check whether we can build the Mirror VFD 
+# Header-check flags set in config/cmake_ext_mod/ConfigureChecks.cmake
+# ----------------------------------------------------------------------
+option (HDF5_ENABLE_MIRROR_VFD "Build the Mirror Virtual File Driver" OFF)
+if (H5FD_ENABLE_MIRROR_VFD)
+  if ( ${HDF_PREFIX}_HAVE_NETINET_IN_H AND
+       ${HDF_PREFIX}_HAVE_NETDB_H      AND
+       ${HDF_PREFIX}_HAVE_ARPA_INET_H  AND
+       ${HDF_PREFIX}_HAVE_SYS_SOCKET_H AND
+       ${HDF_PREFIX}_HAVE_FORK)
+      set (${HDF_PREFIX}_HAVE_MIRROR_VFD 1)
+  else()
+    message(STATUS "The socket-based Mirror VFD was requested but cannot be built. System prerequisites are not met.")
+  endif()
+endif()
+
 #-----------------------------------------------------------------------------
 # Check if C has __float128 extension
 #-----------------------------------------------------------------------------
diff --git a/config/cmake/H5pubconf.h.in b/config/cmake/H5pubconf.h.in
index 0836168..6edf6b4 100644
--- a/config/cmake/H5pubconf.h.in
+++ b/config/cmake/H5pubconf.h.in
@@ -104,6 +104,9 @@
 /* Define if the compiler understands the __func__ keyword */
 #cmakedefine H5_HAVE_C99_FUNC @H5_HAVE_C99_FUNC@
 
+/* Define to 1 if you have the <arpa/inet.h> header file. */
+#cmakedefine H5_HAVE_ARPA_INET_H @H5_HAVE_ARPA_INET_H@
+
 /* Define to 1 if you have the `clock_gettime' function. */
 #cmakedefine H5_HAVE_CLOCK_GETTIME @H5_HAVE_CLOCK_GETTIME@
 
@@ -273,6 +276,9 @@
 /* Define to 1 if you have the <memory.h> header file. */
 #cmakedefine H5_HAVE_MEMORY_H @H5_HAVE_MEMORY_H@
 
+/* Define if we can build the Mirror VFD */
+#cmakedefine H5_HAVE_MIRROR_VFD @H5_HAVE_MIRROR_VFD@
+
 /* Define if we have MPE support */
 #cmakedefine H5_HAVE_MPE @H5_HAVE_MPE@
 
@@ -285,6 +291,12 @@
 /* Define if MPI_Info_c2f and MPI_Info_f2c exists */
 #cmakedefine H5_HAVE_MPI_MULTI_LANG_Info @H5_HAVE_MPI_MULTI_LANG_Info@
 
+/* Define to 1 if you have the <netdb.h> header file. */
+#cmakedefine H5_HAVE_NETDB_H @H5_HAVE_NETDB_H@
+
+/* Define to 1 if you have the <netinet/in.h> header file. */
+#cmakedefine H5_HAVE_NETINET_IN_H @H5_HAVE_NETINET_IN_H@
+
 /* Define to 1 if you have the <openssl/evp.h> header file. */
 #cmakedefine H5_HAVE_OPENSSL_EVP_H @H5_HAVE_OPENSSL_EVP_H@
 
diff --git a/config/cmake/libhdf5.settings.cmake.in b/config/cmake/libhdf5.settings.cmake.in
index e9a8395..b745765 100644
--- a/config/cmake/libhdf5.settings.cmake.in
+++ b/config/cmake/libhdf5.settings.cmake.in
@@ -76,6 +76,7 @@ Parallel Filtered Dataset Writes: @PARALLEL_FILTERED_WRITES@
           I/O filters (external): @EXTERNAL_FILTERS@
                              MPE: @H5_HAVE_LIBLMPE@
                       Direct VFD: @H5_HAVE_DIRECT@
+                      Mirror VFD: @H5_HAVE_MIRROR_VFD@
               (Read-Only) S3 VFD: @H5_HAVE_ROS3_VFD@
             (Read-Only) HDFS VFD: @H5_HAVE_LIBHDFS@
                          dmalloc: @H5_HAVE_LIBDMALLOC@
diff --git a/config/cmake_ext_mod/ConfigureChecks.cmake b/config/cmake_ext_mod/ConfigureChecks.cmake
index 93b977e..0875aad 100644
--- a/config/cmake_ext_mod/ConfigureChecks.cmake
+++ b/config/cmake_ext_mod/ConfigureChecks.cmake
@@ -166,6 +166,8 @@ CHECK_INCLUDE_FILE_CONCAT ("memory.h"        ${HDF_PREFIX}_HAVE_MEMORY_H)
 CHECK_INCLUDE_FILE_CONCAT ("dlfcn.h"         ${HDF_PREFIX}_HAVE_DLFCN_H)
 CHECK_INCLUDE_FILE_CONCAT ("inttypes.h"      ${HDF_PREFIX}_HAVE_INTTYPES_H)
 CHECK_INCLUDE_FILE_CONCAT ("netinet/in.h"    ${HDF_PREFIX}_HAVE_NETINET_IN_H)
+CHECK_INCLUDE_FILE_CONCAT ("netdb.h"         ${HDF_PREFIX}_HAVE_NETDB_H)
+CHECK_INCLUDE_FILE_CONCAT ("arpa/inet.h"     ${HDF_PREFIX}_HAVE_ARPA_INET_H)
 # _Bool type support
 CHECK_INCLUDE_FILE_CONCAT (stdbool.h    ${HDF_PREFIX}_HAVE_STDBOOL_H)
 
@@ -681,3 +683,4 @@ endif ()
 # the cache value is set in it's config file)
 #
 set (${HDF_PREFIX}_CONVERT_DENORMAL_FLOAT 1)
+
diff --git a/configure.ac b/configure.ac
index 5851b32..81e985e 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1090,6 +1090,7 @@ AC_CHECK_HEADERS([stddef.h setjmp.h features.h])
 AC_CHECK_HEADERS([dirent.h])
 AC_CHECK_HEADERS([stdint.h], [C9x=yes])
 AC_CHECK_HEADERS([stdbool.h])
+AC_CHECK_HEADERS([netdb.h netinet/in.h arpa/inet.h])
 
 ## Darwin
 AC_CHECK_HEADERS([mach/mach_time.h])
@@ -2842,6 +2843,50 @@ fi
 AM_CONDITIONAL([DIRECT_VFD_CONDITIONAL], [test "X$DIRECT_VFD" = "Xyes"])
 
 ## ----------------------------------------------------------------------
+## Check whether the Mirror VFD can be built.
+## Auto-enabled if the required libraries are present.
+##
+AC_SUBST([MIRROR_VFD])
+
+## Default is no Mirror VFD
+MIRROR_VFD=no
+
+AC_ARG_ENABLE([mirror-vfd],
+              [AS_HELP_STRING([--enable-mirror-vfd],
+                              [Build the socket-based Mirror virtual file driver (VFD).
+                               [default=no]])],
+              [MIRROR_VFD=$enableval], [MIRROR_VFD=no])
+
+if test "X$MIRROR_VFD" = "Xyes"; then
+
+    AC_CHECK_HEADERS([arpa/inet.h],, [unset MIRROR_VFD])
+    AC_CHECK_HEADERS([netinet/in.h],, [unset MIRROR_VFD])
+    AC_CHECK_HEADERS([netdb.h],, [unset MIRROR_VFD])
+    AC_CHECK_HEADERS([sys/socket.h],, [unset MIRROR_VFD])
+    AC_CHECK_FUNC([fork], [], [unset MIRROR_VFD])
+
+    AC_MSG_CHECKING([if the Mirror virtual file driver (VFD) can be built])
+    if test "X$MIRROR_VFD" = "Xyes"; then
+        AC_DEFINE([HAVE_MIRROR_VFD], [1],
+                [Define whether the Mirror virtual file driver (VFD) will be compiled])
+        AC_MSG_RESULT([yes])
+    else
+        AC_MSG_RESULT([no])
+        MIRROR_VFD=no
+        AC_MSG_ERROR([The Mirror VFD cannot be built.
+                      Missing one or more of: arpa/inet.h, netinet/in.h,
+                      netdb.h, sys/socket.h, fork().])
+    fi
+else
+    AC_MSG_CHECKING([if the Mirror virtual file driver (VFD) is enabled])
+    AC_MSG_RESULT([no])
+    MIRROR_VFD=no
+fi
+
+## Mirror VFD files built only if able.
+AM_CONDITIONAL([MIRROR_VFD_CONDITIONAL], [test "X$MIRROR_VFD" = "Xyes"])
+
+## ----------------------------------------------------------------------
 ## Check if Read-Only S3 virtual file driver is enabled by --enable-ros3-vfd
 ##
 AC_SUBST([ROS3_VFD])
@@ -3720,10 +3765,13 @@ AC_CONFIG_FILES([src/libhdf5.settings
                  test/testvds_env.sh
                  test/testvdsswmr.sh
                  test/test_filter_plugin.sh
+                 test/test_mirror.sh
                  test/test_usecases.sh
                  test/test_vol_plugin.sh
                  testpar/Makefile
                  testpar/testpflush.sh
+                 utils/Makefile
+                 utils/mirror_vfd/Makefile
                  tools/Makefile
                  tools/lib/Makefile
                  tools/libtest/Makefile
diff --git a/release_docs/RELEASE.txt b/release_docs/RELEASE.txt
index 1f1ad60..6c43908 100644
--- a/release_docs/RELEASE.txt
+++ b/release_docs/RELEASE.txt
@@ -298,6 +298,20 @@ New Features
 
     Library:
     --------
+    - Add Mirror VFD
+
+      Use TCP/IP sockets to perform write-only (W/O) file I/O on a remote
+      machine. Must be used in conjunction with the Splitter VFD.
+
+      (JOS - 2020/03/13, TBD)
+
+    - Add Splitter VFD
+
+      Maintain separate R/W and W/O channels for "concurrent" file writes
+      to two files using a single HDF5 file handle.
+
+     (JOS - 2020/03/13, TBD)
+
     - Refactored public exposure of haddr_t type in favor of "object tokens"
 
       To better accommodate HDF5 VOL connectors where "object addresses in a file"
diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index 3682caa..c0915c8 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -231,6 +231,7 @@ set (H5FD_SOURCES
     ${HDF5_SRC_DIR}/H5FDhdfs.c
     ${HDF5_SRC_DIR}/H5FDint.c
     ${HDF5_SRC_DIR}/H5FDlog.c
+    ${HDF5_SRC_DIR}/H5FDmirror.c
     ${HDF5_SRC_DIR}/H5FDmpi.c
     ${HDF5_SRC_DIR}/H5FDmpio.c
     ${HDF5_SRC_DIR}/H5FDmulti.c
@@ -238,6 +239,7 @@ set (H5FD_SOURCES
     ${HDF5_SRC_DIR}/H5FDs3comms.c
     ${HDF5_SRC_DIR}/H5FDsec2.c
     ${HDF5_SRC_DIR}/H5FDspace.c
+    ${HDF5_SRC_DIR}/H5FDsplitter.c
     ${HDF5_SRC_DIR}/H5FDstdio.c
     ${HDF5_SRC_DIR}/H5FDtest.c
     ${HDF5_SRC_DIR}/H5FDwindows.c
@@ -249,6 +251,7 @@ set (H5FD_HDRS
     ${HDF5_SRC_DIR}/H5FDfamily.h
     ${HDF5_SRC_DIR}/H5FDhdfs.h
     ${HDF5_SRC_DIR}/H5FDlog.h
+    ${HDF5_SRC_DIR}/H5FDmirror.h
     ${HDF5_SRC_DIR}/H5FDmpi.h
     ${HDF5_SRC_DIR}/H5FDmpio.h
     ${HDF5_SRC_DIR}/H5FDmulti.h
@@ -256,6 +259,7 @@ set (H5FD_HDRS
     ${HDF5_SRC_DIR}/H5FDros3.h
     ${HDF5_SRC_DIR}/H5FDs3comms.h
     ${HDF5_SRC_DIR}/H5FDsec2.h
+    ${HDF5_SRC_DIR}/H5FDsplitter.h
     ${HDF5_SRC_DIR}/H5FDstdio.h
     ${HDF5_SRC_DIR}/H5FDwindows.h
 )
diff --git a/src/H5FDmirror.c b/src/H5FDmirror.c
new file mode 100644
index 0000000..bcbac08
--- /dev/null
+++ b/src/H5FDmirror.c
@@ -0,0 +1,2001 @@
+/* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
+ * Copyright by The HDF Group.                                               *
+ * All rights reserved.                                                      *
+ *                                                                           *
+ * This file is part of HDF5.  The full HDF5 copyright notice, including     *
+ * terms governing use, modification, and redistribution, is contained in    *
+ * the COPYING file, which can be found at the root of the source code       *
+ * distribution tree, or in https://support.hdfgroup.org/ftp/HDF5/releases.  *
+ * If you do not have access to either file, you may request a copy from     *
+ * help@hdfgroup.org.                                                        *
+ * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */
+
+/*
+ * Purpose: Transmit write-only operations to a receiver/writer process on
+ *          a remote host.
+ */
+
+#include "H5FDdrvr_module.h" /* This source code file is part of the H5FD driver module */
+
+#include "H5private.h"      /* Generic Functions        */
+
+#ifdef H5_HAVE_MIRROR_VFD
+
+#include "H5Eprivate.h"     /* Error handling           */
+#include "H5Fprivate.h"     /* File access              */
+#include "H5FDprivate.h"    /* File drivers             */
+#include "H5FDmirror.h"     /* "Mirror" definitions     */
+#include "H5FLprivate.h"    /* Free Lists               */
+#include "H5Iprivate.h"     /* IDs                      */
+#include "H5MMprivate.h"    /* Memory management        */
+#include "H5Pprivate.h"     /* Property lists           */
+
+/* The driver identification number, initialized at runtime */
+static hid_t H5FD_MIRROR_g = 0;
+
+/* Virtual file structure for a Mirror Driver */
+typedef struct H5FD_mirror_t {
+    H5FD_t             pub;     /* Public stuff, must be first            */
+    H5FD_mirror_fapl_t fa;      /* Configuration structure                */
+    haddr_t            eoa;     /* End of allocated region                */
+    haddr_t            eof;     /* End of file; current file size         */
+    int                sock_fd; /* Handle of socket to remote operator    */
+    H5FD_mirror_xmit_t xmit;    /* Primary communication header           */
+    uint32_t           xmit_i;  /* Counter of transmission sent and rec'd */
+} H5FD_mirror_t;
+
+/*
+ * These macros check for overflow of various quantities.  These macros
+ * assume that HDoff_t is signed and haddr_t and size_t are unsigned.
+ *
+ * ADDR_OVERFLOW:   Checks whether a file address of type `haddr_t'
+ *                  is too large to be represented by the second argument
+ *                  of the file seek function.
+ *
+ * SIZE_OVERFLOW:   Checks whether a buffer size of type `hsize_t' is too
+ *                  large to be represented by the `size_t' type.
+ *
+ * REGION_OVERFLOW: Checks whether an address and size pair describe data
+ *                  which can be addressed entirely by the second
+ *                  argument of the file seek function.
+ */
+#define MAXADDR (((haddr_t)1<<(8*sizeof(HDoff_t)-1))-1)
+#define ADDR_OVERFLOW(A)    (HADDR_UNDEF==(A) || ((A) & ~(haddr_t)MAXADDR))
+
+#ifndef BSWAP_64
+#define BSWAP_64(X)                                \
+    (uint64_t)(  (((X) & 0x00000000000000FF) << 56) \
+               | (((X) & 0x000000000000FF00) << 40) \
+               | (((X) & 0x0000000000FF0000) << 24) \
+               | (((X) & 0x00000000FF000000) <<  8) \
+               | (((X) & 0x000000FF00000000) >>  8) \
+               | (((X) & 0x0000FF0000000000) >> 24) \
+               | (((X) & 0x00FF000000000000) >> 40) \
+               | (((X) & 0xFF00000000000000) >> 56))
+#endif /* BSWAP_64 */
+
+/* Debugging flabs for verbose tracing -- nonzero to enable */
+#define MIRROR_DEBUG_OP_CALLS 0
+#define MIRROR_DEBUG_XMIT_BYTES 0
+
+#if MIRROR_DEBUG_XMIT_BYTES
+#define LOG_XMIT_BYTES(label, buf, len) do {                                  \
+    ssize_t bytes_written = 0;                                                \
+    const unsigned char *b = NULL;                                            \
+                                                                              \
+    HDfprintf(stdout, "%s bytes:\n```\n", (label));                           \
+                                                                              \
+    /* print whole lines */                                                   \
+    while ((len - bytes_written) >= 32) {                                     \
+        b = (const unsigned char *)(buf) + bytes_written;                     \
+        HDfprintf(stdout,                                                     \
+                "%04zX  %02X%02X%02X%02X %02X%02X%02X%02X"                    \
+                      " %02X%02X%02X%02X %02X%02X%02X%02X"                    \
+                      " %02X%02X%02X%02X %02X%02X%02X%02X"                    \
+                      " %02X%02X%02X%02X %02X%02X%02X%02X\n",                 \
+                bytes_written,                                                \
+                b[0], b[1], b[2], b[3],   b[4], b[5], b[6], b[7],             \
+                b[8], b[9], b[10],b[11],  b[12],b[13],b[14],b[15],            \
+                b[16],b[17],b[18],b[19],  b[20],b[21],b[22],b[23],            \
+                b[24],b[25],b[26],b[27],  b[28],b[29],b[30],b[31]);           \
+        bytes_written += 32;                                                  \
+    }                                                                         \
+                                                                              \
+    /* start partial line */                                                  \
+    if (len > bytes_written) {                                                \
+        HDfprintf(stdout, "%04zX ", bytes_written);                           \
+    }                                                                         \
+                                                                              \
+    /* partial line blocks */                                                 \
+    while ((len - bytes_written) >= 4) {                                      \
+        HDfprintf(stdout, " %02X%02X%02X%02X",                                \
+                (buf)[bytes_written],   (buf)[bytes_written+1],               \
+                (buf)[bytes_written+2], (buf)[bytes_written+3]);              \
+        bytes_written += 4;                                                   \
+    }                                                                         \
+                                                                              \
+    /* block separator before partial block */                                \
+    if (len > bytes_written) {                                                \
+        HDfprintf(stdout, " ");                                               \
+    }                                                                         \
+                                                                              \
+    /* partial block individual bytes */                                      \
+    while (len > bytes_written) {                                             \
+        HDfprintf(stdout, "%02X", (buf)[bytes_written++]);                    \
+    }                                                                         \
+                                                                              \
+    /* end partial line */                                                    \
+    HDfprintf(stdout, "\n");                                                  \
+    HDfprintf(stdout, "```\n");                                               \
+    HDfflush(stdout);                                                         \
+} while (0)
+#else
+#define LOG_XMIT_BYTES(label, buf, len) /* no-op */
+#endif /* MIRROR_DEBUG_XMIT_BYTE */
+
+#if MIRROR_DEBUG_OP_CALLS
+#define LOG_OP_CALL(name) do {         \
+    HDprintf("called %s()\n", (name)); \
+    fflush(stdout);                    \
+} while (0)
+#else
+#define LOG_OP_CALL(name) /* no-op */
+#endif /* MIRROR_DEBUG_OP_CALLS */
+
+/* Prototypes */
+static herr_t H5FD_mirror_term(void);
+static void *H5FD_mirror_fapl_get(H5FD_t *_file);
+static void *H5FD_mirror_fapl_copy(const void *_old_fa);
+static herr_t H5FD_mirror_fapl_free(void *_fa);
+static haddr_t H5FD_mirror_get_eoa(const H5FD_t *_file, H5FD_mem_t type);
+static herr_t H5FD_mirror_set_eoa(H5FD_t *_file, H5FD_mem_t type, haddr_t addr);
+static haddr_t H5FD_mirror_get_eof(const H5FD_t *_file, H5FD_mem_t type);
+static H5FD_t *H5FD_mirror_open(const char *name, unsigned flags, \
+        hid_t fapl_id, haddr_t maxaddr);
+static herr_t H5FD_mirror_close(H5FD_t *_file);
+static herr_t H5FD_mirror_query(const H5FD_t *_file, unsigned long *flags);
+static herr_t H5FD_mirror_write(H5FD_t *_file, H5FD_mem_t type, hid_t fapl_id, \
+        haddr_t addr, size_t size, const void *buf);
+static herr_t H5FD_mirror_read(H5FD_t *_file, H5FD_mem_t type, hid_t fapl_id,
+        haddr_t addr, size_t size, void *buf);
+static herr_t H5FD_mirror_truncate(H5FD_t *_file, hid_t dxpl_id, \
+        hbool_t closing);
+static herr_t H5FD_mirror_lock(H5FD_t *_file, hbool_t rw);
+static herr_t H5FD_mirror_unlock(H5FD_t *_file);
+
+static herr_t H5FD__mirror_verify_reply(H5FD_mirror_t *file);
+
+static const H5FD_class_t H5FD_mirror_g = {
+    "mirror",                   /* name                 */
+    MAXADDR,                    /* maxaddr              */
+    H5F_CLOSE_WEAK,             /* fc_degree            */
+    H5FD_mirror_term,           /* terminate            */
+    NULL,                       /* sb_size              */
+    NULL,                       /* sb_encode            */
+    NULL,                       /* sb_decode            */
+    0,                          /* fapl_size            */
+    H5FD_mirror_fapl_get,       /* fapl_get             */
+    H5FD_mirror_fapl_copy,      /* fapl_copy            */
+    H5FD_mirror_fapl_free,      /* fapl_free            */
+    0,                          /* dxpl_size            */
+    NULL,                       /* dxpl_copy            */
+    NULL,                       /* dxpl_free            */
+    H5FD_mirror_open,           /* open                 */
+    H5FD_mirror_close,          /* close                */
+    NULL,                       /* cmp                  */
+    H5FD_mirror_query,          /* query                */
+    NULL,                       /* get_type_map         */
+    NULL,                       /* alloc                */
+    NULL,                       /* free                 */
+    H5FD_mirror_get_eoa,        /* get_eoa              */
+    H5FD_mirror_set_eoa,        /* set_eoa              */
+    H5FD_mirror_get_eof,        /* get_eof              */
+    NULL,                       /* get_handle           */
+    H5FD_mirror_read,           /* read                 */
+    H5FD_mirror_write,          /* write                */
+    NULL,                       /* flush                */
+    H5FD_mirror_truncate,       /* truncate             */
+    H5FD_mirror_lock,           /* lock                 */
+    H5FD_mirror_unlock,         /* unlock               */
+    H5FD_FLMAP_DICHOTOMY        /* fl_map               */
+};
+
+/* Declare a free list to manage the H5FD_mirror_t struct */
+H5FL_DEFINE_STATIC(H5FD_mirror_t);
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD__init_package
+ *
+ * Purpose:     Initializes any interface-specific data or routines.
+ *
+ * Return:      Non-negative on success/Negative on failure
+ *-------------------------------------------------------------------------
+ */
+static herr_t
+H5FD__init_package(void)
+{
+    herr_t ret_value = SUCCEED;
+
+    FUNC_ENTER_STATIC
+
+    LOG_OP_CALL("H5FD__init_package");
+
+    if (H5FD_mirror_init() < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_CANTINIT, FAIL,
+                    "unable to initialize mirror VFD");
+    }
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* H5FD__init_package() */
+
+
+/* -------------------------------------------------------------------------
+ * Function:    H5FD_mirror_init
+ *
+ * Purpose:     Initialize this driver by registering the driver with the
+ *              library.
+ *
+ * Return:      Success:    The driver ID for the mirror driver.
+ *              Failure:    Negative
+ * -------------------------------------------------------------------------
+ */
+hid_t
+H5FD_mirror_init(void)
+{
+    hid_t ret_value = H5I_INVALID_HID;
+
+    FUNC_ENTER_NOAPI(FAIL)
+
+    LOG_OP_CALL("H5FD_mirror_init");
+
+    if (H5I_VFL != H5I_get_type(H5FD_MIRROR_g)) {
+        H5FD_MIRROR_g = H5FD_register(&H5FD_mirror_g, sizeof(H5FD_class_t),
+                FALSE);
+    }
+
+    ret_value = H5FD_MIRROR_g;
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD_mirror_init() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    H5FD_mirror_term
+ *
+ * Purpose:     Shut down the VFD
+ *
+ * Returns:     SUCCEED (Can't fail)
+ * ---------------------------------------------------------------------------
+ */
+static herr_t
+H5FD_mirror_term(void)
+{
+    FUNC_ENTER_NOAPI_NOINIT_NOERR
+
+    /* Reset VFL ID */
+    H5FD_MIRROR_g = 0;
+
+    LOG_OP_CALL("H5FD_mirror_term");
+
+    FUNC_LEAVE_NOAPI(SUCCEED)
+} /* end H5FD_mirror_term() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    H5FD__mirror_xmit_decode_uint16
+ *
+ * Purpose:     Extract a 16-bit integer in "network" (Big-Endian) word order
+ *              from the byte-buffer and return it with the local word order at
+ *              the destination pointer.
+ *
+ *              The programmer must ensure that the received buffer holds
+ *              at least the expected size of data.
+ *
+ * Return:      The number of bytes read from the buffer (2).
+ * ---------------------------------------------------------------------------
+ */
+size_t
+H5FD__mirror_xmit_decode_uint16(uint16_t *out, const unsigned char *_buf)
+{
+    uint16_t n = 0;
+    LOG_OP_CALL("H5FD__mirror_xmit_decode_uint16");
+    HDassert(_buf && out);
+    HDmemcpy(&n, _buf, sizeof(n));
+    *out = (uint16_t)HDntohs(n);
+    return 2; /* number of bytes eaten */
+} /* end H5FD__mirror_xmit_decode_uint16() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    H5FD__mirror_xmit_decode_uint32
+ *
+ * Purpose:     Extract a 32-bit integer in "network" (Big-Endian) word order
+ *              from the byte-buffer and return it with the local word order at
+ *              the destination pointer.
+ *
+ *              The programmer must ensure that the received buffer holds
+ *              at least the expected size of data.
+ *
+ * Return:      The number of bytes read from the buffer (4).
+ * ---------------------------------------------------------------------------
+ */
+size_t
+H5FD__mirror_xmit_decode_uint32(uint32_t *out, const unsigned char *_buf)
+{
+    uint32_t n = 0;
+    LOG_OP_CALL("H5FD__mirror_xmit_decode_uint32");
+    HDassert(_buf && out);
+    HDmemcpy(&n, _buf, sizeof(n));
+    *out = (uint32_t)HDntohl(n);
+    return 4; /* number of bytes eaten */
+} /* end H5FD__mirror_xmit_decode_uint32() */
+
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    is_host_little_endian
+ *
+ * Purpose:     Determine whether the host machine is is little-endian.
+ *
+ *              Store an intger with a known value, re-map the memory to a
+ *              character array, and inspect the array's contents.
+ *
+ * Return:      The number of bytes written to the buffer (8).
+ *
+ * Programmer:  Jacob Smith
+ *              2020-03-05
+ * ---------------------------------------------------------------------------
+ */
+static hbool_t
+is_host_little_endian(void)
+{
+    union {
+        uint32_t u32;
+        uint8_t  u8[4];
+    } echeck;
+    echeck.u32 = 0xA1B2C3D4;
+    if (echeck.u8[0] == 0xD4) {
+        return TRUE;
+    }
+    else {
+        return FALSE;
+    }
+} /* end is_host_little_endian() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    H5FD__mirror_xmit_decode_uint64
+ *
+ * Purpose:     Extract a 64-bit integer in "network" (Big-Endian) word order
+ *              from the byte-buffer and return it with the local word order.
+ *
+ *              The programmer must ensure that the received buffer holds
+ *              at least the expected size of data.
+ *
+ *              WARNING: Does not accommodate other forms of endianness,
+ *              e.g. "middle-endian".
+ *
+ * Return:      The number of bytes written to the buffer (8).
+ * ---------------------------------------------------------------------------
+ */
+size_t
+H5FD__mirror_xmit_decode_uint64(uint64_t *out, const unsigned char *_buf)
+{
+    uint64_t n = 0;
+    LOG_OP_CALL("H5FD__mirror_xmit_decode_uint64");
+    HDassert(_buf && out);
+    HDmemcpy(&n, _buf, sizeof(n));
+    if (TRUE == is_host_little_endian()) {
+        *out = BSWAP_64(n);
+    }
+    else {
+        *out = n;
+    }
+    return 8;
+} /* end H5FD__mirror_xmit_decode_uint64() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    H5FD__mirror_xmit_decode_uint8
+ *
+ * Purpose:     Extract a 8-bit integer in "network" (Big-Endian) word order
+ *              from the byte-buffer and return it with the local word order at
+ *              the destination pointer.
+ *              (yes, it's one byte).
+ *
+ * Return:      The number of bytes read from the buffer (1).
+ * ---------------------------------------------------------------------------
+ */
+size_t
+H5FD__mirror_xmit_decode_uint8(uint8_t *out, const unsigned char *_buf)
+{
+    LOG_OP_CALL("H5FD__mirror_xmit_decode_uint8");
+    HDassert(_buf && out);
+    HDmemcpy(out, _buf, sizeof(uint8_t));
+    return 1; /* number of bytes eaten */
+} /* end H5FD__mirror_xmit_decode_uint8() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    H5FD__mirror_xmit_encode_uint16
+ *
+ * Purpose:     Encode a 16-bit integer in "network" (Big-Endian) word order
+ *              in place in the destination bytes-buffer.
+ *
+ *              The programmer must ensure that the destination buffer is
+ *              large enough to hold the expected data.
+ *
+ * Return:      The number of bytes written to the buffer (2).
+ * ---------------------------------------------------------------------------
+ */
+size_t
+H5FD__mirror_xmit_encode_uint16(unsigned char *_dest, uint16_t v)
+{
+    uint16_t n = 0;
+    LOG_OP_CALL("H5FD__mirror_xmit_encode_uint16");
+    HDassert(_dest);
+    n = (uint16_t)HDhtons(v);
+    HDmemcpy(_dest, &n, sizeof(n));
+    return 2;
+} /* end H5FD__mirror_xmit_encode_uint16() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    H5FD__mirror_xmit_encode_uint32
+ *
+ * Purpose:     Encode a 32-bit integer in "network" (Big-Endian) word order
+ *              in place in the destination bytes-buffer.
+ *
+ *              The programmer must ensure that the destination buffer is
+ *              large enough to hold the expected data.
+ *
+ * Return:      The number of bytes written to the buffer (4).
+ * ---------------------------------------------------------------------------
+ */
+size_t
+H5FD__mirror_xmit_encode_uint32(unsigned char *_dest, uint32_t v)
+{
+    uint32_t n = 0;
+    LOG_OP_CALL("H5FD__mirror_xmit_encode_uint32");
+    HDassert(_dest);
+    n = (uint32_t)HDhtonl(v);
+    HDmemcpy(_dest, &n, sizeof(n));
+    return 4;
+} /* end H5FD__mirror_xmit_encode_uint32() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    H5FD__mirror_xmit_encode_uint64
+ *
+ * Purpose:     Encode a 64-bit integer in "network" (Big-Endian) word order
+ *              in place in the destination bytes-buffer.
+ *
+ *              The programmer must ensure that the destination buffer is
+ *              large enough to hold the expected data.
+ *
+ * Return:      The number of bytes written to the buffer (8).
+ * ---------------------------------------------------------------------------
+ */
+size_t
+H5FD__mirror_xmit_encode_uint64(unsigned char *_dest, uint64_t v)
+{
+    uint64_t n = v;
+    LOG_OP_CALL("H5FD__mirror_xmit_decode_uint64");
+    HDassert(_dest);
+    if (TRUE == is_host_little_endian()) {
+        n = BSWAP_64(v);
+    }
+    HDmemcpy(_dest, &n, sizeof(n));
+    return 8;
+} /* H5FD__mirror_xmit_encode_uint64() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    H5FD__mirror_xmit_encode_uint8
+ *
+ * Purpose:     Encode a 8-bit integer in "network" (Big-Endian) word order
+ *              in place in the destination bytes-buffer.
+ *              (yes, it's one byte).
+ *
+ *              The programmer must ensure that the destination buffer is
+ *              large enough to hold the expected data.
+ *
+ * Return:      The number of bytes read from the buffer (1).
+ * ---------------------------------------------------------------------------
+ */
+size_t
+H5FD__mirror_xmit_encode_uint8(unsigned char *dest, uint8_t v)
+{
+    LOG_OP_CALL("H5FD__mirror_xmit_encode_uint8");
+    HDassert(dest);
+    HDmemcpy(dest, &v, sizeof(v));
+    return 1;
+} /* end H5FD__mirror_xmit_encode_uint8() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    H5FD_mirror_xmit_decode_header
+ *
+ * Purpose:     Extract a mirror_xmit_t "header" from the bytes-buffer.
+ *
+ *              Fields will be lifted from the buffer and stored in the
+ *              target structure, using in the correct location (different
+ *              systems may insert different padding between components) and
+ *              word order (Big- vs Little-Endian).
+ *
+ *              The resulting structure should be sanity-checked with
+ *              H5FD_mirror_xmit_is_xmit() before use.
+ *
+ *              The programmer must ensure that the received buffer holds
+ *              at least the expected size of data.
+ *
+ * Return:      The number of bytes consumed from the buffer.
+ * ---------------------------------------------------------------------------
+ */
+size_t
+H5FD_mirror_xmit_decode_header(H5FD_mirror_xmit_t  *out,
+                               const unsigned char *buf)
+{
+    size_t n_eaten = 0;
+    LOG_OP_CALL("H5FD_mirror_xmit_decode_header");
+    HDassert(out && buf);
+    n_eaten += H5FD__mirror_xmit_decode_uint32(&(out->magic), &buf[n_eaten]);
+    n_eaten += H5FD__mirror_xmit_decode_uint8(&(out->version), &buf[n_eaten]);
+    n_eaten += H5FD__mirror_xmit_decode_uint32(&(out->session_token),
+            &buf[n_eaten]);
+    n_eaten += H5FD__mirror_xmit_decode_uint32(&(out->xmit_count),
+            &buf[n_eaten]);
+    n_eaten += H5FD__mirror_xmit_decode_uint8(&(out->op), &buf[n_eaten]);
+    HDassert(n_eaten == H5FD_MIRROR_XMIT_HEADER_SIZE);
+    return n_eaten;
+} /* end H5FD_mirror_xmit_decode_header() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    H5FD_mirror_xmit_decode_lock
+ *
+ * Purpose:     Extract a mirror_xmit_lock_t from the bytes-buffer.
+ *
+ *              Fields will be lifted from the buffer and stored in the
+ *              target structure, using in the correct location (different
+ *              systems may insert different padding between components) and
+ *              word order (Big- vs Little-Endian).
+ *
+ *              The programmer must ensure that the received buffer holds
+ *              at least the expected size of data.
+ *
+ *              The resulting structure should be sanity-checked with
+ *              H5FD_mirror_xmit_is_lock() before use.
+ *
+ * Return:      The number of bytes consumed from the buffer.
+ * ---------------------------------------------------------------------------
+ */
+size_t
+H5FD_mirror_xmit_decode_lock(H5FD_mirror_xmit_lock_t *out,
+                             const unsigned char     *buf)
+{
+    size_t n_eaten = 0;
+    LOG_OP_CALL("H5FD_mirror_xmit_decode_lock");
+    HDassert(out && buf);
+    n_eaten += H5FD_mirror_xmit_decode_header(&(out->pub), buf);
+    n_eaten += H5FD__mirror_xmit_decode_uint64(&(out->rw), &buf[n_eaten]);
+    HDassert(n_eaten == H5FD_MIRROR_XMIT_LOCK_SIZE);
+    return n_eaten;
+} /* end H5FD_mirror_xmit_decode_lock() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    H5FD_mirror_xmit_decode_open
+ *
+ * Purpose:     Extract a mirror_xmit_open_t from the bytes-buffer.
+ *
+ *              Fields will be lifted from the buffer and stored in the
+ *              target structure, using in the correct location (different
+ *              systems may insert different padding between components) and
+ *              word order (Big- vs Little-Endian).
+ *
+ *              The programmer must ensure that the received buffer holds
+ *              at least the expected size of data.
+ *
+ *              The resulting structure should be sanity-checked with
+ *              H5FD_mirror_xmit_is_open() before use.
+ *
+ * Return:      The maximum number of bytes that this decoding operation might
+ *              have consumed from the buffer.
+ * ---------------------------------------------------------------------------
+ */
+size_t
+H5FD_mirror_xmit_decode_open(H5FD_mirror_xmit_open_t *out,
+                             const unsigned char     *buf)
+{
+    size_t n_eaten = 0;
+    LOG_OP_CALL("H5FD_mirror_xmit_decode_open");
+    HDassert(out && buf);
+    n_eaten += H5FD_mirror_xmit_decode_header(&(out->pub), buf);
+    n_eaten += H5FD__mirror_xmit_decode_uint32(&(out->flags), &buf[n_eaten]);
+    n_eaten += H5FD__mirror_xmit_decode_uint64(&(out->maxaddr), &buf[n_eaten]);
+    n_eaten += H5FD__mirror_xmit_decode_uint64(&(out->size_t_blob),
+            &buf[n_eaten]);
+    HDassert((H5FD_MIRROR_XMIT_OPEN_SIZE - H5FD_MIRROR_XMIT_FILEPATH_MAX)
+             == n_eaten);
+    HDstrncpy(out->filename, (const char *)&buf[n_eaten],
+            H5FD_MIRROR_XMIT_FILEPATH_MAX-1);
+    out->filename[H5FD_MIRROR_XMIT_FILEPATH_MAX-1] = 0; /* force final NULL */
+    return H5FD_MIRROR_XMIT_OPEN_SIZE;
+} /* end H5FD_mirror_xmit_decode_open() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    H5FD_mirror_xmit_decode_reply
+ *
+ * Purpose:     Extract a mirror_xmit_reply_t from the bytes-buffer.
+ *
+ *              Fields will be lifted from the buffer and stored in the
+ *              target structure, using in the correct location (different
+ *              systems may insert different padding between components) and
+ *              word order (Big- vs Little-Endian).
+ *
+ *              The programmer must ensure that the received buffer holds
+ *              at least the expected size of data.
+ *
+ *              The resulting structure should be sanity-checked with
+ *              H5FD_mirror_xmit_is_reply() before use.
+ *
+ * Return:      The maximum number of bytes that this decoding operation might
+ *              have consumed from the buffer.
+ * ---------------------------------------------------------------------------
+ */
+size_t
+H5FD_mirror_xmit_decode_reply(H5FD_mirror_xmit_reply_t *out,
+                              const unsigned char      *buf)
+{
+    size_t n_eaten = 0;
+    LOG_OP_CALL("H5FD_mirror_xmit_decode_reply");
+    HDassert(out && buf);
+    n_eaten += H5FD_mirror_xmit_decode_header(&(out->pub), buf);
+    n_eaten += H5FD__mirror_xmit_decode_uint32(&(out->status), &buf[n_eaten]);
+    HDassert((H5FD_MIRROR_XMIT_REPLY_SIZE - H5FD_MIRROR_STATUS_MESSAGE_MAX)
+             == n_eaten);
+    HDstrncpy(out->message, (const char *)&buf[n_eaten],
+            H5FD_MIRROR_STATUS_MESSAGE_MAX-1);
+    out->message[H5FD_MIRROR_STATUS_MESSAGE_MAX-1] = 0; /* force NULL term */
+    return H5FD_MIRROR_XMIT_REPLY_SIZE;
+} /* end H5FD_mirror_xmit_decode_reply() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    H5FD_mirror_xmit_decode_set_eoa
+ *
+ * Purpose:     Extract a mirror_xmit_eoa_t from the bytes-buffer.
+ *
+ *              Fields will be lifted from the buffer and stored in the
+ *              target structure, using in the correct location (different
+ *              systems may insert different padding between components) and
+ *              word order (Big- vs Little-Endian).
+ *
+ *              The programmer must ensure that the received buffer holds
+ *              at least the expected size of data.
+ *
+ *              The resulting structure should be sanity-checked with
+ *              H5FD_mirror_xmit_is_set_eoa() before use.
+ *
+ * Return:      The number of bytes consumed from the buffer.
+ * ---------------------------------------------------------------------------
+ */
+size_t
+H5FD_mirror_xmit_decode_set_eoa(H5FD_mirror_xmit_eoa_t *out,
+                                const unsigned char    *buf)
+{
+    size_t n_eaten = 0;
+    LOG_OP_CALL("H5FD_mirror_xmit_decode_set_eoa");
+    HDassert(out && buf);
+    n_eaten += H5FD_mirror_xmit_decode_header(&(out->pub), buf);
+    n_eaten += H5FD__mirror_xmit_decode_uint8(&(out->type), &buf[n_eaten]);
+    n_eaten += H5FD__mirror_xmit_decode_uint64(&(out->eoa_addr), &buf[n_eaten]);
+    HDassert(n_eaten == H5FD_MIRROR_XMIT_EOA_SIZE);
+    return n_eaten;
+} /* end H5FD_mirror_xmit_decode_set_eoa() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    H5FD_mirror_xmit_decode_write
+ *
+ * Purpose:     Extract a mirror_xmit_write_t from the bytes-buffer.
+ *
+ *              Fields will be lifted from the buffer and stored in the
+ *              target structure, using in the correct location (different
+ *              systems may insert different padding between components) and
+ *              word order (Big- vs Little-Endian).
+ *
+ *              The programmer must ensure that the received buffer holds
+ *              at least the expected size of data.
+ *
+ *              The resulting structure should be sanity-checked with
+ *              H5FD_mirror_xmit_is_write() before use.
+ *
+ * Return:      The number of bytes consumed from the buffer.
+ * ---------------------------------------------------------------------------
+ */
+size_t
+H5FD_mirror_xmit_decode_write(H5FD_mirror_xmit_write_t *out,
+                              const unsigned char      *buf)
+{
+    size_t n_eaten = 0;
+    LOG_OP_CALL("H5FD_mirror_xmit_decode_write");
+    HDassert(out && buf);
+    n_eaten += H5FD_mirror_xmit_decode_header(&(out->pub), buf);
+    n_eaten += H5FD__mirror_xmit_decode_uint8(&(out->type), &buf[n_eaten]);
+    n_eaten += H5FD__mirror_xmit_decode_uint64(&(out->offset), &buf[n_eaten]);
+    n_eaten += H5FD__mirror_xmit_decode_uint64(&(out->size), &buf[n_eaten]);
+    HDassert(n_eaten == H5FD_MIRROR_XMIT_WRITE_SIZE);
+    return n_eaten;
+} /* end H5FD_mirror_xmit_decode_write() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    H5FD_mirror_xmit_encode_header
+ *
+ * Purpose:     Encode a mirror_xmit_t "header" to the bytes-buffer.
+ *
+ *              Fields will be packed into the buffer in a predictable manner,
+ *              any numbers stored in "network" (Big-Endian) word order.
+ *
+ *              The programmer must ensure that the destination buffer is
+ *              large enough to hold the expected data.
+ *
+ * Return:      The number of bytes written to the buffer.
+ * ---------------------------------------------------------------------------
+ */
+size_t
+H5FD_mirror_xmit_encode_header(unsigned char            *dest,
+                               const H5FD_mirror_xmit_t *x)
+{
+    size_t n_writ = 0;
+    LOG_OP_CALL("H5FD_mirror_xmit_encode_header");
+    HDassert(dest && x);
+    n_writ += H5FD__mirror_xmit_encode_uint32((dest+n_writ), x->magic);
+    n_writ += H5FD__mirror_xmit_encode_uint8((dest+n_writ), x->version);
+    n_writ += H5FD__mirror_xmit_encode_uint32((dest+n_writ), x->session_token);
+    n_writ += H5FD__mirror_xmit_encode_uint32((dest+n_writ), x->xmit_count);
+    n_writ += H5FD__mirror_xmit_encode_uint8((dest+n_writ), x->op);
+    HDassert(n_writ == H5FD_MIRROR_XMIT_HEADER_SIZE);
+    return n_writ;
+} /* end H5FD_mirror_xmit_encode_header() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    H5FD_mirror_xmit_encode_lock
+ *
+ * Purpose:     Encode a mirror_xmit_lock_t to the bytes-buffer.
+ *              Fields will be packed into the buffer in a predictable manner,
+ *              any numbers stored in "network" (Big-Endian) word order.
+ *
+ *              The programmer must ensure that the destination buffer is
+ *              large enough to hold the expected data.
+ *
+ * Return:      The number of bytes written to the buffer.
+ * ---------------------------------------------------------------------------
+ */
+size_t
+H5FD_mirror_xmit_encode_lock(unsigned char *dest,
+                             const H5FD_mirror_xmit_lock_t *x)
+{
+    size_t n_writ = 0;
+    LOG_OP_CALL("H5FD_mirror_xmit_encode_lock");
+    HDassert(dest && x);
+    n_writ += H5FD_mirror_xmit_encode_header(dest,
+            (const H5FD_mirror_xmit_t *)&(x->pub));
+    n_writ += H5FD__mirror_xmit_encode_uint64(&dest[n_writ], x->rw);
+    HDassert(n_writ == H5FD_MIRROR_XMIT_LOCK_SIZE);
+    return n_writ;
+} /* end H5FD_mirror_xmit_encode_lock() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    H5FD_mirror_xmit_encode_open
+ *
+ * Purpose:     Encode a mirror_xmit_open_t to the bytes-buffer.
+ *              Fields will be packed into the buffer in a predictable manner,
+ *              any numbers stored in "network" (Big-Endian) word order.
+ *
+ *              The programmer must ensure that the destination buffer is
+ *              large enough to hold the expected data.
+ *
+ * Return:      The maximum number of bytes that this decoding operation might
+ *              have written into the buffer.
+ * ---------------------------------------------------------------------------
+ */
+size_t
+H5FD_mirror_xmit_encode_open(unsigned char                 *dest,
+                             const H5FD_mirror_xmit_open_t *x)
+{
+    size_t n_writ = 0;
+    LOG_OP_CALL("H5FD_mirror_xmit_encode_open");
+    HDassert(dest && x);
+    /* clear entire structure, but especially its filepath string area */
+    for (n_writ = 0; n_writ < H5FD_MIRROR_XMIT_OPEN_SIZE; n_writ++) {
+        *(dest+n_writ) = 0;
+    }
+    n_writ = 0;
+    n_writ += H5FD_mirror_xmit_encode_header(dest,
+            (const H5FD_mirror_xmit_t *)&(x->pub));
+    n_writ += H5FD__mirror_xmit_encode_uint32(&dest[n_writ], x->flags);
+    n_writ += H5FD__mirror_xmit_encode_uint64(&dest[n_writ], x->maxaddr);
+    n_writ += H5FD__mirror_xmit_encode_uint64(&dest[n_writ], x->size_t_blob);
+    HDassert((H5FD_MIRROR_XMIT_OPEN_SIZE - H5FD_MIRROR_XMIT_FILEPATH_MAX)
+             == n_writ);
+    HDstrncpy((char *)&dest[n_writ], x->filename,
+            H5FD_MIRROR_XMIT_FILEPATH_MAX);
+    return H5FD_MIRROR_XMIT_OPEN_SIZE;
+} /* end H5FD_mirror_xmit_encode_open() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    H5FD_mirror_xmit_encode_reply
+ *
+ * Purpose:     Encode a mirror_xmit_reply_t to the bytes-buffer.
+ *
+ *              Fields will be packed into the buffer in a predictable manner,
+ *              any numbers stored in "network" (Big-Endian) word order.
+ *
+ *              The programmer must ensure that the destination buffer is
+ *              large enough to hold the expected data.
+ *
+ * Return:      The maximum number of bytes that this decoding operation might
+ *              have written into the buffer.
+ * ---------------------------------------------------------------------------
+ */
+size_t
+H5FD_mirror_xmit_encode_reply(unsigned char                  *dest,
+                              const H5FD_mirror_xmit_reply_t *x)
+{
+    size_t n_writ = 0;
+    LOG_OP_CALL("H5FD_mirror_xmit_encode_reply");
+    HDassert(dest && x);
+    /* clear entire structure, but especially its message string area */
+    for (n_writ = 0; n_writ < H5FD_MIRROR_XMIT_REPLY_SIZE; n_writ++) {
+        *(dest+n_writ) = 0;
+    }
+    n_writ = 0;
+    n_writ += H5FD_mirror_xmit_encode_header(dest,
+            (const H5FD_mirror_xmit_t *)&(x->pub));
+    n_writ += H5FD__mirror_xmit_encode_uint32(&dest[n_writ], x->status);
+    HDassert((H5FD_MIRROR_XMIT_REPLY_SIZE - H5FD_MIRROR_STATUS_MESSAGE_MAX)
+             == n_writ);
+    HDstrncpy((char *)&dest[n_writ], x->message,
+            H5FD_MIRROR_STATUS_MESSAGE_MAX);
+    return H5FD_MIRROR_XMIT_REPLY_SIZE;
+} /* end H5FD_mirror_xmit_encode_reply() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    H5FD_mirror_xmit_encode_set_eoa
+ *
+ * Purpose:     Encode a mirror_xmit_eoa_t to the bytes-buffer.
+ *
+ *              Fields will be packed into the buffer in a predictable manner,
+ *              any numbers stored in "network" (Big-Endian) word order.
+ *
+ *              The programmer must ensure that the destination buffer is
+ *              large enough to hold the expected data.
+ *
+ * Return:      The number of bytes written to the buffer.
+ * ---------------------------------------------------------------------------
+ */
+size_t
+H5FD_mirror_xmit_encode_set_eoa(unsigned char                *dest,
+                                const H5FD_mirror_xmit_eoa_t *x)
+{
+    size_t n_writ = 0;
+    LOG_OP_CALL("H5FD_mirror_xmit_encode_set_eoa");
+    HDassert(dest && x);
+    n_writ += H5FD_mirror_xmit_encode_header(dest,
+            (const H5FD_mirror_xmit_t *)&(x->pub));
+    n_writ += H5FD__mirror_xmit_encode_uint8(&dest[n_writ], x->type);
+    n_writ += H5FD__mirror_xmit_encode_uint64(&dest[n_writ], x->eoa_addr);
+    HDassert(n_writ == H5FD_MIRROR_XMIT_EOA_SIZE);
+    return n_writ;
+} /* end H5FD_mirror_xmit_encode_set_eoa() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    H5FD_mirror_xmit_encode_write
+ *
+ * Purpose:     Encode a mirror_xmit_write_t to the bytes-buffer.
+ *
+ *              Fields will be packed into the buffer in a predictable manner,
+ *              any numbers stored in "network" (Big-Endian) word order.
+ *
+ *              The programmer must ensure that the destination buffer is
+ *              large enough to hold the expected data.
+ *
+ * Return:      The number of bytes written to the buffer.
+ * ---------------------------------------------------------------------------
+ */
+size_t
+H5FD_mirror_xmit_encode_write(unsigned char                  *dest,
+                              const H5FD_mirror_xmit_write_t *x)
+{
+    size_t n_writ = 0;
+    LOG_OP_CALL("H5FD_mirror_xmit_encode_write");
+    HDassert(dest && x);
+    n_writ += H5FD_mirror_xmit_encode_header(dest,
+            (const H5FD_mirror_xmit_t *)&(x->pub));
+    n_writ += H5FD__mirror_xmit_encode_uint8(&dest[n_writ], x->type);
+    n_writ += H5FD__mirror_xmit_encode_uint64(&dest[n_writ], x->offset);
+    n_writ += H5FD__mirror_xmit_encode_uint64(&dest[n_writ], x->size);
+    HDassert(n_writ == H5FD_MIRROR_XMIT_WRITE_SIZE);
+    return n_writ;
+} /* end H5FD_mirror_xmit_encode_write() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    H5FD_mirror_xmit_is_close
+ *
+ * Purpose:     Verify that a mirror_xmit_t is a valid CLOSE xmit.
+ *
+ *              Checks header validity and op code.
+ *
+ * Return:      TRUE if valid; else FALSE.
+ * ---------------------------------------------------------------------------
+ */
+hbool_t
+H5FD_mirror_xmit_is_close(const H5FD_mirror_xmit_t *xmit)
+{
+    LOG_OP_CALL("H5FD_mirror_xmit_is_close");
+    HDassert(xmit);
+    if ( (TRUE == H5FD_mirror_xmit_is_xmit(xmit)) &&
+         (H5FD_MIRROR_OP_CLOSE == xmit->op) )
+    {
+        return TRUE;
+    }
+    return FALSE;
+} /* end H5FD_mirror_xmit_is_close() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    H5FD_mirror_xmit_is_lock
+ *
+ * Purpose:     Verify that a mirror_xmit_lock_t is a valid LOCK xmit.
+ *
+ *              Checks header validity and op code.
+ *
+ * Return:      TRUE if valid; else FALSE.
+ * ---------------------------------------------------------------------------
+ */
+hbool_t
+H5FD_mirror_xmit_is_lock(const H5FD_mirror_xmit_lock_t *xmit)
+{
+    LOG_OP_CALL("H5FD_mirror_xmit_is_lock");
+    HDassert(xmit);
+    if ( (TRUE == H5FD_mirror_xmit_is_xmit(&(xmit->pub))) &&
+         (H5FD_MIRROR_OP_LOCK == xmit->pub.op) )
+    {
+        return TRUE;
+    }
+    return FALSE;
+} /* end H5FD_mirror_xmit_is_lock() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    H5FD_mirror_xmit_is_open
+ *
+ * Purpose:     Verify that a mirror_xmit_open_t is a valid OPEN xmit.
+ *
+ *              Checks header validity and op code.
+ *
+ * Return:      TRUE if valid; else FALSE.
+ * ---------------------------------------------------------------------------
+ */
+hbool_t
+H5FD_mirror_xmit_is_open(const H5FD_mirror_xmit_open_t *xmit)
+{
+    LOG_OP_CALL("H5FD_mirror_xmit_is_open");
+    HDassert(xmit);
+    if ( (TRUE == H5FD_mirror_xmit_is_xmit(&(xmit->pub))) &&
+         (H5FD_MIRROR_OP_OPEN == xmit->pub.op) )
+    {
+        return TRUE;
+    }
+    return FALSE;
+} /* end H5FD_mirror_xmit_is_open() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    H5FD_mirror_xmit_is_eoa
+ *
+ * Purpose:     Verify that a mirror_xmit_eoa_t is a valid SET-EOA xmit.
+ *
+ *              Checks header validity and op code.
+ *
+ * Return:      TRUE if valid; else FALSE.
+ * ---------------------------------------------------------------------------
+ */
+hbool_t
+H5FD_mirror_xmit_is_set_eoa(const H5FD_mirror_xmit_eoa_t *xmit)
+{
+    LOG_OP_CALL("H5FD_mirror_xmit_is_set_eoa");
+    HDassert(xmit);
+    if ( (TRUE == H5FD_mirror_xmit_is_xmit(&(xmit->pub))) &&
+         (H5FD_MIRROR_OP_SET_EOA == xmit->pub.op) )
+    {
+        return TRUE;
+    }
+    return FALSE;
+} /* end H5FD_mirror_xmit_is_eoa() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    H5FD_mirror_xmit_is_reply
+ *
+ * Purpose:     Verify that a mirror_xmit_reply_t is a valid REPLY xmit.
+ *
+ *              Checks header validity and op code.
+ *
+ * Return:      TRUE if valid; else FALSE.
+ * ---------------------------------------------------------------------------
+ */
+hbool_t
+H5FD_mirror_xmit_is_reply(const H5FD_mirror_xmit_reply_t *xmit)
+{
+    LOG_OP_CALL("H5FD_mirror_xmit_is_reply");
+    HDassert(xmit);
+    if ( (TRUE == H5FD_mirror_xmit_is_xmit(&(xmit->pub))) &&
+         (H5FD_MIRROR_OP_REPLY == xmit->pub.op) )
+    {
+        return TRUE;
+    }
+    return FALSE;
+} /* end H5FD_mirror_xmit_is_reply() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    H5FD_mirror_xmit_is_write
+ *
+ * Purpose:     Verify that a mirror_xmit_write_t is a valid WRITE xmit.
+ *
+ *              Checks header validity and op code.
+ *
+ * Return:      TRUE if valid; else FALSE.
+ * ---------------------------------------------------------------------------
+ */
+hbool_t
+H5FD_mirror_xmit_is_write(const H5FD_mirror_xmit_write_t *xmit)
+{
+    LOG_OP_CALL("H5FD_mirror_xmit_is_write");
+    HDassert(xmit);
+    if ( (TRUE == H5FD_mirror_xmit_is_xmit(&(xmit->pub))) &&
+         (H5FD_MIRROR_OP_WRITE == xmit->pub.op) )
+    {
+        return TRUE;
+    }
+    return FALSE;
+} /* end H5FD_mirror_xmit_is_write() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    H5FD_mirror_xmit_is_xmit
+ *
+ * Purpose:     Verify that a mirror_xmit_t is well-formed.
+ *
+ *              Checks magic number and structure version.
+ *
+ * Return:      TRUE if valid; else FALSE.
+ * ---------------------------------------------------------------------------
+ */
+hbool_t
+H5FD_mirror_xmit_is_xmit(const H5FD_mirror_xmit_t *xmit)
+{
+    LOG_OP_CALL("H5FD_mirror_xmit_is_xmit");
+    HDassert(xmit);
+    if ( (H5FD_MIRROR_XMIT_MAGIC != xmit->magic) || 
+         (H5FD_MIRROR_XMIT_CURR_VERSION != xmit->version) )
+    {
+        return FALSE;
+    }
+    return TRUE;
+} /* end H5FD_mirror_xmit_is_xmit() */
+
+
+/* ----------------------------------------------------------------------------
+ * Function:    H5FD__mirror_verify_reply
+ *
+ * Purpose:     Wait for and read reply data from remote processes.
+ *              Sanity-check that a reply is well-formed and valid.
+ *              If all checks pass, inspect the reply contents and handle
+ *              reported error, if not an OK reply.
+ *
+ * Return:      SUCCEED if ok, else FAIL.
+ * ----------------------------------------------------------------------------
+ */
+herr_t
+H5FD__mirror_verify_reply(H5FD_mirror_t *file)
+{
+    char                                xmit_buf[H5FD_MIRROR_XMIT_REPLY_SIZE];
+    struct H5FD_mirror_xmit_reply_t     reply;
+    ssize_t                             read_ret = 0;
+    herr_t                              ret_value = SUCCEED;
+
+    FUNC_ENTER_NOAPI_NOINIT;
+
+    LOG_OP_CALL("H5FD__mirror_verify_reply");
+
+    HDassert(file && file->sock_fd);
+
+    read_ret = HDread(file->sock_fd, xmit_buf, H5FD_MIRROR_XMIT_REPLY_SIZE);
+    if (read_ret < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_READERROR, FAIL, "unable to read reply");
+    }
+    if (read_ret != H5FD_MIRROR_XMIT_REPLY_SIZE) {
+        HGOTO_ERROR(H5E_VFL, H5E_READERROR, FAIL, "unexpected read size");
+    }
+
+    LOG_XMIT_BYTES("reply", xmit_buf, read_ret);
+
+    if (H5FD_mirror_xmit_decode_reply(&reply, (const unsigned char *)xmit_buf)
+        != H5FD_MIRROR_XMIT_REPLY_SIZE)
+    {
+        HGOTO_ERROR(H5E_VFL, H5E_BADVALUE, FAIL, "unable to decode reply xmit");
+    }
+
+    if (H5FD_mirror_xmit_is_reply(&reply) != TRUE) {
+        HGOTO_ERROR(H5E_VFL, H5E_BADVALUE, FAIL, "xmit op code was not REPLY");
+    }
+
+    if (reply.pub.session_token != file->xmit.session_token) {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADVALUE, FAIL, "wrong session");
+    }
+
+    if (reply.pub.xmit_count != (file->xmit_i)++) {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADVALUE, FAIL, "xmit out of sync");
+    }
+
+    if (reply.status != H5FD_MIRROR_STATUS_OK) {
+        HGOTO_ERROR(H5E_VFL, H5E_BADVALUE, FAIL,
+                    "%s", (const char *)(reply.message));
+    }
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value);
+} /* end H5FD__mirror_verify_reply() */
+
+
+/* -------------------------------------------------------------------------
+ * Function:    H5FD_mirror_fapl_get
+ *
+ * Purpose:     Get the file access propety list which could be used to create
+ *              an identical file.
+ *
+ * Return:      Success: pointer to the new file access property list value.
+ *              Failure: NULL
+ * -------------------------------------------------------------------------
+ */
+static void *
+H5FD_mirror_fapl_get(H5FD_t *_file)
+{
+    H5FD_mirror_t      *file      = (H5FD_mirror_t *)_file;
+    H5FD_mirror_fapl_t *fa        = NULL;
+    void               *ret_value = NULL;
+
+    FUNC_ENTER_NOAPI_NOINIT
+
+    LOG_OP_CALL("H5FD_mirror_fapl_get");
+
+    fa = (H5FD_mirror_fapl_t *)H5MM_calloc(sizeof(H5FD_mirror_fapl_t));
+    if (NULL == fa) {
+        HGOTO_ERROR(H5E_RESOURCE, H5E_NOSPACE, NULL, "calloc failed");
+    }
+
+    HDmemcpy(fa, &(file->fa), sizeof(H5FD_mirror_fapl_t));
+
+    ret_value = fa;
+
+done:
+    if (ret_value == NULL) {
+        if (fa != NULL) {
+            H5MM_xfree(fa);
+        }
+    }
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD_mirror_fapl_get() */
+
+
+/* -------------------------------------------------------------------------
+ * Function:    H5FD_mirror_fapl_copy
+ *
+ * Purpose:     Copies the mirror vfd-specific file access properties.
+ *
+ * Return:      Success:        Pointer to a new property list
+ *              Failure:        NULL
+ * -------------------------------------------------------------------------
+ */
+static void *
+H5FD_mirror_fapl_copy(const void *_old_fa)
+{
+    const H5FD_mirror_fapl_t *old_fa    = (const H5FD_mirror_fapl_t *)_old_fa;
+    H5FD_mirror_fapl_t       *new_fa    = NULL;
+    void                     *ret_value = NULL;
+
+    FUNC_ENTER_NOAPI_NOINIT
+
+    LOG_OP_CALL("H5FD_mirror_fapl_copy");
+
+    new_fa = (H5FD_mirror_fapl_t *)H5MM_malloc(sizeof(H5FD_mirror_fapl_t));
+    if (new_fa == NULL) {
+        HGOTO_ERROR(H5E_RESOURCE, H5E_NOSPACE, NULL,
+                    "memory allocation failed");
+    }
+
+    HDmemcpy(new_fa, old_fa, sizeof(H5FD_mirror_fapl_t));
+    ret_value = new_fa;
+
+done:
+    if (ret_value == NULL) {
+        if (new_fa != NULL) {
+            H5MM_xfree(new_fa);
+        }
+    }
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD_mirror_fapl_copy() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_mirror_fapl_free
+ *
+ * Purpose:     Frees the mirror VFD-specific file access properties.
+ *
+ * Return:      SUCCEED (cannot fail)
+ *-------------------------------------------------------------------------
+ */
+static herr_t
+H5FD_mirror_fapl_free(void *_fa)
+{
+    H5FD_mirror_fapl_t *fa = (H5FD_mirror_fapl_t*)_fa;
+
+    FUNC_ENTER_NOAPI_NOINIT_NOERR
+
+    LOG_OP_CALL("H5FD_mirror_fapl_free");
+
+    /* sanity check */
+    HDassert(fa != NULL);
+    HDassert(fa->magic == H5FD_MIRROR_FAPL_MAGIC);
+
+    fa->magic += 1; /* invalidate */
+    H5MM_xfree(fa);
+
+    FUNC_LEAVE_NOAPI(SUCCEED)
+} /* end H5FD_mirror_fapl_free() */
+
+
+/* -------------------------------------------------------------------------
+ * Function:    H5Pget_fapl_mirror
+ *
+ * Purpose:     Get the configuration information for this fapl.
+ *              Data is memcopied into the fa_out pointer.
+ *
+ * Return:      SUCCEED/FAIL
+ * -------------------------------------------------------------------------
+ */
+herr_t
+H5Pget_fapl_mirror(hid_t fapl_id, H5FD_mirror_fapl_t *fa_out)
+{
+    const H5FD_mirror_fapl_t *fa        = NULL;
+    H5P_genplist_t           *plist     = NULL;
+    herr_t                    ret_value = SUCCEED;
+
+    FUNC_ENTER_API(FAIL)
+    H5TRACE2("e", "i*x", fapl_id, fa_out);
+
+    LOG_OP_CALL("H5Pget_fapl_mirror");
+
+    if (NULL == fa_out) {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADVALUE, FAIL, "fa_out is NULL");
+    }
+
+    plist = H5P_object_verify(fapl_id, H5P_FILE_ACCESS);
+    if (NULL == plist) {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADTYPE, FAIL,
+                    "not a file access property list");
+    }
+
+    if (H5P_peek_driver(plist) != H5FD_MIRROR) {
+        HGOTO_ERROR(H5E_PLIST, H5E_BADVALUE, FAIL, "incorrect VFL driver");
+    }
+
+    fa = (const H5FD_mirror_fapl_t *)H5P_peek_driver_info(plist);
+    if (NULL == fa) {
+        HGOTO_ERROR(H5E_PLIST, H5E_BADVALUE, FAIL, "bad VFL driver info");
+    }
+
+    HDassert(fa->magic == H5FD_MIRROR_FAPL_MAGIC); /* sanity check */
+
+    HDmemcpy(fa_out, fa, sizeof(H5FD_mirror_fapl_t));
+
+done:
+    FUNC_LEAVE_API(ret_value);
+} /* end H5Pget_fapl_mirror() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5Pset_fapl_mirror
+ *
+ * Purpose:     Modify the file access property list to use the mirror
+ *              driver (H5FD_MIRROR) defined in this source file.
+ *
+ * Return:      SUCCEED/FAIL
+ *-------------------------------------------------------------------------
+ */
+herr_t
+H5Pset_fapl_mirror(hid_t fapl_id, H5FD_mirror_fapl_t *fa)
+{
+    H5P_genplist_t *plist = NULL;
+    herr_t ret_value = FAIL;
+
+    FUNC_ENTER_API(FAIL)
+    H5TRACE2("e", "i*x", fapl_id, fa);
+
+    LOG_OP_CALL("H5Pset_fapl_mirror");
+
+    plist = H5P_object_verify(fapl_id, H5P_FILE_ACCESS);
+    if (NULL == plist) {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADTYPE, FAIL,
+                    "not a file access property list");
+    }
+    if (NULL == fa) {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADVALUE, FAIL, "null fapl_t pointer");
+    }
+    if (H5FD_MIRROR_FAPL_MAGIC != fa->magic) {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADVALUE, FAIL, "invalid fapl_t magic");
+    }
+    if (H5FD_MIRROR_CURR_FAPL_T_VERSION != fa->version) {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADVALUE, FAIL, "unknown fapl_t version");
+    }
+
+    ret_value = H5P_set_driver(plist, H5FD_MIRROR, (const void *)fa);
+
+done:
+    FUNC_LEAVE_API(ret_value)
+} /* end H5Pset_fapl_mirror() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_mirror_open
+ *
+ * Purpose:     Create and/or opens a file as an HDF5 file.
+ *
+ *              Initiate connection with remote Server/Writer.
+ *              If successful, the remote file is open.
+ *
+ * Return:      Success:    A pointer to a new file data structure. The
+ *                          public fields will be initialized by the
+ *                          caller, which is always H5FD_open().
+ *              Failure:    NULL
+ *-------------------------------------------------------------------------
+ */
+static H5FD_t *
+H5FD_mirror_open(const char *name,
+                 unsigned    flags,
+                 hid_t       fapl_id,
+                 haddr_t     maxaddr)
+{
+#define MIRR_OPEN_MAXBUF 16 /* local symbol to give meaning to magic number  */
+                            /* #defined because it is needed at compile time */
+                            /* Large enough to hold a port number string     */
+    int                      live_socket = -1;
+    struct sockaddr_in       target_addr;
+    socklen_t                addr_size;
+    char                     xmit_buf[H5FD_MIRROR_XMIT_OPEN_SIZE];
+    H5FD_mirror_fapl_t       fa;
+    H5FD_mirror_t           *file      = NULL;
+    H5FD_mirror_xmit_open_t  open_xmit;
+    H5FD_t                  *ret_value = NULL;
+
+    FUNC_ENTER_NOAPI_NOINIT
+
+    LOG_OP_CALL("H5FD_mirror_open");
+
+    /* --------------- */
+    /* Check arguments */
+    /* --------------- */
+
+    if (!name || !*name) {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADVALUE, NULL, "invalid file name");
+    }
+    if (HDstrlen(name) >= H5FD_MIRROR_XMIT_FILEPATH_MAX) {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADVALUE, NULL, "filename is too long");
+    }
+    if (0 == maxaddr || HADDR_UNDEF == maxaddr) {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADRANGE, NULL, "bogus maxaddr");
+    }
+    if (ADDR_OVERFLOW(maxaddr)) {
+        HGOTO_ERROR(H5E_ARGS, H5E_OVERFLOW, NULL, "bogus maxaddr");
+    }
+
+    if (H5Pget_fapl_mirror(fapl_id, &fa) == FAIL) {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADVALUE, NULL, "can't get config info");
+    }
+
+    if (H5FD_MIRROR_FAPL_MAGIC != fa.magic) {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADVALUE, NULL, "invalid fapl magic");
+    }
+
+    if (H5FD_MIRROR_CURR_FAPL_T_VERSION != fa.version) {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADVALUE, NULL, "invalid fapl version");
+    }
+
+    /* --------------------- */
+    /* Handshake with remote */
+    /* --------------------- */
+
+    live_socket = HDsocket(AF_INET, SOCK_STREAM, 0);
+    if (live_socket < 0) {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADVALUE, NULL, "can't create socket");
+    }
+
+    target_addr.sin_family = AF_INET;
+    target_addr.sin_port = HDhtons((uint16_t)fa.handshake_port);
+    target_addr.sin_addr.s_addr = HDinet_addr(fa.remote_ip);
+    HDmemset(target_addr.sin_zero, '\0', sizeof target_addr.sin_zero);
+
+    addr_size = sizeof(target_addr);
+    if (HDconnect(live_socket, (struct sockaddr *)&target_addr, addr_size) < 0)
+    {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADVALUE, NULL,
+                    "can't connect to remote server");
+    }
+
+    /* ------------- */
+    /* Open the file */
+    /* ------------- */
+
+    file = (H5FD_mirror_t *)H5FL_CALLOC(H5FD_mirror_t);
+    if (NULL == file) {
+        HGOTO_ERROR(H5E_RESOURCE, H5E_NOSPACE, NULL,
+                    "unable to allocate file struct");
+    }
+
+    file->sock_fd = live_socket;
+    file->xmit_i = 0;
+
+    file->xmit.magic = H5FD_MIRROR_XMIT_MAGIC;
+    file->xmit.version = H5FD_MIRROR_XMIT_CURR_VERSION;
+    file->xmit.xmit_count = file->xmit_i++;
+    file->xmit.session_token = (uint32_t)(0x01020304 ^ file->sock_fd); /* TODO: hashing? */
+    /* int --> uint32_t may truncate on some systems... shouldn't matter? */
+
+    file->xmit.op = H5FD_MIRROR_OP_OPEN;
+    open_xmit.pub = file->xmit;
+    open_xmit.flags = (uint32_t)flags;
+    open_xmit.maxaddr = (uint64_t)maxaddr;
+    open_xmit.size_t_blob = (uint64_t)((size_t)(-1));
+    HDsnprintf(open_xmit.filename, H5FD_MIRROR_XMIT_FILEPATH_MAX-1, "%s", name);
+
+    if (H5FD_mirror_xmit_encode_open((unsigned char *)xmit_buf,
+            (const H5FD_mirror_xmit_open_t *)&open_xmit)
+        != H5FD_MIRROR_XMIT_OPEN_SIZE)
+    {
+        HGOTO_ERROR(H5E_VFL, H5E_WRITEERROR, NULL, "unable to encode open");
+    }
+
+    LOG_XMIT_BYTES("open", xmit_buf, H5FD_MIRROR_XMIT_OPEN_SIZE);
+
+    if (HDwrite(file->sock_fd, xmit_buf, H5FD_MIRROR_XMIT_OPEN_SIZE) < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_WRITEERROR, NULL, "unable to transmit open");
+    }
+
+    if (H5FD__mirror_verify_reply(file) == FAIL) {
+        HGOTO_ERROR(H5E_VFL, H5E_BADVALUE, NULL, "invalid reply");
+    }
+
+    ret_value = (H5FD_t *)file;
+
+done:
+    if (NULL == ret_value) {
+        if (file) {
+            file = H5FL_FREE(H5FD_mirror_t, file);
+        }
+        if (live_socket >= 0 && HDclose(live_socket) < 0) {
+            HDONE_ERROR(H5E_VFL, H5E_CANTCLOSEFILE, NULL,
+                        "can't close socket");
+        }
+    }
+    FUNC_LEAVE_NOAPI(ret_value)
+#undef MIRR_OPEN_MAXBUF
+} /* end H5FD_mirror_open() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_mirror_close
+ *
+ * Purpose:     Closes the HDF5 file.
+ *
+ *              Tries to send a CLOSE op to the remote Writer and expects
+ *              a valid reply, then closes the socket.
+ *              In error, attempts to send a deliberately invalid xmit to the
+ *              Writer to get it to close/abort, then attempts to close the
+ *              socket.
+ *
+ * Return:      Success:    SUCCEED
+ *              Failure:    FAIL, file possibly not closed but resources freed.
+ *-------------------------------------------------------------------------
+ */
+static herr_t
+H5FD_mirror_close(H5FD_t *_file)
+{
+    H5FD_mirror_t *file = (H5FD_mirror_t *)_file;
+    unsigned char  xmit_buf[H5FD_MIRROR_XMIT_HEADER_SIZE];
+    int xmit_encoded = 0; /* monitor point of failure */
+    herr_t         ret_value = SUCCEED;
+
+    FUNC_ENTER_NOAPI_NOINIT
+
+    LOG_OP_CALL("H5FD_mirror_close");
+
+    /* Sanity check */
+    HDassert(file);
+    HDassert(file->sock_fd >= 0);
+
+    file->xmit.xmit_count = (file->xmit_i)++;
+    file->xmit.op = H5FD_MIRROR_OP_CLOSE;
+
+    if (H5FD_mirror_xmit_encode_header(xmit_buf,
+            (const H5FD_mirror_xmit_t *)&(file->xmit))
+        != H5FD_MIRROR_XMIT_HEADER_SIZE)
+    {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADVALUE, FAIL, "unable to encode close");
+    }
+    xmit_encoded = 1;
+
+    LOG_XMIT_BYTES("close", xmit_buf, H5FD_MIRROR_XMIT_HEADER_SIZE);
+
+    if (HDwrite(file->sock_fd, xmit_buf, H5FD_MIRROR_XMIT_HEADER_SIZE) < 0) {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADVALUE, FAIL, "unable to transmit close");
+    }
+
+    if (H5FD__mirror_verify_reply(file) == FAIL) {
+        HGOTO_ERROR(H5E_VFL, H5E_BADVALUE, FAIL, "invalid reply");
+    }
+
+    if (HDclose(file->sock_fd) < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_CANTCLOSEFILE, FAIL, "can't close socket");
+    }
+
+done:
+    if (ret_value == FAIL) {
+        if (xmit_encoded == 0) {
+            /* Encode failed; send GOODBYE to force writer halt.
+             * We can ignore any response from the writer, if we receive
+             * any reply at all.
+             */
+            if (HDwrite(file->sock_fd, "GOODBYE", HDstrlen("GOODBYE")) < 0) {
+                HDONE_ERROR(H5E_ARGS, H5E_BADVALUE, FAIL,
+                            "unable to transmit close");
+                if (HDclose(file->sock_fd) < 0) {
+                    HDONE_ERROR(H5E_VFL, H5E_CANTCLOSEFILE, FAIL,
+                                "can't close socket");
+                }
+                file->sock_fd = -1; /* invalidate for later */
+            } /* end if problem writing goodbye; go down hard */
+            else
+            if (HDshutdown(file->sock_fd, SHUT_WR) < 0) {
+                HDONE_ERROR(H5E_VFL, H5E_BADVALUE, FAIL,
+                            "can't shutdown socket write: %s",
+                            HDstrerror(errno));
+            } /* end else-if problem shutting down socket */
+        } /* end if xmit encode failed */
+
+        if (file->sock_fd >= 0) {
+            if (HDclose(file->sock_fd) < 0) {
+                HDONE_ERROR(H5E_VFL, H5E_CANTCLOSEFILE, FAIL,
+                            "can't close socket");
+            }
+        } /* end if socket not closed by going down hard */
+    } /* end if error */
+
+    file = H5FL_FREE(H5FD_mirror_t, file); /* always release resources */
+
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD_mirror_close() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_mirror_query
+ *
+ * Purpose:     Get the driver feature flags implemented by the driver.
+ *
+ * Return:      SUCCEED (non-negative) (can't fail)
+ *-------------------------------------------------------------------------
+ */
+static herr_t
+H5FD_mirror_query(const H5FD_t H5_ATTR_UNUSED *_file, unsigned long *flags)
+{
+    FUNC_ENTER_NOAPI_NOINIT_NOERR;
+
+    LOG_OP_CALL("H5FD_mirror_query");
+
+    /* Notice: the Mirror VFD Writer currently uses only the Sec2 driver as
+     * the underying driver -- as such, the Mirror VFD implementation copies
+     * the Sec2 feature flags as its own.
+     *
+     * File pointer is always NULL/unused -- the H5FD_FEAT_IGNORE_DRVRINFO flag
+     * is never included.
+     * -- JOS 2020-01-13
+     */
+    if (flags) {
+        *flags = 0                                      \
+               | H5FD_FEAT_AGGREGATE_METADATA           \
+               | H5FD_FEAT_ACCUMULATE_METADATA          \
+               | H5FD_FEAT_DATA_SIEVE                   \
+               | H5FD_FEAT_AGGREGATE_SMALLDATA          \
+               | H5FD_FEAT_POSIX_COMPAT_HANDLE          \
+               | H5FD_FEAT_SUPPORTS_SWMR_IO             \
+               | H5FD_FEAT_DEFAULT_VFD_COMPATIBLE;
+    }
+
+    FUNC_LEAVE_NOAPI(SUCCEED);
+} /* end H5FD_mirror_query() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_mirror_get_eoa
+ *
+ * Purpose:     Gets the end-of-address marker for the file. The EOA marker
+ *              is the first address past the last byte allocated in the
+ *              format address space.
+ *
+ *              Required to register the driver.
+ *
+ * Return:      The end-of-address marker.
+ *-------------------------------------------------------------------------
+ */
+static haddr_t
+H5FD_mirror_get_eoa(const H5FD_t *_file, H5FD_mem_t H5_ATTR_UNUSED type)
+{
+    const H5FD_mirror_t *file = (const H5FD_mirror_t *)_file;
+
+    FUNC_ENTER_NOAPI_NOINIT_NOERR
+
+    LOG_OP_CALL("H5FD_mirror_get_eoa");
+
+    HDassert(file);
+
+    FUNC_LEAVE_NOAPI(file->eoa)
+} /* end H5FD_mirror_get_eoa() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_mirror_set_eoa
+ *
+ * Purpose:     Set the end-of-address marker for the file. This function is
+ *              called shortly after an existing HDF5 file is opened in order
+ *              to tell the driver where the end of the HDF5 data is located.
+ *
+ * Return:      SUCCEED / FAIL
+ *-------------------------------------------------------------------------
+ */
+static herr_t
+H5FD_mirror_set_eoa(H5FD_t *_file, H5FD_mem_t type, haddr_t addr)
+{
+    H5FD_mirror_xmit_eoa_t      xmit_eoa;
+    unsigned char       xmit_buf[H5FD_MIRROR_XMIT_EOA_SIZE];
+    H5FD_mirror_t      *file      = (H5FD_mirror_t *)_file;
+    herr_t              ret_value = SUCCEED;
+
+    FUNC_ENTER_NOAPI_NOINIT
+
+    LOG_OP_CALL("H5FD_mirror_set_eoa");
+
+    HDassert(file);
+
+    file->eoa = addr; /* local copy */
+
+    file->xmit.xmit_count = (file->xmit_i)++;
+    file->xmit.op = H5FD_MIRROR_OP_SET_EOA;
+
+    xmit_eoa.pub = file->xmit;
+    xmit_eoa.type = (uint8_t)type;
+    xmit_eoa.eoa_addr = (uint64_t)addr;
+
+    if (H5FD_mirror_xmit_encode_set_eoa(xmit_buf,
+            (const H5FD_mirror_xmit_eoa_t *)&xmit_eoa)
+        != H5FD_MIRROR_XMIT_EOA_SIZE)
+    {
+        HGOTO_ERROR(H5E_VFL, H5E_WRITEERROR, FAIL, "unable to encode set-eoa");
+    }
+
+    LOG_XMIT_BYTES("set-eoa", xmit_buf, H5FD_MIRROR_XMIT_EOA_SIZE);
+
+    if (HDwrite(file->sock_fd, xmit_buf, H5FD_MIRROR_XMIT_EOA_SIZE) < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_WRITEERROR, FAIL,
+                    "unable to transmit set-eoa");
+    }
+
+    if (H5FD__mirror_verify_reply(file) == FAIL) {
+        HGOTO_ERROR(H5E_VFL, H5E_BADVALUE, FAIL, "invalid reply");
+    }
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD_mirror_set_eoa() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_mirror_get_eof
+ *
+ * Purpose:     Returns the end-of-file marker, which is the greater of
+ *              either the filesystem end-of-file or the HDF5 end-of-address
+ *              markers.
+ *
+ *              Required to register the driver.
+ *
+ * Return:      End of file address, the first address past the end of the
+ *              "file", either the filesystem file or the HDF5 file.
+ *-------------------------------------------------------------------------
+ */
+static haddr_t
+H5FD_mirror_get_eof(const H5FD_t *_file, H5FD_mem_t H5_ATTR_UNUSED type)
+{
+    const H5FD_mirror_t *file = (const H5FD_mirror_t *)_file;
+
+    FUNC_ENTER_NOAPI_NOINIT_NOERR
+
+    LOG_OP_CALL("H5FD_mirror_get_eof");
+
+    HDassert(file);
+
+    FUNC_LEAVE_NOAPI(file->eof)
+} /* end H5FD_mirror_get_eof() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_mirror_read
+ *
+ * Purpose:     Required to register the driver.
+ *              If called, MUST fail.
+ *
+ * Return:      FAIL
+ *-------------------------------------------------------------------------
+ */
+static herr_t
+H5FD_mirror_read(H5FD_t     H5_ATTR_UNUSED *_file,
+                 H5FD_mem_t H5_ATTR_UNUSED  type,
+                 hid_t      H5_ATTR_UNUSED  fapl_id,
+                 haddr_t    H5_ATTR_UNUSED  addr,
+                 size_t     H5_ATTR_UNUSED  size,
+                 void       H5_ATTR_UNUSED *buf)
+{
+    herr_t ret_value = FAIL;
+
+    FUNC_ENTER_NOAPI_NOINIT_NOERR
+
+    LOG_OP_CALL("H5FD_mirror_read");
+
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD_mirror_read() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_mirror_write
+ *
+ * Purpose:     Writes SIZE bytes of data to FILE beginning at address ADDR
+ *              from buffer BUF according to data transfer properties in
+ *              DXPL_ID.
+ *
+ *              Send metadata regarding the write (location, size) to the
+ *              remote Writer, then separately transmits the data.
+ *              Both transmission expect an OK reply from the Writer.
+ *              This two-exchange approach incurs significant overhead,
+ *              but is a simple and modular approach.
+ *              Start optimizations here.
+ *
+ * Return:      SUCCEED/FAIL
+ *-------------------------------------------------------------------------
+ */
+static herr_t
+H5FD_mirror_write(H5FD_t               *_file,
+                  H5FD_mem_t            type,
+                  hid_t H5_ATTR_UNUSED  dxpl_id,
+                  haddr_t               addr,
+                  size_t                size,
+                  const void           *buf)
+{
+    H5FD_mirror_xmit_write_t    xmit_write;
+    unsigned char               xmit_buf[H5FD_MIRROR_XMIT_WRITE_SIZE];
+    H5FD_mirror_t              *file        = (H5FD_mirror_t *)_file;
+    herr_t                      ret_value   = SUCCEED;
+
+    FUNC_ENTER_NOAPI_NOINIT
+
+    LOG_OP_CALL("H5FD_mirror_write");
+
+    HDassert(file);
+    HDassert(buf);
+
+    file->xmit.xmit_count = (file->xmit_i)++;
+    file->xmit.op = H5FD_MIRROR_OP_WRITE;
+
+    xmit_write.pub = file->xmit;
+    xmit_write.size = (uint64_t)size;
+    xmit_write.offset = (uint64_t)addr;
+    xmit_write.type = (uint8_t)type;
+
+
+    /* Notify Writer of incoming data to write. */
+    if (H5FD_mirror_xmit_encode_write(xmit_buf,
+            (const H5FD_mirror_xmit_write_t *)&xmit_write)
+        != H5FD_MIRROR_XMIT_WRITE_SIZE)
+    {
+        HGOTO_ERROR(H5E_VFL, H5E_WRITEERROR, FAIL, "unable to encode write");
+    }
+
+    LOG_XMIT_BYTES("write", xmit_buf, H5FD_MIRROR_XMIT_WRITE_SIZE);
+
+    if (HDwrite(file->sock_fd, xmit_buf, H5FD_MIRROR_XMIT_WRITE_SIZE) < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_WRITEERROR, FAIL, "unable to transmit write");
+    }
+
+    /* Check that our write xmission was received */
+    if (H5FD__mirror_verify_reply(file) == FAIL) {
+        HGOTO_ERROR(H5E_VFL, H5E_BADVALUE, FAIL, "invalid reply");
+    }
+
+    /* Send the data to be written */
+    if (HDwrite(file->sock_fd, buf, size) < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_WRITEERROR, FAIL, "unable to transmit data");
+    }
+
+    /* Writer should reply that it got the data and is still okay/ready */
+    if (H5FD__mirror_verify_reply(file) == FAIL) {
+        HGOTO_ERROR(H5E_VFL, H5E_BADVALUE, FAIL, "invalid reply");
+    }
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD_mirror_write() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_mirror_truncate
+ *
+ * Purpose:     Makes sure that the true file size is the same (or larger)
+ *              than the end-of-address.
+ *
+ * Return:      SUCCEED/FAIL
+ *-------------------------------------------------------------------------
+ */
+static herr_t
+H5FD_mirror_truncate(H5FD_t                 *_file,
+                     hid_t   H5_ATTR_UNUSED  dxpl_id,
+                     hbool_t H5_ATTR_UNUSED  closing)
+{
+    unsigned char       xmit_buf[H5FD_MIRROR_XMIT_HEADER_SIZE];
+    H5FD_mirror_t      *file = (H5FD_mirror_t *)_file;
+    herr_t      ret_value = SUCCEED;
+
+    FUNC_ENTER_NOAPI_NOINIT
+
+    LOG_OP_CALL("H5FD_mirror_truncate");
+
+    file->xmit.xmit_count = (file->xmit_i)++;
+    file->xmit.op = H5FD_MIRROR_OP_TRUNCATE;
+
+    if (H5FD_mirror_xmit_encode_header(xmit_buf, &(file->xmit))
+        != H5FD_MIRROR_XMIT_HEADER_SIZE)
+    {
+        HGOTO_ERROR(H5E_VFL, H5E_WRITEERROR, FAIL, "unable to encode truncate");
+    }
+
+    LOG_XMIT_BYTES("truncate", xmit_buf, H5FD_MIRROR_XMIT_HEADER_SIZE);
+
+    if (HDwrite(file->sock_fd, xmit_buf, H5FD_MIRROR_XMIT_HEADER_SIZE) < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_WRITEERROR, FAIL,
+                    "unable to transmit truncate");
+    }
+
+    if (H5FD__mirror_verify_reply(file) == FAIL) {
+        HGOTO_ERROR(H5E_VFL, H5E_BADVALUE, FAIL, "invalid reply");
+    }
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD_mirror_truncate() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_mirror_lock
+ *
+ * Purpose:     To place an advisory lock on a file.
+ *              The lock type to apply depends on the parameter "rw":
+ *                      TRUE--opens for write: an exclusive lock
+ *                      FALSE--opens for read: a shared lock
+ *
+ * Return:      SUCCEED/FAIL
+ *-------------------------------------------------------------------------
+ */
+static herr_t
+H5FD_mirror_lock(H5FD_t *_file, hbool_t rw)
+{
+    H5FD_mirror_xmit_lock_t xmit_lock;
+    unsigned char       xmit_buf[H5FD_MIRROR_XMIT_LOCK_SIZE];
+    H5FD_mirror_t      *file = (H5FD_mirror_t *)_file;
+    herr_t ret_value = SUCCEED;
+
+    FUNC_ENTER_NOAPI_NOINIT;
+
+    LOG_OP_CALL("H5FD_mirror_lock");
+
+    file->xmit.xmit_count = (file->xmit_i)++;
+    file->xmit.op = H5FD_MIRROR_OP_LOCK;
+
+    xmit_lock.pub = file->xmit;
+    xmit_lock.rw = (uint64_t)rw;
+
+    if (H5FD_mirror_xmit_encode_lock(xmit_buf,
+            (const H5FD_mirror_xmit_lock_t *)&xmit_lock)
+        != H5FD_MIRROR_XMIT_LOCK_SIZE)
+    {
+        HGOTO_ERROR(H5E_VFL, H5E_WRITEERROR, FAIL, "unable to encode lock");
+    }
+
+    LOG_XMIT_BYTES("lock", xmit_buf, H5FD_MIRROR_XMIT_LOCK_SIZE);
+
+    if (HDwrite(file->sock_fd, xmit_buf, H5FD_MIRROR_XMIT_LOCK_SIZE) < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_WRITEERROR, FAIL, "unable to transmit lock");
+    }
+
+    if (H5FD__mirror_verify_reply(file) == FAIL) {
+        HGOTO_ERROR(H5E_VFL, H5E_BADVALUE, FAIL, "invalid reply");
+    }
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value);
+} /* end H5FD_mirror_lock */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_mirror_unlock
+ *
+ * Purpose:     Remove the existing lock on the file.
+ *
+ * Return:      SUCCEED/FAIL
+ *-------------------------------------------------------------------------
+ */
+static herr_t
+H5FD_mirror_unlock(H5FD_t *_file)
+{
+    unsigned char       xmit_buf[H5FD_MIRROR_XMIT_HEADER_SIZE];
+    H5FD_mirror_t      *file = (H5FD_mirror_t *)_file;
+    herr_t ret_value = SUCCEED;
+
+    FUNC_ENTER_NOAPI_NOINIT;
+
+    LOG_OP_CALL("H5FD_mirror_unlock");
+
+    file->xmit.xmit_count = (file->xmit_i)++;
+    file->xmit.op = H5FD_MIRROR_OP_UNLOCK;
+
+    if (H5FD_mirror_xmit_encode_header(xmit_buf, &(file->xmit))
+        != H5FD_MIRROR_XMIT_HEADER_SIZE)
+    {
+        HGOTO_ERROR(H5E_VFL, H5E_WRITEERROR, FAIL, "unable to encode unlock");
+    }
+
+    LOG_XMIT_BYTES("unlock", xmit_buf, H5FD_MIRROR_XMIT_HEADER_SIZE);
+
+    if (HDwrite(file->sock_fd, xmit_buf, H5FD_MIRROR_XMIT_HEADER_SIZE) < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_WRITEERROR, FAIL, "unable to transmit unlock");
+    }
+
+    if (H5FD__mirror_verify_reply(file) == FAIL) {
+        HGOTO_ERROR(H5E_VFL, H5E_BADVALUE, FAIL, "invalid reply");
+    }
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value);
+} /* end H5FD_mirror_unlock */
+
+#endif /* H5_HAVE_MIRROR_VFD */
+
diff --git a/src/H5FDmirror.h b/src/H5FDmirror.h
new file mode 100644
index 0000000..fb66b7b
--- /dev/null
+++ b/src/H5FDmirror.h
@@ -0,0 +1,371 @@
+/* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
+ * Copyright by The HDF Group.                                               *
+ * All rights reserved.                                                      *
+ *                                                                           *
+ * This file is part of HDF5.  The full HDF5 copyright notice, including     *
+ * terms governing use, modification, and redistribution, is contained in    *
+ * the COPYING file, which can be found at the root of the source code       *
+ * distribution tree, or in https://support.hdfgroup.org/ftp/HDF5/releases.  *
+ * If you do not have access to either file, you may request a copy from     *
+ * help@hdfgroup.org.                                                        *
+ * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */
+
+/*
+ * Purpose: Public, shared definitions for Mirror VFD & remote Writer.
+ */
+
+#ifndef H5FDmirror_H
+#define H5FDmirror_H
+
+#ifdef H5_HAVE_MIRROR_VFD
+
+#define H5FD_MIRROR (H5FD_mirror_init())
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/* ============================================================================
+ * Mirror VFD use and operation.
+ * ============================================================================
+ */
+
+/* ---------------------------------------------------------------------------
+ * Structure:   H5FD_mirror_fapl_t
+ *
+ * Used to pass configuraiton information to the Mirror VFD.
+ * Populate components as appropriate and pass structure pointer to
+ * `H5Pset_fapl_mirror()`.
+ *
+ * `magic` (uint32_t)
+ *      Semi-unique number to sanity-check pointers to this structure type.
+ *      MUST equal H5FD_MIRROR_FAPL_MAGIC to be considered valid.
+ *
+ * `version` (uint32_t)
+ *      Indicates expected components of the structure.
+ *
+ * `handshake_port (int)
+ *      Port number to expect to reach the "Mirror Server" on the remote host.
+ *
+ * `remote_ip` (char[])
+ *      IP address string of "Mirror Server" remote host.
+ * ---------------------------------------------------------------------------
+ */
+#define H5FD_MIRROR_FAPL_MAGIC 0xF8DD514C
+#define H5FD_MIRROR_CURR_FAPL_T_VERSION 1
+#define H5FD_MIRROR_MAX_IP_LEN 32
+typedef struct H5FD_mirror_fapl_t {
+    uint32_t magic;
+    uint32_t version;
+    int     handshake_port;
+    char    remote_ip[H5FD_MIRROR_MAX_IP_LEN + 1];
+} H5FD_mirror_fapl_t;
+
+H5_DLL hid_t  H5FD_mirror_init(void);
+H5_DLL herr_t H5Pget_fapl_mirror(hid_t fapl_id, H5FD_mirror_fapl_t *fa_out);
+H5_DLL herr_t H5Pset_fapl_mirror(hid_t fapl_id, H5FD_mirror_fapl_t *fa);
+
+/* = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = =
+ * IPC - Mirror VFD and Remote Worker application.
+ * = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = =
+ */
+
+
+/* The maximum allowed size for a receiving buffer when accepting bytes to
+ * write. Writes larger than this size are performed by multiple accept-write
+ * steps by the Writer. */
+#define H5FD_MIRROR_DATA_BUFFER_MAX H5_GB /* 1 Gigabyte */
+
+#define H5FD_MIRROR_XMIT_CURR_VERSION 1
+#define H5FD_MIRROR_XMIT_MAGIC 0x87F8005B
+
+#define H5FD_MIRROR_OP_OPEN     1
+#define H5FD_MIRROR_OP_CLOSE    2
+#define H5FD_MIRROR_OP_WRITE    3
+#define H5FD_MIRROR_OP_TRUNCATE 4
+#define H5FD_MIRROR_OP_REPLY    5
+#define H5FD_MIRROR_OP_SET_EOA  6
+#define H5FD_MIRROR_OP_LOCK     7
+#define H5FD_MIRROR_OP_UNLOCK   8
+
+#define H5FD_MIRROR_STATUS_OK 0
+#define H5FD_MIRROR_STATUS_ERROR 1
+#define H5FD_MIRROR_STATUS_MESSAGE_MAX 256 /* Dedicated error message size */
+
+/* Maximum length of a path/filename string, including the NULL-terminator.
+ * Must not be smaller than H5FD_SPLITTER_PATH_MAX. */
+#define H5FD_MIRROR_XMIT_FILEPATH_MAX 4097
+
+/* Define the exact sizes of the various xmit blobs as sent over the wire.
+ * This is used to minimize the number of bytes transmitted as well as to
+ * sanity-check received bytes.
+ * Any modifications to the xmit structures and/or the encode/decode functions
+ * must be reflected here.
+ * */
+#define H5FD_MIRROR_XMIT_HEADER_SIZE 14
+#define H5FD_MIRROR_XMIT_EOA_SIZE (H5FD_MIRROR_XMIT_HEADER_SIZE + 9)
+#define H5FD_MIRROR_XMIT_LOCK_SIZE (H5FD_MIRROR_XMIT_HEADER_SIZE + 8)
+#define H5FD_MIRROR_XMIT_OPEN_SIZE (H5FD_MIRROR_XMIT_HEADER_SIZE + 20 + H5FD_MIRROR_XMIT_FILEPATH_MAX)
+#define H5FD_MIRROR_XMIT_REPLY_SIZE (H5FD_MIRROR_XMIT_HEADER_SIZE + 4 + H5FD_MIRROR_STATUS_MESSAGE_MAX)
+#define H5FD_MIRROR_XMIT_WRITE_SIZE (H5FD_MIRROR_XMIT_HEADER_SIZE + 17)
+
+/* Maximum length of any xmit. */
+#define H5FD_MIRROR_XMIT_BUFFER_MAX MAX2( MAX3(H5FD_MIRROR_XMIT_HEADER_SIZE,  \
+                                               H5FD_MIRROR_XMIT_EOA_SIZE,     \
+                                               H5FD_MIRROR_XMIT_LOCK_SIZE),   \
+                                          MAX3(H5FD_MIRROR_XMIT_OPEN_SIZE,    \
+                                               H5FD_MIRROR_XMIT_REPLY_SIZE,   \
+                                               H5FD_MIRROR_XMIT_WRITE_SIZE) ) \
+
+/* ---------------------------------------------------------------------------
+ * Structure:   H5FD_mirror_xmit_t
+ *
+ * Common structure 'header' for all mirror VFD/worker IPC.
+ * Must be the first component of a derived operation xmit structure,
+ * such as file-open or write command.
+ *
+ * `magic` (uint32_t)
+ *      A "unique" number identifying the structure and endianness of
+ *      transmitting maching.
+ *      Must be set to H5FD_MIRROR_XMIT_MAGIC native to the VFD "sender".
+ *
+ * `version` (uint8_t)
+ *      Number used to identify the structure membership.
+ *      Allows sane modifications to this structure in the future.
+ *      Must be set to H5FD_MIRROR_XMIT_CURR_VERSION.
+ *
+ * `session_token` (uint32_t)
+ *      A "unique" number identifying the session between VFD sender and
+ *      remote receiver/worker/writer. Exists to help sanity-check.
+ *
+ * `xmit_count` (uint32_t)
+ *      Which transmission this is since the session began.
+ *      Used to sanity-check transmission errors.
+ *      First xmit (file-open) must be 0.
+ *
+ * `op` (uint8_t)
+ *      Number identifying which operation to perform.
+ *      Corresponds with the extended structure outside of this xmit header.
+ *      Possible values are all defined H5FD_MIRROR_OP_* constants.
+ *
+ * ---------------------------------------------------------------------------
+ */
+typedef struct H5FD_mirror_xmit_t {
+    uint32_t magic;
+    uint8_t  version;
+    uint32_t session_token;
+    uint32_t xmit_count;
+    uint8_t  op;
+} H5FD_mirror_xmit_t;
+
+/* ---------------------------------------------------------------------------
+ * Structure:   H5FD_mirror_xmit_eoa_t
+ *
+ * Structure containing eoa-set information from VFD sender.
+ *
+ * `pub` (H5FD_mirror_xmit_t)
+ *      Common transmission header, containing session information.
+ *      Must be first.
+ *
+ * `type` (uint8_t)
+ *      System-independent alias for H5F[D]_mem_t.
+ *      Specifies datatype to be written.
+ *
+ * `eoa_addr` (uint64_t)
+ *      New address for eoa.
+ *      (Natively 'haddr_t', always a 64-bit field)
+ *
+ * ---------------------------------------------------------------------------
+ */
+typedef struct H5FD_mirror_xmit_eoa_t {
+    H5FD_mirror_xmit_t pub;
+    uint8_t            type;
+    uint64_t           eoa_addr;
+} H5FD_mirror_xmit_eoa_t;
+
+/* ---------------------------------------------------------------------------
+ * Structure:   H5FD_mirror_xmit_lock_t
+ *
+ * Structure containing eoa-set information from VFD sender.
+ *
+ * `pub` (H5FD_mirror_xmit_t)
+ *      Common transmission header, containing session information.
+ *      Must be first.
+ *
+ * `rw` (uint64_t)
+ *      The Read/Write mode flag passed into H5FDlock().
+ *      (Natively `hbool_t`, an 'int') TODO: native int may be 64-bit?
+ *
+ * ---------------------------------------------------------------------------
+ */
+typedef struct H5FD_mirror_xmit_lock_t {
+    H5FD_mirror_xmit_t pub;
+    uint64_t           rw;
+} H5FD_mirror_xmit_lock_t;
+
+/* ---------------------------------------------------------------------------
+ * Structure:   H5FD_mirror_xmit_open_t
+ *
+ * Structure containing file-open information from the VFD sender.
+ *
+ * `pub` (H5FD_mirror_xmit_t)
+ *      Common transmission header, containing session information.
+ *      Must be first.
+ *
+ * `flags` (uint32_t)
+ *      VFL-layer file-open flags passed directly to H5FDopen().
+ *      (Natively 'unsigned [int]') TODO: native int may be 64-bit?
+ *
+ * `maxaddr` (uint64_t)
+ *      VFL-layer maximum allowed address space for the file to open passed
+ *      directly to H5FDopen().
+ *      (Natively 'haddr_t', always a 64-bit field)
+ *
+ * `size_t_blob` (uint64_t)
+ *      A number indicating how large a size_t is on the sending system.
+ *      Must be set to (uint64_t)((size_t)(-1))
+ *          (maximum possible value of size_t, cast to uint64_t).
+ *      The receiving system inspects this value -- if the local (remote)
+ *      size_t is smaller than that of the Sender, issues a warning.
+ *      Not an error, as:
+ *      1. It is assumed that underlying file systems/drivers have become
+ *         smart enough to handle file sizes that otherwise might be
+ *         constrained.
+ *      2. The Mirror Writer ingests bytes to write multiple 'slices' if the
+ *         size is greater than H5FD_MIRROR_DATA_BUFFER_MAX, regardless of
+ *         any size_t storage size disparity.
+ *
+ * `filename` (char[])
+ *      String giving the filename and path of file to open.
+ *
+ * ---------------------------------------------------------------------------
+ */
+typedef struct H5FD_mirror_xmit_open_t {
+    H5FD_mirror_xmit_t pub;
+    uint32_t           flags;
+    uint64_t           maxaddr;
+    uint64_t           size_t_blob;
+    char               filename[H5FD_MIRROR_XMIT_FILEPATH_MAX];
+} H5FD_mirror_xmit_open_t;
+
+/* ---------------------------------------------------------------------------
+ * Structure: H5FD_mirror_xmit_reply_t
+ *
+ * Structure used by the remote receiver/worker/writer to respond to
+ * a command from the VFD sender.
+ *
+ * `pub` (H5FD_mirror_xmit_t)
+ *      Common transmission header, containing session information.
+ *      Must be first.
+ *
+ * `status` (uint32_t)
+ *      Number indicating whether the command was successful or if an
+ *      occured.
+ *      Allowed values are H5FD_MIRROR_STATUS_OK and
+ *      H5FD_MIRROR_STATUS_ERROR.
+ *
+ * `message` (char[])
+ *      Error message. Populated if and only if there was a problem.
+ *      It is possible that a message may reach the end of the alloted
+ *      space without a NULL terminator -- the onus is on the programmer to
+ *      handle this situation.
+ *
+ * ---------------------------------------------------------------------------
+ */
+typedef struct H5FD_mirror_xmit_reply_t {
+    H5FD_mirror_xmit_t pub;
+    uint32_t           status;
+    char               message[H5FD_MIRROR_STATUS_MESSAGE_MAX];
+} H5FD_mirror_xmit_reply_t;
+
+/* ---------------------------------------------------------------------------
+ * Structure:   H5FD_mirror_xmit_write_t
+ *
+ * Structure containing data-write information from VFD sender.
+ *
+ * The data to be written is transmitted in subsequent, packets
+ * and may be broken up into more than one transmission buffer.
+ * The VFD sender and remote receiver/worker/writer must coordinate
+ * the receipt of data.
+ *
+ * `pub` (H5FD_mirror_xmit_t)
+ *      Common transmission header, containing session information.
+ *      Must be first.
+ *
+ * `type` (uint8_t)
+ *      Specifies datatype to be written.
+ *      (Natively 'H5FD_mem_t', an enumerated type in H5Fpublic.h)
+ *
+ * `offset` (uint64_t)
+ *      Start location of write in file.
+ *      (Natively 'haddr_t', always a 64-bit field)
+ *
+ * `size` (uint64_t)
+ *      Size of the data to be written, in bytes.
+ *      (Natively 'size_t', accommodate the largest possible as 64-bits)
+ *
+ * ---------------------------------------------------------------------------
+ */
+typedef struct H5FD_mirror_xmit_write_t {
+    H5FD_mirror_xmit_t  pub;
+    uint8_t             type;
+    uint64_t            offset;
+    uint64_t            size;
+} H5FD_mirror_xmit_write_t;
+
+
+
+/* Encode/decode routines are required to "pack" the xmit data into a known
+ * byte format for transmission over the wire.
+ *
+ * All component numbers must be stored in "network" word order (Big-Endian).
+ *
+ * All components must be packed in the order given in the structure definition.
+ *
+ * All components must be packed with zero padding between.
+ */
+
+H5_DLL size_t H5FD__mirror_xmit_decode_uint16(uint16_t *out, const unsigned char *buf);
+H5_DLL size_t H5FD__mirror_xmit_decode_uint32(uint32_t *out, const unsigned char *buf);
+H5_DLL size_t H5FD__mirror_xmit_decode_uint64(uint64_t *out, const unsigned char *buf);
+H5_DLL size_t H5FD__mirror_xmit_decode_uint8(uint8_t *out, const unsigned char *buf);
+H5_DLL size_t H5FD__mirror_xmit_encode_uint16(unsigned char *dest, uint16_t v);
+H5_DLL size_t H5FD__mirror_xmit_encode_uint32(unsigned char *dest, uint32_t v);
+H5_DLL size_t H5FD__mirror_xmit_encode_uint64(unsigned char *dest, uint64_t v);
+H5_DLL size_t H5FD__mirror_xmit_encode_uint8(unsigned char *dest, uint8_t v);
+
+H5_DLL size_t H5FD_mirror_xmit_decode_header(H5FD_mirror_xmit_t *out, const unsigned char *buf);
+H5_DLL size_t H5FD_mirror_xmit_decode_lock(H5FD_mirror_xmit_lock_t *out, const unsigned char *buf);
+H5_DLL size_t H5FD_mirror_xmit_decode_open(H5FD_mirror_xmit_open_t *out, const unsigned char *buf);
+H5_DLL size_t H5FD_mirror_xmit_decode_reply(H5FD_mirror_xmit_reply_t *out, const unsigned char *buf);
+H5_DLL size_t H5FD_mirror_xmit_decode_set_eoa(H5FD_mirror_xmit_eoa_t *out, const unsigned char *buf);
+H5_DLL size_t H5FD_mirror_xmit_decode_write(H5FD_mirror_xmit_write_t *out, const unsigned char *buf);
+
+H5_DLL size_t H5FD_mirror_xmit_encode_header(unsigned char *dest, const H5FD_mirror_xmit_t *x);
+H5_DLL size_t H5FD_mirror_xmit_encode_lock(unsigned char *dest, const H5FD_mirror_xmit_lock_t *x);
+H5_DLL size_t H5FD_mirror_xmit_encode_open(unsigned char *dest, const H5FD_mirror_xmit_open_t *x);
+H5_DLL size_t H5FD_mirror_xmit_encode_reply(unsigned char *dest, const H5FD_mirror_xmit_reply_t *x);
+H5_DLL size_t H5FD_mirror_xmit_encode_set_eoa(unsigned char *dest, const H5FD_mirror_xmit_eoa_t *x);
+H5_DLL size_t H5FD_mirror_xmit_encode_write(unsigned char *dest, const H5FD_mirror_xmit_write_t *x);
+
+H5_DLL hbool_t H5FD_mirror_xmit_is_close(const H5FD_mirror_xmit_t *xmit);
+H5_DLL hbool_t H5FD_mirror_xmit_is_lock(const H5FD_mirror_xmit_lock_t *xmit);
+H5_DLL hbool_t H5FD_mirror_xmit_is_open(const H5FD_mirror_xmit_open_t *xmit);
+H5_DLL hbool_t H5FD_mirror_xmit_is_reply(const H5FD_mirror_xmit_reply_t *xmit);
+H5_DLL hbool_t H5FD_mirror_xmit_is_set_eoa(const H5FD_mirror_xmit_eoa_t *xmit);
+H5_DLL hbool_t H5FD_mirror_xmit_is_write(const H5FD_mirror_xmit_write_t *xmit);
+H5_DLL hbool_t H5FD_mirror_xmit_is_xmit(const H5FD_mirror_xmit_t *xmit);
+
+#ifdef __cplusplus
+}
+#endif
+
+#else /* H5_HAVE_MIRROR_VFD */
+
+#define H5FD_MIRROR (H5I_INAVLID_HID)
+
+#endif /* H5_HAVE_MIRROR_VFD */
+
+#endif /* H5FDmirror_H */
+
+
diff --git a/src/H5FDpublic.h b/src/H5FDpublic.h
index 514d1bf..124bac6 100644
--- a/src/H5FDpublic.h
+++ b/src/H5FDpublic.h
@@ -255,6 +255,8 @@ typedef enum H5F_mem_t	H5FD_mem_t;
      * that creates a file which is compatible with the default VFD.
      * Generally, this means that the VFD creates a single file that follows
      * the canonical HDF5 file format.
+     * Regarding the Splitter VFD specifically, only drivers with this flag
+     * enabled may be used as the Write-Only (W/O) channel driver.
      */
 #define H5FD_FEAT_DEFAULT_VFD_COMPATIBLE        0x00008000
 
diff --git a/src/H5FDsec2.c b/src/H5FDsec2.c
index a393490..d718a93 100644
--- a/src/H5FDsec2.c
+++ b/src/H5FDsec2.c
@@ -521,6 +521,12 @@ H5FD_sec2_query(const H5FD_t *_file, unsigned long *flags /* out */)
     FUNC_ENTER_NOAPI_NOINIT_NOERR
 
     /* Set the VFL feature flags that this driver supports */
+    /* Notice: the Mirror VFD Writer currently uses only the Sec2 driver as
+     * the underying driver -- as such, the Mirror VFD implementation copies
+     * these feature flags as its own. Any modifications made here must be
+     * reflected in H5FDmirror.c
+     * -- JOS 2020-01-13
+     */
     if(flags) {
         *flags = 0;
         *flags |= H5FD_FEAT_AGGREGATE_METADATA;     /* OK to aggregate metadata allocations                             */
diff --git a/src/H5FDsplitter.c b/src/H5FDsplitter.c
new file mode 100644
index 0000000..13816a5
--- /dev/null
+++ b/src/H5FDsplitter.c
@@ -0,0 +1,1467 @@
+/* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
+ * Copyright by The HDF Group.                                               *
+ * All rights reserved.                                                      *
+ *                                                                           *
+ * This file is part of HDF5.  The full HDF5 copyright notice, including     *
+ * terms governing use, modification, and redistribution, is contained in    *
+ * the COPYING file, which can be found at the root of the source code       *
+ * distribution tree, or in https://support.hdfgroup.org/ftp/HDF5/releases.  *
+ * If you do not have access to either file, you may request a copy from     *
+ * help@hdfgroup.org.                                                        *
+ * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */
+
+/*
+ * Purpose:     The Splitter VFD implements a file driver which relays all the
+ *              VFD calls to an underlying VFD, and send all the write calls to
+ *              another underlying VFD. Maintains two files simultaneously.
+ */
+
+/* This source code file is part of the H5FD driver module */
+#include "H5FDdrvr_module.h"
+
+#include "H5private.h"      /* Generic Functions        */
+#include "H5Eprivate.h"     /* Error handling           */
+#include "H5Fprivate.h"     /* File access              */
+#include "H5FDprivate.h"    /* File drivers             */
+#include "H5FDsplitter.h"   /* Splitter file driver     */
+#include "H5FLprivate.h"    /* Free Lists               */
+#include "H5Iprivate.h"     /* IDs                      */
+#include "H5MMprivate.h"    /* Memory management        */
+#include "H5FDsec2.h"       /* Generic Functions        */
+#include "H5FDstdio.h"      /* Generic Functions        */
+#include "H5Pprivate.h"     /* Property lists           */
+
+/* The driver identification number, initialized at runtime */
+static hid_t H5FD_SPLITTER_g = 0;
+
+/* Driver-specific file access properties */
+typedef struct H5FD_splitter_fapl_t {
+    hid_t rw_fapl_id;   /* fapl for the R/W channel       */
+    hid_t wo_fapl_id;   /* fapl for the W/O channel       */
+    char wo_path[H5FD_SPLITTER_PATH_MAX + 1]; /* file name for the W/O channel */
+    char log_file_path[H5FD_SPLITTER_PATH_MAX + 1]; /* file to record errors reported by the W/O channel */
+    hbool_t ignore_wo_errs; /* TRUE to ignore errors on the W/O channel */
+} H5FD_splitter_fapl_t;
+
+/* The information of this splitter */
+typedef struct H5FD_splitter_t {
+    H5FD_t pub;         /* public stuff, must be first    */
+    unsigned version;   /* version of the H5FD_splitter_vfd_config_t structure used */
+    H5FD_splitter_fapl_t fa; /* driver-specific file access properties */
+    H5FD_t  *rw_file;   /* pointer of R/W channel */
+    H5FD_t  *wo_file;   /* pointer of W/O channel */
+    FILE    *logfp;     /* Log file pointer */
+} H5FD_splitter_t;
+
+/*
+ * These macros check for overflow of various quantities.  These macros
+ * assume that HDoff_t is signed and haddr_t and size_t are unsigned.
+ *
+ * ADDR_OVERFLOW:   Checks whether a file address of type `haddr_t'
+ *                  is too large to be represented by the second argument
+ *                  of the file seek function.
+ *
+ * SIZE_OVERFLOW:   Checks whether a buffer size of type `hsize_t' is too
+ *                  large to be represented by the `size_t' type.
+ *
+ * REGION_OVERFLOW: Checks whether an address and size pair describe data
+ *                  which can be addressed entirely by the second
+ *                  argument of the file seek function.
+ */
+#define MAXADDR (((haddr_t)1<<(8*sizeof(HDoff_t)-1))-1)
+#define ADDR_OVERFLOW(A)    (HADDR_UNDEF==(A) || ((A) & ~(haddr_t)MAXADDR))
+#define SIZE_OVERFLOW(Z)    ((Z) & ~(hsize_t)MAXADDR)
+#define REGION_OVERFLOW(A,Z)    (ADDR_OVERFLOW(A) || SIZE_OVERFLOW(Z) ||    \
+                                 HADDR_UNDEF==(A)+(Z) ||                    \
+                                (HDoff_t)((A)+(Z))<(HDoff_t)(A))
+
+/* This macro provides a wrapper for shared fail-log-ignore behavior
+ * for errors arising in the splitter's W/O channel.
+ * Logs an error entry in a log file, if the file exists.
+ * If not set to ignore errors, registers an error with the library.
+ */
+#define H5FD_SPLITTER_WO_ERROR(file, funcname, errmajor, errminor, ret, mesg) \
+{                                                                             \
+    H5FD__splitter_log_error((file), (funcname), (mesg));                     \
+    if (FALSE == (file)->fa.ignore_wo_errs) {                                 \
+        HGOTO_ERROR((errmajor), (errminor), (ret), (mesg))                    \
+    }                                                                         \
+}
+
+#define H5FD_SPLITTER_DEBUG_OP_CALLS 0 /* debugging print toggle; 0 disables */
+
+#if H5FD_SPLITTER_DEBUG_OP_CALLS
+#define H5FD_SPLITTER_LOG_CALL(name) do { \
+    HDprintf("called %s()\n", (name));    \
+    fflush(stdout);                       \
+} while (0)
+#else
+#define H5FD_SPLITTER_LOG_CALL(name) /* no-op */
+#endif /* H5FD_SPLITTER_DEBUG_OP_CALLS */
+
+/* Private functions */
+
+/* Print error messages from W/O channel to log file */
+static herr_t H5FD__splitter_log_error(const H5FD_splitter_t *file, const char *atfunc, const char *msg);
+
+static int H5FD__copy_plist(hid_t fapl_id, hid_t *id_out_ptr);
+
+/* Prototypes */
+static herr_t H5FD_splitter_term(void);
+static hsize_t H5FD_splitter_sb_size(H5FD_t *_file);
+static herr_t H5FD_splitter_sb_encode(H5FD_t *_file, char *name/*out*/, unsigned char *buf/*out*/);
+static herr_t H5FD_splitter_sb_decode(H5FD_t *_file, const char *name, const unsigned char *buf);
+static void *H5FD_splitter_fapl_get(H5FD_t *_file);
+static void *H5FD_splitter_fapl_copy(const void *_old_fa);
+static herr_t H5FD_splitter_fapl_free(void *_fapl);
+static H5FD_t *H5FD_splitter_open(const char *name, unsigned flags, hid_t fapl_id, haddr_t maxaddr);
+static herr_t H5FD_splitter_close(H5FD_t *_file);
+static int H5FD_splitter_cmp(const H5FD_t *_f1, const H5FD_t *_f2);
+static herr_t H5FD_splitter_query(const H5FD_t *_file, unsigned long *flags /* out */);
+static herr_t H5FD_splitter_get_type_map(const H5FD_t *_file, H5FD_mem_t *type_map);
+static haddr_t H5FD_splitter_alloc(H5FD_t *file, H5FD_mem_t type, hid_t dxpl_id, hsize_t size);
+static herr_t H5FD_splitter_free(H5FD_t *_file, H5FD_mem_t type, hid_t dxpl_id, haddr_t addr, hsize_t size);
+static haddr_t H5FD_splitter_get_eoa(const H5FD_t *_file, H5FD_mem_t H5_ATTR_UNUSED type);
+static herr_t H5FD_splitter_set_eoa(H5FD_t *_file, H5FD_mem_t H5_ATTR_UNUSED type, haddr_t addr);
+static haddr_t H5FD_splitter_get_eof(const H5FD_t *_file, H5FD_mem_t H5_ATTR_UNUSED type);
+static herr_t  H5FD_splitter_get_handle(H5FD_t *_file, hid_t H5_ATTR_UNUSED fapl, void** file_handle);
+static herr_t H5FD_splitter_read(H5FD_t *_file, H5FD_mem_t type, hid_t dxpl_id, haddr_t addr, size_t size, void *buf);
+static herr_t H5FD_splitter_write(H5FD_t *_file, H5FD_mem_t type, hid_t dxpl_id, haddr_t addr, size_t size, const void *buf);
+static herr_t H5FD_splitter_flush(H5FD_t *_file, hid_t dxpl_id, hbool_t closing);
+static herr_t H5FD_splitter_truncate(H5FD_t *_file, hid_t dxpl_id, hbool_t closing);
+static herr_t H5FD_splitter_lock(H5FD_t *_file, hbool_t rw);
+static herr_t H5FD_splitter_unlock(H5FD_t *_file);
+
+static const H5FD_class_t H5FD_splitter_g = {
+    "splitter",                 /* name                 */
+    MAXADDR,                    /* maxaddr              */
+    H5F_CLOSE_WEAK,             /* fc_degree            */
+    H5FD_splitter_term,         /* terminate            */
+    H5FD_splitter_sb_size,      /* sb_size              */
+    H5FD_splitter_sb_encode,    /* sb_encode            */
+    H5FD_splitter_sb_decode,    /* sb_decode            */
+    sizeof(H5FD_splitter_fapl_t), /* fapl_size          */
+    H5FD_splitter_fapl_get,     /* fapl_get             */
+    H5FD_splitter_fapl_copy,    /* fapl_copy            */
+    H5FD_splitter_fapl_free,    /* fapl_free            */
+    0,                          /* dxpl_size            */
+    NULL,                       /* dxpl_copy            */
+    NULL,                       /* dxpl_free            */
+    H5FD_splitter_open,         /* open                 */
+    H5FD_splitter_close,        /* close                */
+    H5FD_splitter_cmp,          /* cmp                  */
+    H5FD_splitter_query,        /* query                */
+    H5FD_splitter_get_type_map, /* get_type_map         */
+    H5FD_splitter_alloc,        /* alloc                */
+    H5FD_splitter_free,         /* free                 */
+    H5FD_splitter_get_eoa,      /* get_eoa              */
+    H5FD_splitter_set_eoa,      /* set_eoa              */
+    H5FD_splitter_get_eof,      /* get_eof              */
+    H5FD_splitter_get_handle,   /* get_handle           */
+    H5FD_splitter_read,         /* read                 */
+    H5FD_splitter_write,        /* write                */
+    H5FD_splitter_flush,        /* flush                */
+    H5FD_splitter_truncate,     /* truncate             */
+    H5FD_splitter_lock,         /* lock                 */
+    H5FD_splitter_unlock,       /* unlock               */
+    H5FD_FLMAP_DICHOTOMY        /* fl_map               */
+};
+
+/* Declare a free list to manage the H5FD_splitter_t struct */
+H5FL_DEFINE_STATIC(H5FD_splitter_t);
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD__init_package
+ *
+ * Purpose:     Initializes any interface-specific data or routines.
+ *
+ * Return:      SUCCEED/FAIL
+ *-------------------------------------------------------------------------
+ */
+static herr_t
+H5FD__init_package(void)
+{
+    herr_t ret_value = SUCCEED;
+
+    FUNC_ENTER_STATIC
+
+    H5FD_SPLITTER_LOG_CALL("H5FD__init_package");
+
+    if (H5FD_splitter_init() < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_CANTINIT, FAIL, "unable to initialize splitter VFD")
+    }
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* H5FD__init_package() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_splitter_init
+ *
+ * Purpose:     Initialize the splitter driver by registering it with the
+ *              library.
+ *
+ * Return:      Success:    The driver ID for the splitter driver.
+ *              Failure:    Negative
+ *-------------------------------------------------------------------------
+ */
+hid_t
+H5FD_splitter_init(void)
+{
+    hid_t ret_value = H5I_INVALID_HID;
+
+    FUNC_ENTER_NOAPI(FAIL)
+
+    H5FD_SPLITTER_LOG_CALL("H5FD_splitter_init");
+
+    if (H5I_VFL != H5I_get_type(H5FD_SPLITTER_g)) {
+        H5FD_SPLITTER_g = H5FDregister(&H5FD_splitter_g);
+    }
+
+    ret_value = H5FD_SPLITTER_g;
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD_splitter_init() */
+
+
+/*---------------------------------------------------------------------------
+ * Function:    H5FD_splitter_term
+ *
+ * Purpose:     Shut down the splitter VFD.
+ *
+ * Returns:     SUCCEED (Can't fail)
+ *---------------------------------------------------------------------------
+ */
+static herr_t
+H5FD_splitter_term(void)
+{
+    FUNC_ENTER_NOAPI_NOINIT_NOERR
+
+    H5FD_SPLITTER_LOG_CALL("H5FD_splitter_term");
+
+    /* Reset VFL ID */
+    H5FD_SPLITTER_g = 0;
+
+    FUNC_LEAVE_NOAPI(SUCCEED)
+} /* end H5FD_splitter_term() */
+
+
+ /*-------------------------------------------------------------------------
+ * Function:    H5FD__copy_plist
+ *
+ * Purpose:     Sanity-wrapped H5P_copy_plist() for each channel.
+ *              Utility function for operation in multiple locations.
+ *
+ * Return:      0 on success, -1 on error.
+ *-------------------------------------------------------------------------
+ */
+static int
+H5FD__copy_plist(hid_t   fapl_id,
+                 hid_t  *id_out_ptr)
+{
+    int                 ret_value = 0;
+    H5P_genplist_t     *plist_ptr = NULL;
+
+    FUNC_ENTER_NOAPI_NOINIT
+
+    H5FD_SPLITTER_LOG_CALL("H5FD__copy_plist");
+
+    HDassert(id_out_ptr != NULL);
+
+    if (FALSE == H5P_isa_class(fapl_id, H5P_FILE_ACCESS)) {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADTYPE, -1, "not a file access property list");
+    }
+
+    plist_ptr = (H5P_genplist_t *)H5I_object(fapl_id);
+    if (NULL == plist_ptr) {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADTYPE, -1, "unable to get property list");
+    }
+
+    *id_out_ptr = H5P_copy_plist(plist_ptr, FALSE);
+    if (H5I_INVALID_HID == *id_out_ptr) {
+        HGOTO_ERROR(H5E_VFL, H5E_BADTYPE, -1, "unable to copy file access property list");
+    }
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value);
+} /* end H5FD__copy_plist() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5Pset_fapl_splitter
+ *
+ * Purpose:     Sets the file access property list to use the
+ *              splitter driver.
+ *
+ * Return:      SUCCEED/FAIL
+ *-------------------------------------------------------------------------
+ */
+herr_t
+H5Pset_fapl_splitter(hid_t fapl_id, H5FD_splitter_vfd_config_t *vfd_config)
+{
+    H5FD_splitter_fapl_t  info;
+    H5P_genplist_t       *plist_ptr = NULL;
+    herr_t                ret_value = SUCCEED;
+
+    H5Eclear2(H5E_DEFAULT);
+
+    FUNC_ENTER_API(FAIL)
+    H5TRACE2("e", "i*Dr", fapl_id, vfd_config);
+
+    H5FD_SPLITTER_LOG_CALL("H5Pset_fapl_splitter");
+
+    if (H5FD_SPLITTER_MAGIC != vfd_config->magic) {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADVALUE, FAIL, "invalid configuration (magic number mismatch)")
+    }
+    if (H5FD_CURR_SPLITTER_VFD_CONFIG_VERSION != vfd_config->version) {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADVALUE, FAIL, "invlaid config (version number mismatch)")
+    }
+
+    if (NULL == (plist_ptr = (H5P_genplist_t *)H5I_object(fapl_id))) {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADTYPE, FAIL, "not a valid property list")
+    }
+
+
+    /* Make sure that the W/O channel supports write-only capability.
+     * Some drivers (e.g. family or multi) do revision of the superblock
+     * in-memory, causing problems in that channel.
+     * Uses the feature flag H5FD_FEAT_DEFAULT_VFD_COMPATIBLE as the
+     * determining attribute.
+     */
+    if (H5P_DEFAULT != vfd_config->wo_fapl_id) {
+        H5FD_class_t       *wo_driver = NULL;
+        H5FD_driver_prop_t  wo_driver_prop;
+        H5P_genplist_t     *wo_plist_ptr = NULL;
+        unsigned long       wo_driver_flags = 0;
+
+        wo_plist_ptr = (H5P_genplist_t *)H5I_object(vfd_config->wo_fapl_id);
+        if (NULL == wo_plist_ptr) {
+            HGOTO_ERROR(H5E_ARGS, H5E_BADTYPE, FAIL, "not a file access property list")
+        }
+        if (H5P_peek(wo_plist_ptr, H5F_ACS_FILE_DRV_NAME, &wo_driver_prop) < 0) {
+            HGOTO_ERROR(H5E_PLIST, H5E_CANTGET, FAIL, "can't get driver ID & info")
+        }
+        wo_driver = (H5FD_class_t *)H5I_object(wo_driver_prop.driver_id);
+        if (NULL == wo_driver) {
+            HGOTO_ERROR(H5E_VFL, H5E_BADVALUE, FAIL, "invalid driver ID in file access property list")
+        }
+        if (H5FD_driver_query(wo_driver, &wo_driver_flags) < 0) {
+            HGOTO_ERROR(H5E_VFL, H5E_BADVALUE, FAIL, "can't query VFD flags")
+        }
+        if (0 == (H5FD_FEAT_DEFAULT_VFD_COMPATIBLE & wo_driver_flags)) {
+            HGOTO_ERROR(H5E_VFL, H5E_BADVALUE, FAIL, "unsuitable W/O driver")
+        }
+    } /* end if W/O VFD is non-default */
+
+
+    info.ignore_wo_errs = vfd_config->ignore_wo_errs;
+    HDstrncpy(info.wo_path, vfd_config->wo_path, H5FD_SPLITTER_PATH_MAX);
+    HDstrncpy(info.log_file_path, vfd_config->log_file_path, H5FD_SPLITTER_PATH_MAX);
+    info.rw_fapl_id = H5P_FILE_ACCESS_DEFAULT; /* pre-set value */
+    info.wo_fapl_id = H5P_FILE_ACCESS_DEFAULT; /* pre-set value */
+
+    /* Set non-default channel FAPL IDs in splitter configuration info */
+    if (H5P_DEFAULT != vfd_config->rw_fapl_id) {
+        if (FALSE == H5P_isa_class(vfd_config->rw_fapl_id, H5P_FILE_ACCESS)) {
+            HGOTO_ERROR(H5E_ARGS, H5E_BADTYPE, FAIL, "not a file access list")
+        }
+        info.rw_fapl_id = vfd_config->rw_fapl_id;
+    }
+    if (H5P_DEFAULT != vfd_config->wo_fapl_id) {
+        if (FALSE == H5P_isa_class(vfd_config->wo_fapl_id, H5P_FILE_ACCESS)) {
+            HGOTO_ERROR(H5E_ARGS, H5E_BADTYPE, FAIL, "not a file access list")
+        }
+        info.wo_fapl_id = vfd_config->wo_fapl_id;
+    }
+
+    ret_value = H5P_set_driver(plist_ptr, H5FD_SPLITTER, &info);
+
+done:
+    FUNC_LEAVE_API(ret_value)
+} /* end H5Pset_fapl_splitter() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5Pget_fapl_splitter
+ *
+ * Purpose:     Returns information about the splitter file access property
+ *              list through the structure config_out.
+ *
+ *              Will fail if config_out is received without pre-set valid
+ *              magic and version information.
+ *
+ * Return:      SUCCEED/FAIL
+ *-------------------------------------------------------------------------
+ */
+herr_t
+H5Pget_fapl_splitter(hid_t fapl_id, H5FD_splitter_vfd_config_t *config_out)
+{
+    const H5FD_splitter_fapl_t *fapl_ptr  = NULL;
+    H5P_genplist_t             *plist_ptr = NULL;
+    herr_t                      ret_value = SUCCEED;
+
+    FUNC_ENTER_API(FAIL)
+    H5TRACE2("e", "i*Dr", fapl_id, config_out);
+
+    H5FD_SPLITTER_LOG_CALL("H5Pget_fapl_splitter");
+
+    /* Check arguments */
+    if (TRUE != H5P_isa_class(fapl_id, H5P_FILE_ACCESS)) {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADTYPE, FAIL, "not a file access property list")
+    }
+    if (config_out == NULL) {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADVALUE, FAIL, "config_out pointer is null")
+    }
+    if (H5FD_SPLITTER_MAGIC != config_out->magic) {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADVALUE, FAIL, "info-out pointer invalid (magic number mismatch)")
+    }
+    if (H5FD_CURR_SPLITTER_VFD_CONFIG_VERSION != config_out->version) {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADVALUE, FAIL, "info-out pointer invalid (version unsafe)")
+    }
+
+    /* Pre-set out FAPL IDs with intent to replace these values */
+    config_out->rw_fapl_id = H5I_INVALID_HID;
+    config_out->wo_fapl_id = H5I_INVALID_HID;
+
+    /* Check and get the splitter fapl */
+    if (NULL == (plist_ptr = H5P_object_verify(fapl_id, H5P_FILE_ACCESS))) {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADTYPE, FAIL, "not a file access property list")
+    }
+    if (H5FD_SPLITTER != H5P_peek_driver(plist_ptr)) {
+        HGOTO_ERROR(H5E_PLIST, H5E_BADVALUE, FAIL, "incorrect VFL driver")
+    }
+    if (NULL == (fapl_ptr = (const H5FD_splitter_fapl_t *)H5P_peek_driver_info(plist_ptr))) {
+        HGOTO_ERROR(H5E_PLIST, H5E_BADVALUE, FAIL, "unable to get specific-driver info")
+    }
+
+    HDstrncpy(config_out->wo_path, fapl_ptr->wo_path, H5FD_SPLITTER_PATH_MAX);
+    HDstrncpy(config_out->log_file_path, fapl_ptr->log_file_path, H5FD_SPLITTER_PATH_MAX);
+    config_out->ignore_wo_errs = fapl_ptr->ignore_wo_errs;
+
+    /* Copy R/W and W/O FAPLs */
+    if (H5FD__copy_plist(fapl_ptr->rw_fapl_id, &(config_out->rw_fapl_id)) < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_BADVALUE, FAIL, "can't copy R/W FAPL");
+    }
+    if (H5FD__copy_plist(fapl_ptr->wo_fapl_id, &(config_out->wo_fapl_id)) < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_BADVALUE, FAIL, "can't copy W/O FAPL");
+    }
+
+done:
+    FUNC_LEAVE_API(ret_value)
+} /* end H5Pget_fapl_splitter() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_splitter_flush
+ *
+ * Purpose:     Flushes all data to disk for both channels.
+ *
+ * Return:      SUCCEED/FAIL
+ *-------------------------------------------------------------------------
+ */
+static herr_t
+H5FD_splitter_flush(H5FD_t *_file, hid_t H5_ATTR_UNUSED dxpl_id, hbool_t closing)
+{
+    H5FD_splitter_t *file = (H5FD_splitter_t *)_file;
+    herr_t      ret_value = SUCCEED;       /* Return value */
+
+    FUNC_ENTER_NOAPI_NOINIT
+
+    H5FD_SPLITTER_LOG_CALL("H5FD_splitter_flush");
+
+    /* Public API for dxpl "context" */
+    if (H5FDflush(file->rw_file, dxpl_id, closing) < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_CANTFLUSH, FAIL, "unable to flush R/W file")
+    }
+    if (H5FDflush(file->wo_file, dxpl_id, closing) < 0) {
+       H5FD_SPLITTER_WO_ERROR(file, "H5FD_splitter_flush",
+                              H5E_VFL, H5E_CANTFLUSH, FAIL,
+                              "unable to flush W/O file")
+    }
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD_splitter_flush() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_splitter_read
+ *
+ * Purpose:     Reads SIZE bytes of data from the R/W channel, beginning at
+ *              address ADDR into buffer BUF according to data transfer
+ *              properties in DXPL_ID.
+ *
+ * Return:      Success:    SUCCEED
+ *                          The read result is written into the BUF buffer
+ *                          which should be allocated by the caller.
+ *              Failure:    FAIL
+ *                          The contents of BUF are undefined.
+ *-------------------------------------------------------------------------
+ */
+static herr_t
+H5FD_splitter_read(
+        H5FD_t     *_file,
+        H5FD_mem_t  H5_ATTR_UNUSED type,
+        hid_t       H5_ATTR_UNUSED dxpl_id,
+        haddr_t     addr,
+        size_t      size,
+        void       *buf /*out*/)
+{
+    H5FD_splitter_t *file       = (H5FD_splitter_t *)_file;
+    herr_t          ret_value   = SUCCEED;
+
+    FUNC_ENTER_NOAPI_NOINIT
+
+    H5FD_SPLITTER_LOG_CALL("H5FD_splitter_read");
+
+    HDassert(file && file->pub.cls);
+    HDassert(buf);
+
+    /* Check for overflow conditions */
+    if (!H5F_addr_defined(addr)) {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADVALUE, FAIL, "addr undefined, addr = %llu", (unsigned long long)addr)
+    }
+    if (REGION_OVERFLOW(addr, size)) {
+        HGOTO_ERROR(H5E_ARGS, H5E_OVERFLOW, FAIL, "addr overflow, addr = %llu", (unsigned long long)addr)
+    }
+
+    /* Only read from R/W channel */
+    /* Public API for dxpl "context" */
+    if (H5FDread(file->rw_file, type, dxpl_id, addr, size, buf) < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_READERROR, FAIL, "Reading from R/W channel failed")
+    }
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD_splitter_read() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_splitter_write
+ *
+ * Purpose:     Writes SIZE bytes of data to R/W and W/O channels, beginning
+ *              at address ADDR from buffer BUF according to data transfer
+ *              properties in DXPL_ID.
+ *
+ * Return:      SUCCEED/FAIL
+ *-------------------------------------------------------------------------
+ */
+static herr_t
+H5FD_splitter_write(H5FD_t *_file, H5FD_mem_t type, hid_t dxpl_id, haddr_t addr, size_t size, const void *buf)
+{
+    H5FD_splitter_t *file      = (H5FD_splitter_t *)_file;
+    H5P_genplist_t  *plist_ptr     = NULL;
+    herr_t           ret_value = SUCCEED;
+
+    FUNC_ENTER_NOAPI_NOINIT
+
+    H5FD_SPLITTER_LOG_CALL("H5FD_splitter_write");
+
+    if (NULL == (plist_ptr = (H5P_genplist_t *)H5I_object(dxpl_id))) {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADTYPE, FAIL, "not a property list")
+    }
+
+    /* Write to each file */
+    /* Public API for dxpl "context" */
+    if (H5FDwrite(file->rw_file, type, dxpl_id, addr, size, buf) < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_WRITEERROR, FAIL, "R/W file write failed")
+    }
+    if (H5FDwrite(file->wo_file, type, dxpl_id, addr, size, buf) < 0) {
+       H5FD_SPLITTER_WO_ERROR(file, "H5FD_splitter_write",
+                              H5E_VFL, H5E_WRITEERROR, FAIL,
+                              "unable to write W/O file")
+    }
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD_splitter_write() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_splitter_fapl_get
+ *
+ * Purpose:     Returns a file access property list which indicates how the
+ *              specified file is being accessed. The return list could be
+ *              used to access another file the same way.
+ *
+ * Return:      Success:    Ptr to new file access property list with all
+ *                          members copied from the file struct.
+ *              Failure:    NULL
+ *-------------------------------------------------------------------------
+ */
+static void *
+H5FD_splitter_fapl_get(H5FD_t *_file)
+{
+    H5FD_splitter_t *file      = (H5FD_splitter_t *)_file;
+    void            *ret_value = NULL;
+
+    FUNC_ENTER_NOAPI_NOINIT_NOERR
+
+    H5FD_SPLITTER_LOG_CALL("H5FD_splitter_fapl_get");
+
+    ret_value = H5FD_splitter_fapl_copy(&(file->fa));
+
+    FUNC_LEAVE_NOAPI(ret_value)
+}
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_splitter_fapl_copy
+ *
+ * Purpose:     Copies the file access properties.
+ *
+ * Return:      Success:    Pointer to a new property list info structure.
+ *              Failure:    NULL
+ *-------------------------------------------------------------------------
+ */
+static void *
+H5FD_splitter_fapl_copy(const void *_old_fa)
+{
+    const H5FD_splitter_fapl_t *old_fa_ptr = (const H5FD_splitter_fapl_t *)_old_fa;
+    H5FD_splitter_fapl_t       *new_fa_ptr = NULL;
+    void                       *ret_value  = NULL;
+
+    FUNC_ENTER_NOAPI_NOINIT
+
+    H5FD_SPLITTER_LOG_CALL("H5FD_splitter_fapl_copy");
+
+    HDassert(old_fa_ptr);
+
+    new_fa_ptr = (H5FD_splitter_fapl_t *)H5MM_calloc(sizeof(H5FD_splitter_fapl_t));
+    if (NULL == new_fa_ptr) {
+        HGOTO_ERROR(H5E_VFL, H5E_CANTALLOC, NULL, "unable to allocate log file FAPL")
+    }
+
+    if (HDmemcpy(new_fa_ptr, old_fa_ptr, sizeof(H5FD_splitter_fapl_t)) == NULL) {
+        HGOTO_ERROR(H5E_ARGS, H5E_CANTCOPY, NULL, "unable to shallow-copy info")
+    }
+    if (HDstrncpy(new_fa_ptr->wo_path, old_fa_ptr->wo_path, H5FD_SPLITTER_PATH_MAX) == NULL) {
+        HGOTO_ERROR(H5E_ARGS, H5E_CANTCOPY, NULL, "unable to copy write-only channel file path")
+    }
+    if (HDstrncpy(new_fa_ptr->log_file_path, old_fa_ptr->log_file_path, H5FD_SPLITTER_PATH_MAX) == NULL) {
+        HGOTO_ERROR(H5E_ARGS, H5E_CANTCOPY, NULL, "unable to copy log file path")
+    }
+
+    /* Copy R/W and W/O FAPLs */
+    if (H5FD__copy_plist(old_fa_ptr->rw_fapl_id, &(new_fa_ptr->rw_fapl_id)) < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_BADVALUE, NULL, "can't copy R/W FAPL");
+    }
+    if (H5FD__copy_plist(old_fa_ptr->wo_fapl_id, &(new_fa_ptr->wo_fapl_id)) < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_BADVALUE, NULL, "can't copy W/O FAPL");
+    }
+
+    ret_value = (void *)new_fa_ptr;
+
+done:
+    if (NULL == ret_value) {
+        if (new_fa_ptr) {
+            H5MM_free(new_fa_ptr);
+        }
+    }
+
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD_splitter_fapl_copy() */
+
+
+/*--------------------------------------------------------------------------
+ * Function:    H5FD_splitter_fapl_free
+ *
+ * Purpose:     Releases the file access lists
+ *
+ * Return:      SUCCEED/FAIL
+ *--------------------------------------------------------------------------
+ */
+static herr_t
+H5FD_splitter_fapl_free(void *_fapl)
+{
+    H5FD_splitter_fapl_t *fapl      = (H5FD_splitter_fapl_t*)_fapl;
+    herr_t                ret_value = SUCCEED;
+
+    FUNC_ENTER_NOAPI_NOINIT
+
+    H5FD_SPLITTER_LOG_CALL("H5FD_splitter_fapl_free");
+
+    /* Check arguments */
+    HDassert(fapl);
+
+    if (H5I_dec_ref(fapl->rw_fapl_id) < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_CANTDEC, FAIL, "can't close R/W FAPL ID")
+    }
+    if (H5I_dec_ref(fapl->wo_fapl_id) < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_CANTDEC, FAIL, "can't close W/O FAPL ID")
+    }
+
+    /* Free the property list */
+    H5MM_free(fapl);
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD_splitter_fapl_free() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_splitter_open
+ *
+ * Purpose:     Create and/or opens a file as an HDF5 file.
+ *
+ * Return:      Success:    A pointer to a new file data structure. The
+ *                          public fields will be initialized by the
+ *                          caller, which is always H5FD_open().
+ *              Failure:    NULL
+ *-------------------------------------------------------------------------
+ */
+static H5FD_t *
+H5FD_splitter_open(const char *name, unsigned flags, hid_t splitter_fapl_id, haddr_t maxaddr)
+{
+    H5FD_splitter_t            *file_ptr  = NULL; /* Splitter VFD info */
+    const H5FD_splitter_fapl_t *fapl_ptr  = NULL; /* Driver-specific property list */
+    H5P_genplist_t     *plist_ptr = NULL;
+    H5FD_t     *ret_value = NULL;
+
+    FUNC_ENTER_NOAPI_NOINIT
+
+    H5FD_SPLITTER_LOG_CALL("H5FD_splitter_open");
+
+    /* Check arguments */
+    if (!name || !*name) {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADVALUE, NULL, "invalid file name")
+    }
+    if (0 == maxaddr || HADDR_UNDEF == maxaddr) {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADRANGE, NULL, "bogus maxaddr")
+    }
+    if (ADDR_OVERFLOW(maxaddr)) {
+        HGOTO_ERROR(H5E_ARGS, H5E_OVERFLOW, NULL, "bogus maxaddr")
+    }
+    if ( (H5P_FILE_ACCESS_DEFAULT == splitter_fapl_id) ||
+         (H5FD_SPLITTER != H5Pget_driver(splitter_fapl_id)) )
+    {
+        /* presupposes that H5P_FILE_ACCESS_DEFAULT is not a splitter */
+        HGOTO_ERROR(H5E_ARGS, H5E_BADVALUE, NULL, "driver is not splitter")
+    }
+
+    file_ptr = (H5FD_splitter_t *)H5FL_CALLOC(H5FD_splitter_t);
+    if (NULL == file_ptr) {
+        HGOTO_ERROR(H5E_RESOURCE, H5E_NOSPACE, NULL, "unable to allocate file struct")
+    }
+    file_ptr->fa.rw_fapl_id = H5I_INVALID_HID;
+    file_ptr->fa.wo_fapl_id = H5I_INVALID_HID;
+
+    /* Get the driver-specific file access properties */
+    plist_ptr = (H5P_genplist_t *)H5I_object(splitter_fapl_id);
+    if (NULL == plist_ptr) {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADTYPE, NULL, "not a file access property list")
+    }
+    fapl_ptr = (const H5FD_splitter_fapl_t *)H5P_peek_driver_info(plist_ptr);
+    if (NULL == fapl_ptr) {
+        HGOTO_ERROR(H5E_PLIST, H5E_CANTGET, NULL, "unable to get VFL driver info")
+    }
+
+    /* Copy simpler info */
+    if (HDstrncpy(file_ptr->fa.wo_path, fapl_ptr->wo_path, H5FD_SPLITTER_PATH_MAX) == NULL) {
+        HGOTO_ERROR(H5E_VFL, H5E_CANTCOPY, NULL, "unable to copy write-only path")
+    }
+    if (HDstrncpy(file_ptr->fa.log_file_path, fapl_ptr->log_file_path, H5FD_SPLITTER_PATH_MAX) == NULL) {
+        HGOTO_ERROR(H5E_VFL, H5E_CANTCOPY, NULL, "unable to copy logfile path")
+    }
+    file_ptr->fa.ignore_wo_errs = fapl_ptr->ignore_wo_errs;
+
+    /* Copy R/W and W/O channel FAPLs. */
+    if (H5FD__copy_plist(fapl_ptr->rw_fapl_id, &(file_ptr->fa.rw_fapl_id)) < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_BADVALUE, NULL, "can't copy R/W FAPL");
+    }
+    if (H5FD__copy_plist(fapl_ptr->wo_fapl_id, &(file_ptr->fa.wo_fapl_id)) < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_BADVALUE, NULL, "can't copy W/O FAPL");
+    }
+
+    /* Prepare log file if necessary.
+     * If application wants to ignore the errors from W/O channel and
+     * provided a name for the log file, then open it
+     */
+    if (!file_ptr->logfp) {
+        if (file_ptr->fa.log_file_path[0] != '\0') {
+            file_ptr->logfp = HDfopen(file_ptr->fa.log_file_path, "w");
+            if (file_ptr->logfp == NULL) {
+                HGOTO_ERROR(H5E_VFL, H5E_CANTOPENFILE, NULL, "unable to open log file")
+            }
+        } /* end if logfile path given */
+    } /* end if logfile pointer/handle does not exist */
+
+    file_ptr->rw_file = H5FD_open(name, flags, fapl_ptr->rw_fapl_id, HADDR_UNDEF);
+    if (!file_ptr->rw_file) {
+        HGOTO_ERROR(H5E_VFL, H5E_CANTOPENFILE, NULL, "unable to open R/W file")
+    }
+
+    file_ptr->wo_file = H5FD_open(fapl_ptr->wo_path, flags, fapl_ptr->wo_fapl_id, HADDR_UNDEF);
+    if (!file_ptr->wo_file) {
+       H5FD_SPLITTER_WO_ERROR(file_ptr, "H5FD_splitter_open",
+                              H5E_VFL, H5E_CANTOPENFILE, NULL,
+                              "unable to open W/O file")
+    }
+
+    ret_value = (H5FD_t*)file_ptr;
+
+done:
+    if (NULL == ret_value) {
+        if (file_ptr) {
+            if (H5I_INVALID_HID != file_ptr->fa.rw_fapl_id) {
+                H5I_dec_ref(file_ptr->fa.rw_fapl_id);
+            }
+            if (H5I_INVALID_HID != file_ptr->fa.wo_fapl_id) {
+                H5I_dec_ref(file_ptr->fa.wo_fapl_id);
+            }
+            if (file_ptr->rw_file) {
+                H5FD_close(file_ptr->rw_file);
+            }
+            if (file_ptr->wo_file) {
+                H5FD_close(file_ptr->wo_file);
+            }
+            H5FL_FREE(H5FD_splitter_t, file_ptr);
+        }
+    } /* end if error */
+
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD_splitter_open() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_splitter_close
+ *
+ * Purpose:     Closes files on both read-write and write-only channels.
+ *
+ * Return:      Success:    SUCCEED
+ *              Failure:    FAIL, file not closed.
+ *-------------------------------------------------------------------------
+ */
+static herr_t
+H5FD_splitter_close(H5FD_t *_file)
+{
+    H5FD_splitter_t *file      = (H5FD_splitter_t *)_file;
+    herr_t           ret_value = SUCCEED;
+
+    FUNC_ENTER_NOAPI_NOINIT
+
+    H5FD_SPLITTER_LOG_CALL("H5FD_splitter_close");
+
+    /* Sanity check */
+    HDassert(file);
+
+    if (H5I_dec_ref(file->fa.rw_fapl_id) < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_ARGS, FAIL, "can't close R/W FAPL")
+    }
+    if (H5I_dec_ref(file->fa.wo_fapl_id) < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_ARGS, FAIL, "can't close W/O FAPL")
+    }
+
+    if (file->rw_file) {
+        if (H5FD_close(file->rw_file) == FAIL) {
+            HGOTO_ERROR(H5E_VFL, H5E_CANTCLOSEFILE, FAIL, "unable to close R/W file")
+        }
+    }
+    if (file->wo_file) {
+        if (H5FD_close(file->wo_file) == FAIL) {
+            H5FD_SPLITTER_WO_ERROR(file, "H5FD_splitter_close",
+                              H5E_VFL, H5E_CANTCLOSEFILE, FAIL,
+                              "unable to close W/O file")
+        }
+    }
+
+    if (file->logfp) {
+        HDfclose(file->logfp);
+        file->logfp = NULL;
+    }
+
+    /* Release the file info */
+    file = H5FL_FREE(H5FD_splitter_t, file);
+    file = NULL;
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD_splitter_close() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_splitter_get_eoa
+ *
+ * Purpose:     Returns the end-of-address marker for the file. The EOA
+ *              marker is the first address past the last byte allocated in
+ *              the format address space.
+ *
+ * Return:      Success:    The end-of-address-marker
+ *
+ *              Failure:    HADDR_UNDEF
+ *-------------------------------------------------------------------------
+ */
+static haddr_t
+H5FD_splitter_get_eoa(const H5FD_t *_file, H5FD_mem_t H5_ATTR_UNUSED type)
+{
+    const H5FD_splitter_t *file      = (const H5FD_splitter_t *)_file;
+    haddr_t                ret_value = HADDR_UNDEF;
+
+    FUNC_ENTER_NOAPI_NOINIT
+
+    H5FD_SPLITTER_LOG_CALL("H5FD_splitter_get_eoa");
+
+    /* Sanity check */
+    HDassert(file);
+    HDassert(file->rw_file);
+
+    if ((ret_value = H5FD_get_eoa(file->rw_file, type)) == HADDR_UNDEF) {
+        HGOTO_ERROR(H5E_VFL, H5E_BADVALUE, HADDR_UNDEF, "unable to get eoa")
+    }
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD_splitter_get_eoa */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_splitter_set_eoa
+ *
+ * Purpose:     Set the end-of-address marker for the file. This function is
+ *              called shortly after an existing HDF5 file is opened in order
+ *              to tell the driver where the end of the HDF5 data is located.
+ *
+ * Return:      SUCCEED/FAIL
+ *-------------------------------------------------------------------------
+ */
+static herr_t
+H5FD_splitter_set_eoa(H5FD_t *_file, H5FD_mem_t H5_ATTR_UNUSED type, haddr_t addr)
+{
+    H5FD_splitter_t *file = (H5FD_splitter_t *)_file;
+    herr_t      ret_value = SUCCEED;                /* Return value */
+
+    FUNC_ENTER_NOAPI_NOINIT
+
+    H5FD_SPLITTER_LOG_CALL("H5FD_splitter_set_eoa";)
+
+    /* Sanity check */
+    HDassert(file);
+    HDassert(file->rw_file);
+    HDassert(file->wo_file);
+
+    if (H5FD_set_eoa(file->rw_file, type, addr) < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_CANTSET, FAIL, "H5FDset_eoa failed for R/W file")
+    }
+
+    if (H5FD_set_eoa(file->wo_file, type, addr) < 0) {
+       H5FD_SPLITTER_WO_ERROR(file, "H5FD_splitter_set_eoa",
+                              H5E_VFL, H5E_CANTSET, FAIL,
+                              "unable to set EOA for W/O file")
+    }
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD_splitter_set_eoa() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_splitter_get_eof
+ *
+ * Purpose:     Returns the end-of-address marker for the file. The EOA
+ *              marker is the first address past the last byte allocated in
+ *              the format address space.
+ *
+ * Return:      Success:    The end-of-address-marker
+ *
+ *              Failure:    HADDR_UNDEF
+ *-------------------------------------------------------------------------
+ */
+static haddr_t
+H5FD_splitter_get_eof(const H5FD_t *_file, H5FD_mem_t H5_ATTR_UNUSED type)
+{
+    const H5FD_splitter_t *file = (const H5FD_splitter_t *)_file;
+    haddr_t ret_value = HADDR_UNDEF;        /* Return value */
+
+    FUNC_ENTER_NOAPI_NOINIT
+
+    H5FD_SPLITTER_LOG_CALL("H5FD_splitter_get_eof");
+
+    /* Sanity check */
+    HDassert(file);
+    HDassert(file->rw_file);
+
+    if (HADDR_UNDEF == (ret_value = H5FD_get_eof(file->rw_file, type))) {
+        HGOTO_ERROR(H5E_VFL, H5E_CANTGET, HADDR_UNDEF, "unable to get eof")
+    }
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD_splitter_get_eof */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_splitter_truncate
+ *
+ * Purpose:     Notify driver to truncate the file back to the allocated size.
+ *
+ * Return:      SUCCEED/FAIL
+ *-------------------------------------------------------------------------
+ */
+static herr_t
+H5FD_splitter_truncate(H5FD_t *_file, hid_t dxpl_id, hbool_t closing)
+{
+    H5FD_splitter_t *file = (H5FD_splitter_t *)_file;
+    herr_t           ret_value = SUCCEED;       /* Return value */
+
+    FUNC_ENTER_NOAPI_NOINIT
+
+    H5FD_SPLITTER_LOG_CALL("H5FD_splitter_truncate");
+
+    HDassert(file);
+    HDassert(file->rw_file);
+    HDassert(file->wo_file);
+
+    if (H5FDtruncate(file->rw_file, dxpl_id, closing) < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_CANTUPDATE, FAIL, "unable to truncate R/W file")
+    }
+
+    if (H5FDtruncate(file->wo_file, dxpl_id, closing) < 0) {
+       H5FD_SPLITTER_WO_ERROR(file, "H5FD_splitter_truncate",
+                              H5E_VFL, H5E_CANTUPDATE, FAIL,
+                              "unable to truncate W/O file")
+    }
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD_splitter_truncate */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_splitter_sb_size
+ *
+ * Purpose:     Obtains the number of bytes required to store the driver file
+ *              access data in the HDF5 superblock.
+ *
+ * Return:      Success:    Number of bytes required.
+ *
+ *              Failure:    0 if an error occurs or if the driver has no
+ *                          data to store in the superblock.
+ *
+ * NOTE: no public API for H5FD_sb_size, it needs to be added
+ *-------------------------------------------------------------------------
+ */
+static hsize_t
+H5FD_splitter_sb_size(H5FD_t *_file)
+{
+    H5FD_splitter_t *file      = (H5FD_splitter_t *)_file;
+    hsize_t          ret_value = 0;
+
+    FUNC_ENTER_NOAPI_NOINIT_NOERR
+
+    H5FD_SPLITTER_LOG_CALL("H5FD_splitter_sb_size");
+
+    /* Sanity check */
+    HDassert(file);
+    HDassert(file->rw_file);
+
+    if (file->rw_file) {
+        ret_value = H5FD_sb_size(file->rw_file);
+    }
+
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD_splitter_sb_size */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_splitter_sb_encode
+ *
+ * Purpose:     Encode driver-specific data into the output arguments.
+ *
+ * Return:      SUCCEED/FAIL
+ *-------------------------------------------------------------------------
+ */
+static herr_t
+H5FD_splitter_sb_encode(H5FD_t *_file, char *name/*out*/, unsigned char *buf/*out*/)
+{
+    H5FD_splitter_t *file = (H5FD_splitter_t *)_file;
+    herr_t           ret_value = SUCCEED;       /* Return value */
+
+    FUNC_ENTER_NOAPI_NOINIT
+
+    H5FD_SPLITTER_LOG_CALL("H5FD_splitter_sb_encode");
+
+    /* Sanity check */
+    HDassert(file);
+    HDassert(file->rw_file);
+
+    if (file->rw_file && H5FD_sb_encode(file->rw_file, name, buf) < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_CANTENCODE, FAIL, "unable to encode the superblock in R/W file")
+    }
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD_splitter_sb_encode */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_splitter_sb_decode
+ *
+ * Purpose:     Decodes the driver information block.
+ *
+ * Return:      SUCCEED/FAIL
+ *
+ * NOTE: no public API for H5FD_sb_size, need to add
+ *-------------------------------------------------------------------------
+ */
+static herr_t
+H5FD_splitter_sb_decode(H5FD_t *_file, const char *name, const unsigned char *buf)
+{
+    H5FD_splitter_t *file = (H5FD_splitter_t *)_file;
+    herr_t           ret_value = SUCCEED;       /* Return value */
+
+    FUNC_ENTER_NOAPI_NOINIT
+
+    H5FD_SPLITTER_LOG_CALL("H5FD_splitter_sb_decode");
+
+    /* Sanity check */
+    HDassert(file);
+    HDassert(file->rw_file);
+
+    if (H5FD_sb_load(file->rw_file, name, buf) < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_CANTDECODE, FAIL, "unable to decode the superblock in R/W file")
+    }
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD_splitter_sb_decode */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_splitter_cmp
+ *
+ * Purpose:     Compare the keys of two files.
+ *
+ * Return:      Success:    A value like strcmp()
+ *              Failure:    Must never fail
+ *-------------------------------------------------------------------------
+ */
+static int
+H5FD_splitter_cmp(const H5FD_t *_f1, const H5FD_t *_f2)
+{
+    const H5FD_splitter_t  *f1 = (const H5FD_splitter_t *)_f1;
+    const H5FD_splitter_t  *f2 = (const H5FD_splitter_t *)_f2;
+    herr_t ret_value = 0;   /* Return value */
+
+    FUNC_ENTER_NOAPI_NOINIT_NOERR
+
+    H5FD_SPLITTER_LOG_CALL("H5FD_splitter_cmp");
+
+    HDassert(f1);
+    HDassert(f2);
+
+    ret_value = H5FD_cmp(f1->rw_file, f2->rw_file);
+
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD_splitter_cmp */
+
+
+/*--------------------------------------------------------------------------
+ * Function:    H5FD_splitter_get_handle
+ *
+ * Purpose:     Returns a pointer to the file handle of low-level virtual
+ *              file driver.
+ *
+ * Return:      SUCCEED/FAIL
+ *--------------------------------------------------------------------------
+ */
+static herr_t
+H5FD_splitter_get_handle(
+        H5FD_t                *_file,
+        hid_t H5_ATTR_UNUSED   fapl,
+        void                 **file_handle)
+{
+    H5FD_splitter_t  *file = (H5FD_splitter_t*)_file;
+    herr_t ret_value = SUCCEED;   /* Return value */
+
+    FUNC_ENTER_NOAPI_NOINIT
+
+    H5FD_SPLITTER_LOG_CALL("H5FD_splitter_get_handle");
+
+    /* Check arguments */
+    HDassert(file);
+    HDassert(file->rw_file);
+    HDassert(file_handle);
+
+    /* Only do for R/W channel */
+    if (H5FD_get_vfd_handle(file->rw_file, file->fa.rw_fapl_id, file_handle) < 0)
+    {
+        HGOTO_ERROR(H5E_VFL, H5E_CANTGET, FAIL, "unable to get handle of R/W file")
+    }
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD_splitter_get_handle */
+
+
+/*--------------------------------------------------------------------------
+ * Function:    H5FD_splitter_lock
+ *
+ * Purpose:     Sets a file lock.
+ *
+ * Return:      SUCCEED/FAIL
+ *--------------------------------------------------------------------------
+ */
+static herr_t
+H5FD_splitter_lock(H5FD_t *_file, hbool_t rw)
+{
+    H5FD_splitter_t *file = (H5FD_splitter_t *)_file;   /* VFD file struct */
+    herr_t ret_value = SUCCEED;         /* Return value */
+
+    FUNC_ENTER_NOAPI_NOINIT
+
+    H5FD_SPLITTER_LOG_CALL("H5FD_splitter_lock");
+
+    HDassert(file);
+    HDassert(file->rw_file);
+
+    /* Place the lock on each file */
+    if (H5FD_lock(file->rw_file, rw) < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_CANTLOCK, FAIL, "unable to lock R/W file")
+    }
+    if (file->wo_file != NULL) {
+        if (H5FD_lock(file->wo_file, rw) < 0) {
+           H5FD_SPLITTER_WO_ERROR(file, "H5FD_splitter_lock",
+                                  H5E_VFL, H5E_CANTLOCK, FAIL,
+                                  "unable to lock W/O file")
+        }
+    }
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD_splitter_lock */
+
+
+/*--------------------------------------------------------------------------
+ * Function:    H5FD_splitter_unlock
+ *
+ * Purpose:     Removes a file lock.
+ *
+ * Return:      SUCCEED/FAIL
+ *--------------------------------------------------------------------------
+ */
+static herr_t
+H5FD_splitter_unlock(H5FD_t *_file)
+{
+    H5FD_splitter_t *file = (H5FD_splitter_t *)_file;   /* VFD file struct */
+    herr_t ret_value = SUCCEED;         /* Return value */
+
+    FUNC_ENTER_NOAPI_NOINIT
+
+    H5FD_SPLITTER_LOG_CALL("H5FD_splitter_unlock");
+
+    /* Check arguments */
+    HDassert(file);
+    HDassert(file->rw_file);
+
+    /* Remove the lock on each file */
+    if (H5FD_unlock(file->rw_file) < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_CANTUNLOCK, FAIL, "unable to unlock R/W file")
+    }
+    if (file->wo_file != NULL) {
+        if (H5FD_unlock(file->wo_file) < 0) {
+            HGOTO_ERROR(H5E_VFL, H5E_CANTUNLOCK, FAIL,
+                        "unable to unlock W/O file")
+        }
+    }
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD_splitter_unlock */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_splitter_query
+ *
+ * Purpose:     Set the flags that this VFL driver is capable of supporting.
+ *              (listed in H5FDpublic.h)
+ *
+ * Return:      SUCCEED/FAIL
+ *-------------------------------------------------------------------------
+ */
+static herr_t
+H5FD_splitter_query(const H5FD_t *_file, unsigned long *flags /* out */)
+{
+    const H5FD_splitter_t *file      = (const H5FD_splitter_t *)_file;
+    herr_t                 ret_value = SUCCEED;
+
+    FUNC_ENTER_NOAPI_NOINIT
+
+    H5FD_SPLITTER_LOG_CALL("H5FD_splitter_query");
+
+    if (file) {
+        HDassert(file);
+        HDassert(file->rw_file);
+
+        if (H5FDquery(file->rw_file, flags) < 0) {
+            HGOTO_ERROR(H5E_VFL, H5E_CANTLOCK, FAIL,
+                        "unable to query R/W file");
+        }
+    }
+    else {
+        /* There is no file. Because this is a pure passthrough VFD,
+         * it has no features of its own.
+         */
+        if (flags) {
+            *flags = 0;
+        }
+    }
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD_splitter_query() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_splitter_alloc
+ *
+ * Purpose:     Allocate file memory.
+ *
+ * Return:      Address of allocated space (HADDR_UNDEF if error).
+ *-------------------------------------------------------------------------
+ */
+static haddr_t
+H5FD_splitter_alloc(H5FD_t *_file, H5FD_mem_t type, hid_t dxpl_id, hsize_t size)
+{
+    H5FD_splitter_t *file = (H5FD_splitter_t *)_file;   /* VFD file struct */
+    haddr_t ret_value = HADDR_UNDEF;         /* Return value */
+
+    FUNC_ENTER_NOAPI_NOINIT
+
+    H5FD_SPLITTER_LOG_CALL("H5FD_splitter_alloc");
+
+    /* Check arguments */
+    HDassert(file);
+    HDassert(file->rw_file);
+
+    /* Allocate memory for each file, only return the return value for R/W file. */
+    if ((ret_value = H5FDalloc(file->rw_file, type, dxpl_id, size)) == HADDR_UNDEF) {
+        HGOTO_ERROR(H5E_VFL, H5E_CANTINIT, HADDR_UNDEF, "unable to allocate for R/W file")
+    }
+
+    if (H5FDalloc(file->wo_file, type, dxpl_id, size) == HADDR_UNDEF) {
+       H5FD_SPLITTER_WO_ERROR(file, "H5FD_splitter_alloc",
+                              H5E_VFL, H5E_CANTINIT, HADDR_UNDEF,
+                              "unable to alloc for W/O file")
+    }
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD_splitter_alloc() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_splitter_get_type_map
+ *
+ * Purpose:     Retrieve the memory type mapping for this file
+ *
+ * Return:      SUCCEED/FAIL
+ *-------------------------------------------------------------------------
+ */
+static herr_t
+H5FD_splitter_get_type_map(const H5FD_t *_file, H5FD_mem_t *type_map)
+{
+    const H5FD_splitter_t *file      = (const H5FD_splitter_t *)_file;
+    herr_t                 ret_value = SUCCEED;
+
+    FUNC_ENTER_NOAPI_NOINIT
+
+    H5FD_SPLITTER_LOG_CALL("H5FD_splitter_get_type_map");
+
+    /* Check arguments */
+    HDassert(file);
+    HDassert(file->rw_file);
+
+    /* Retrieve memory type mapping for R/W channel only */
+    if (H5FD_get_fs_type_map(file->rw_file, type_map) < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_CANTGET, FAIL, "unable to allocate for R/W file")
+    }
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD_splitter_get_type_map() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_splitter_free
+ *
+ * Purpose:     Free the resources for the splitter VFD.
+ *
+ * Return:      SUCCEED/FAIL
+ *-------------------------------------------------------------------------
+ */
+static herr_t
+H5FD_splitter_free(H5FD_t *_file, H5FD_mem_t type, hid_t dxpl_id, haddr_t addr, hsize_t size)
+{
+    H5FD_splitter_t *file = (H5FD_splitter_t *)_file;   /* VFD file struct */
+    herr_t ret_value = SUCCEED;         /* Return value */
+
+    FUNC_ENTER_NOAPI_NOINIT
+
+    H5FD_SPLITTER_LOG_CALL("H5FD_splitter_free");
+
+    /* Check arguments */
+    HDassert(file);
+    HDassert(file->rw_file);
+
+    if (H5FDfree(file->rw_file, type, dxpl_id, addr, size) < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_CANTFREE, FAIL, "unable to free for R/W file")
+    }
+
+    if (H5FDfree(file->wo_file, type, dxpl_id, addr, size) < 0) {
+       H5FD_SPLITTER_WO_ERROR(file, "H5FD_splitter_free",
+                              H5E_VFL, H5E_CANTINIT, FAIL,
+                              "unable to free for W/O file")
+    }
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD_splitter_free() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD__splitter_log_error
+ *
+ * Purpose:     Log an error from the W/O channel appropriately.
+ *
+ * Return:      SUCCEED/FAIL
+ *-------------------------------------------------------------------------
+ */
+static herr_t
+H5FD__splitter_log_error(const H5FD_splitter_t *file, const char *atfunc, const char *msg)
+{
+    size_t size = 0;
+    char  *s = NULL;
+    herr_t ret_value = SUCCEED;
+
+    FUNC_ENTER_STATIC_NOERR
+
+    H5FD_SPLITTER_LOG_CALL("H5FD__splitter_log_error");
+
+    /* Check arguments */
+    HDassert(file);
+    HDassert(atfunc && *atfunc);
+    HDassert(msg && *msg);
+
+    if (file->logfp != NULL) {
+        size = strlen(atfunc) + strlen(msg) + 3; /* ':', ' ', '\n' */
+        s = (char *)malloc(sizeof(char) * (size+1));
+        if (NULL == s) {
+            ret_value = FAIL;
+        }
+        else
+        if (size < (size_t)HDsnprintf(s, size+1, "%s: %s\n", atfunc, msg)) {
+            ret_value = FAIL;
+        }
+        else
+        if (size != HDfwrite(s, 1, size, file->logfp)) {
+            ret_value = FAIL;
+        }
+        HDfree(s);
+    }
+
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD__splitter_log_error() */
+
diff --git a/src/H5FDsplitter.h b/src/H5FDsplitter.h
new file mode 100644
index 0000000..5a5ef29
--- /dev/null
+++ b/src/H5FDsplitter.h
@@ -0,0 +1,99 @@
+/* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
+ * Copyright by The HDF Group.                                               *
+ * All rights reserved.                                                      *
+ *                                                                           *
+ * This file is part of HDF5.  The full HDF5 copyright notice, including     *
+ * terms governing use, modification, and redistribution, is contained in    *
+ * the COPYING file, which can be found at the root of the source code       *
+ * distribution tree, or in https://support.hdfgroup.org/ftp/HDF5/releases.  *
+ * If you do not have access to either file, you may request a copy from     *
+ * help@hdfgroup.org.                                                        *
+ * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */
+
+/*
+ * Purpose:	The public header file for the "splitter" driver.
+ */
+
+#ifndef H5FDsplitter_H
+#define H5FDsplitter_H
+
+#define H5FD_SPLITTER (H5FD_splitter_init())
+
+/* The version of the H5FD_splitter_vfd_config_t structure used */
+#define H5FD_CURR_SPLITTER_VFD_CONFIG_VERSION 1
+
+/* Maximum length of a filename/path string in the Write-Only channel,
+ * including the NULL-terminator.
+ */
+#define H5FD_SPLITTER_PATH_MAX 4096
+
+/* Semi-unique constant used to help identify structure pointers */
+#define H5FD_SPLITTER_MAGIC 0x2B916880
+
+/* ----------------------------------------------------------------------------
+ * Structure:   H5FD_spliiter_vfd_config_t
+ *
+ * One-stop shopping for configuring a Splitter VFD (rather than many
+ * paramaters passed into H5Pset/get functions).
+ *
+ * magic (int32_t)
+ *      Semi-unique number, used to sanity-check that a given pointer is
+ *      likely (or not) to be this structure type. MUST be first.
+ *      If magic is not H5FD_SPLITTER_MAGIC, the structure (and/or pointer to)
+ *      must be considered invalid.
+ *
+ * version (unsigned int)
+ *      Version number of this structure -- informs component membership.
+ *      If not H5FD_CURR_SPLITTER_VFD_CONFIG_VERSION, the structure (and/or
+ *      pointer to) must be considered invalid.
+ *
+ * rw_fapl_id (hid_t)
+ *      Library-given identification number of the Read/Write channel driver
+ *      File Access Property List.
+ *      The driver must support read/write access.
+ *      Must be set to H5P_DEFAULT or a valid FAPL ID.
+ *
+ * wo_fapl_id (hid_t)
+ *      Library-given identification number of the Read/Write channel driver
+ *      File Access Property List.
+ *      The driver feature flags must include H5FD_FEAT_DEFAULT_VFD_COMPAITBLE.
+ *      Must be set to H5P_DEFAULT or a valid FAPL ID.
+ *
+ * wo_file_path (char[H5FD_SPLITTER_PATH_MAX + 1])
+ *      String buffer for the Write-Only channel target file.
+ *      Must be null-terminated, cannot be empty.
+ *
+ * log_file_path (char[H5FD_SPLITTER_PATH_MAX + 1])
+ *      String buffer for the Splitter VFD logging output.
+ *      Must be null-terminated.
+ *      If null, no logfile is created.
+ *
+ * ignore_wo_errors (hbool_t)
+ *      Toggle flag for how judiciously to respond to errors on the Write-Only
+ *      channel.
+ *
+ * ----------------------------------------------------------------------------
+ */
+typedef struct H5FD_splitter_vfd_config_t {
+    int32_t magic;
+    unsigned int version;
+    hid_t rw_fapl_id;
+    hid_t wo_fapl_id;
+    char wo_path[H5FD_SPLITTER_PATH_MAX + 1];
+    char log_file_path[H5FD_SPLITTER_PATH_MAX + 1];
+    hbool_t ignore_wo_errs;
+} H5FD_splitter_vfd_config_t;
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+H5_DLL hid_t H5FD_splitter_init(void);
+H5_DLL herr_t H5Pset_fapl_splitter(hid_t fapl_id, H5FD_splitter_vfd_config_t *config_ptr);
+H5_DLL herr_t H5Pget_fapl_splitter(hid_t fapl_id, H5FD_splitter_vfd_config_t *config_ptr);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif
+
diff --git a/src/H5private.h b/src/H5private.h
index f570723..95d4885 100644
--- a/src/H5private.h
+++ b/src/H5private.h
@@ -365,6 +365,22 @@
 #endif /* __cplusplus */
 
 /*
+ * Networking headers used by the mirror VFD and related tests and utilities.
+ */
+#ifdef H5_HAVE_ARPA_INET_H
+#   include <arpa/inet.h>
+#endif
+#ifdef H5_HAVE_NETDB_H
+#   include <netdb.h>
+#endif
+#ifdef H5_HAVE_NETINET_IN_H
+#   include <netinet/in.h>
+#endif
+#ifdef H5_HAVE_SYS_SOCKET_H
+#   include <sys/socket.h>
+#endif
+
+/*
  * Status return values for the `herr_t' type.
  * Since some unix/c routines use 0 and -1 (or more precisely, non-negative
  * vs. negative) as their return code, and some assumption had been made in
@@ -646,6 +662,9 @@ typedef struct {
 #ifndef HDabs
     #define HDabs(X)    abs(X)
 #endif /* HDabs */
+#ifndef HDaccept
+    #define HDaccept(A,B,C)    accept((A),(B),(C)) /* mirror VFD */
+#endif /* HDaccept */
 #ifndef HDaccess
     #define HDaccess(F,M)    access(F, M)
 #endif /* HDaccess */
@@ -692,6 +711,9 @@ typedef struct {
 #ifndef HDatoll
     #define HDatoll(S)   atoll(S)
 #endif /* HDatol */
+#ifndef HDbind
+    #define HDbind(A,B,C)   bind((A),(B),(C)) /* mirror VFD */
+#endif /* HDbind */
 #ifndef HDbsearch
     #define HDbsearch(K,B,N,Z,F)  bsearch(K,B,N,Z,F)
 #endif /* HDbsearch */
@@ -734,6 +756,9 @@ typedef struct {
 #ifndef HDclosedir
     #define HDclosedir(D)    closedir(D)
 #endif /* HDclosedir */
+#ifndef HDconnect
+    #define HDconnect(A,B,C)    connect((A),(B),(C)) /* mirror VFD */
+#endif /* HDconnect */
 #ifndef HDcos
     #define HDcos(X)    cos(X)
 #endif /* HDcos */
@@ -978,9 +1003,12 @@ typedef off_t               h5_stat_size_t;
 #ifndef HDgetgroups
     #define HDgetgroups(Z,G)  getgroups(Z,G)
 #endif /* HDgetgroups */
+#ifndef HDgethostbyaddr
+    #define HDgethostbyaddr(A,B,C)  gethostbyaddr((A),(B),(C)) /* mirror VFD */
+#endif /* HDgethostbyaddr */
 #ifndef HDgethostname
     #define HDgethostname(N,L)    gethostname(N,L)
-#endif /* HDgetlogin */
+#endif /* HDgethostname */
 #ifndef HDgetlogin
     #define HDgetlogin()    getlogin()
 #endif /* HDgetlogin */
@@ -1014,6 +1042,18 @@ typedef off_t               h5_stat_size_t;
 #ifndef HDgmtime
     #define HDgmtime(T)    gmtime(T)
 #endif /* HDgmtime */
+#ifndef HDhtonl
+    #define HDhtonl(X)    htonl((X)) /* mirror VFD */
+#endif /* HDhtonl */
+#ifndef HDhtons
+    #define HDhtons(X)    htons((X)) /* mirror VFD */
+#endif /* HDhtons */
+#ifndef HDinet_addr
+    #define HDinet_addr(C)    inet_addr((C)) /* mirror VFD */
+#endif /* HDinet_addr */
+#ifndef HDinet_ntoa
+    #define HDinet_ntoa(C)    inet_ntoa((C)) /* mirror VFD */
+#endif /* HDinet_ntoa */
 #ifndef HDisalnum
     #define HDisalnum(C)    isalnum((int)(C)) /*cast for solaris warning*/
 #endif /* HDisalnum */
@@ -1068,6 +1108,9 @@ typedef off_t               h5_stat_size_t;
 #ifndef HDlink
     #define HDlink(OLD,NEW)    link(OLD,NEW)
 #endif /* HDlink */
+#ifndef HDlisten
+    #define HDlisten(A,B)    listen((A),(B)) /* mirror VFD */
+#endif /* HDlisten */
 #ifndef HDllround
     #define HDllround(V)     llround(V)
 #endif /* HDround */
@@ -1149,6 +1192,12 @@ typedef off_t               h5_stat_size_t;
 #ifndef HDnanosleep
     #define HDnanosleep(N, O)    nanosleep(N, O)
 #endif /* HDnanosleep */
+#ifndef HDntohl
+    #define HDntohl(A)    ntohl((A)) /* mirror VFD */
+#endif /* HDntohl */
+#ifndef HDntohs
+    #define HDntohs(A)    ntohs((A)) /* mirror VFD */
+#endif /* HDntohs */
 #ifndef HDopen
     #define HDopen(F,...)    open(F,__VA_ARGS__)
 #endif /* HDopen */
@@ -1296,12 +1345,21 @@ typedef off_t               h5_stat_size_t;
 #ifndef HDsetsid
     #define HDsetsid()    setsid()
 #endif /* HDsetsid */
+#ifndef HDsetsockopt
+    #define HDsetsockopt(A,B,C,D,E)    setsockopt((A),(B),(C),(D),(E)) /* mirror VFD */
+#endif /* HDsetsockopt */
 #ifndef HDsetuid
     #define HDsetuid(U)    setuid(U)
 #endif /* HDsetuid */
 #ifndef HDsetvbuf
     #define HDsetvbuf(F,S,M,Z)  setvbuf(F,S,M,Z)
 #endif /* HDsetvbuf */
+#ifndef HDshutdown
+    #define HDshutdown(A, B)    shutdown((A),(B)) /* mirror VFD */
+#endif /* HDshutdown */
+#ifndef HDsigaction
+    #define HDsigaction(S,A,O)  sigaction((S),(A),(O))
+#endif /* HDsigaction */
 #ifndef HDsigaddset
     #define HDsigaddset(S,N)  sigaddset(S,N)
 #endif /* HDsigaddset */
@@ -1347,6 +1405,9 @@ typedef off_t               h5_stat_size_t;
 #ifndef HDsnprintf
     #define HDsnprintf    snprintf /*varargs*/
 #endif /* HDsnprintf */
+#ifndef HDsocket
+    #define HDsocket(A,B,C)    socket((A),(B),(C)) /* mirror VFD */
+#endif /* HDsocket */
 #ifndef HDsprintf
     #define HDsprintf    sprintf /*varargs*/
 #endif /* HDsprintf */
diff --git a/src/Makefile.am b/src/Makefile.am
index 0c07c1b..4f40e60 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -62,7 +62,8 @@ libhdf5_la_SOURCES= H5.c H5checksum.c H5dbg.c H5lib_settings.c H5system.c \
         H5FA.c H5FAcache.c H5FAdbg.c H5FAdblock.c H5FAdblkpage.c H5FAhdr.c \
         H5FAint.c H5FAstat.c H5FAtest.c \
         H5FD.c H5FDcore.c H5FDfamily.c H5FDhdfs.c H5FDint.c H5FDlog.c \
-        H5FDmulti.c H5FDsec2.c H5FDspace.c H5FDstdio.c H5FDtest.c \
+        H5FDmirror.c H5FDmulti.c H5FDros3.c H5FDsec2.c H5FDspace.c \
+        H5FDsplitter.c H5FDstdio.c H5FDtest.c \
         H5FL.c H5FO.c H5FS.c H5FScache.c H5FSdbg.c H5FSint.c H5FSsection.c \
         H5FSstat.c H5FStest.c \
         H5G.c H5Gbtree2.c H5Gcache.c H5Gcompact.c H5Gdense.c H5Gdeprec.c \
@@ -135,9 +136,9 @@ include_HEADERS = hdf5.h H5api_adpt.h H5overflow.h H5pubconf.h H5public.h H5vers
         H5Apublic.h H5ACpublic.h \
         H5Cpublic.h H5Dpublic.h \
         H5Epubgen.h H5Epublic.h H5ESpublic.h H5Fpublic.h \
-        H5FDpublic.h H5FDcore.h H5FDdirect.h \
-        H5FDfamily.h H5FDhdfs.h H5FDlog.h H5FDmpi.h H5FDmpio.h \
-        H5FDmulti.h H5FDros3.h H5FDsec2.h H5FDstdio.h H5FDwindows.h \
+        H5FDpublic.h H5FDcore.h H5FDdirect.h  H5FDfamily.h H5FDhdfs.h \
+        H5FDlog.h H5FDmirror.h H5FDmpi.h H5FDmpio.h  H5FDmulti.h H5FDros3.h \
+        H5FDsec2.h H5FDsplitter.h H5FDstdio.h H5FDwindows.h \
         H5Gpublic.h  H5Ipublic.h H5Lpublic.h \
         H5Mpublic.h H5MMpublic.h H5Opublic.h H5Ppublic.h \
         H5PLextern.h H5PLpublic.h \
diff --git a/src/hdf5.h b/src/hdf5.h
index 8367122..eaaf8ae 100644
--- a/src/hdf5.h
+++ b/src/hdf5.h
@@ -46,10 +46,12 @@
 #include "H5FDfamily.h"         /* File families                            */
 #include "H5FDhdfs.h"           /* Hadoop HDFS                              */
 #include "H5FDlog.h"            /* sec2 driver with I/O logging (for debugging) */
+#include "H5FDmirror.h"         /* Mirror VFD and IPC definitions           */
 #include "H5FDmpi.h"            /* MPI-based file drivers                   */
 #include "H5FDmulti.h"          /* Usage-partitioned file family            */
 #include "H5FDros3.h"           /* R/O S3 "file" I/O                        */
 #include "H5FDsec2.h"           /* POSIX unbuffered file I/O                */
+#include "H5FDsplitter.h"       /* Twin-channel (R/W & R/O) I/O passthrough */
 #include "H5FDstdio.h"          /* Standard C buffered I/O                  */
 #ifdef H5_HAVE_WINDOWS
 #include "H5FDwindows.h"        /* Win32 I/O                                */
diff --git a/src/libhdf5.settings.in b/src/libhdf5.settings.in
index 0c2be75..192fc6f 100644
--- a/src/libhdf5.settings.in
+++ b/src/libhdf5.settings.in
@@ -80,6 +80,7 @@ Parallel Filtered Dataset Writes: @PARALLEL_FILTERED_WRITES@
                              MPE: @MPE@
                    Map (H5M) API: @MAP_API@
                       Direct VFD: @DIRECT_VFD@
+                      Mirror VFD: @MIRROR_VFD@
               (Read-Only) S3 VFD: @ROS3_VFD@
             (Read-Only) HDFS VFD: @HAVE_LIBHDFS@
                          dmalloc: @HAVE_DMALLOC@
diff --git a/test/CMakeLists.txt b/test/CMakeLists.txt
index 7a0b2dd..e3bfeb3 100644
--- a/test/CMakeLists.txt
+++ b/test/CMakeLists.txt
@@ -211,6 +211,11 @@ set (cache_image_SOURCES
     ${HDF5_TEST_SOURCE_DIR}/genall5.c
 )
 
+set(mirror_vfd_SOURCES
+    ${HDF5_TEST_SOURCE_DIR}/mirror_vfd.c
+    ${HDF5_TEST_SOURCE_DIR}/genall5.c
+)
+
 set (ttsafe_SOURCES
     ${HDF5_TEST_SOURCE_DIR}/ttsafe.c
     ${HDF5_TEST_SOURCE_DIR}/ttsafe_dcreate.c
@@ -273,6 +278,7 @@ set (H5_TESTS
     ros3
     s3comms
     hdfs
+    mirror_vfd
     ntypes
     dangle
     dtransform
@@ -312,6 +318,7 @@ set (H5_TESTS_MULTIPLE
     cache_image
     ttsafe
     thread_id # special link
+    mirror_vfd
 )
 # Only build single source tests here
 foreach (h5_test ${H5_TESTS})
@@ -415,6 +422,18 @@ else ()
 endif ()
 set_target_properties (thread_id PROPERTIES FOLDER test)
 
+#-- Adding test for mirror_vfd
+add_executable (mirror_vfd ${mirror_vfd_SOURCES})
+target_include_directories (mirror_vfd PRIVATE "${HDF5_SRC_DIR};${HDF5_BINARY_DIR};$<$<BOOL:${HDF5_ENABLE_PARALLEL}>:${MPI_C_INCLUDE_DIRS}>")
+if (NOT BUILD_SHARED_LIBS)
+  TARGET_C_PROPERTIES (mirror_vfd STATIC)
+  target_link_libraries (mirror_vfd PRIVATE ${HDF5_TEST_LIB_TARGET})
+else ()
+  TARGET_C_PROPERTIES (mirror_vfd SHARED)
+  target_link_libraries (mirror_vfd PRIVATE ${HDF5_TEST_LIBSH_TARGET})
+endif ()
+set_target_properties (mirror_vfd PROPERTIES FOLDER test)
+
 ##############################################################################
 ###           A D D I T I O N A L   T E S T S                              ###
 ##############################################################################
diff --git a/test/CMakeTests.cmake b/test/CMakeTests.cmake
index b7eaa56..c73aeda 100644
--- a/test/CMakeTests.cmake
+++ b/test/CMakeTests.cmake
@@ -446,6 +446,10 @@ set (test_CLEANFILES
     vds_swmr_src_*.h5
     tmp_vds_env/vds_src_2.h5
     direct_chunk.h5
+    splitter*.h5
+    splitter.log
+    mirror_rw/*
+    mirror_wo/*
     native_vol_test.h5
 )
 
diff --git a/test/Makefile.am b/test/Makefile.am
index dd0a579..6bea16a 100644
--- a/test/Makefile.am
+++ b/test/Makefile.am
@@ -33,9 +33,11 @@ AM_CPPFLAGS+=-I$(top_srcdir)/src -I$(top_builddir)/src
 #       testvdsswmr.sh: vds_swmr*
 #       testabort_fail.sh: filenotclosed.c and del_many_dense_attrs.c
 #       test_filter_plugin.sh: filter_plugin.c
+#       test_mirror.sh: mirror_vfd ../utils/mirror_vfd/*
 #       test_usecases.sh: use_append_chunk, use_append_mchunks, use_disable_mdc_flushes
 TEST_SCRIPT = testerror.sh testlibinfo.sh testcheck_version.sh testlinks_env.sh testexternal_env.sh \
-    testswmr.sh testvds_env.sh testvdsswmr.sh testflushrefresh.sh test_usecases.sh testabort_fail.sh
+    testswmr.sh testvds_env.sh testvdsswmr.sh testflushrefresh.sh test_usecases.sh testabort_fail.sh \
+    test_mirror.sh
 SCRIPT_DEPEND = error_test$(EXEEXT) err_compat$(EXEEXT) links_env$(EXEEXT) \
     external_env$(EXEEXT) filenotclosed$(EXEEXT) del_many_dense_attrs$(EXEEXT) \
     flushrefresh$(EXEEXT) use_append_chunk$(EXEEXT) use_append_mchunks$(EXEEXT) use_disable_mdc_flushes$(EXEEXT) \
@@ -78,16 +80,18 @@ TEST_PROG= testhdf5 \
 # swmr_* files (besides swmr.c) are used by testswmr.sh.
 # vds_swmr_* files are used by testvdsswmr.sh
 # vds_env is used by testvds_env.sh
+# mirror_vfd is used by test_mirror.sh
 # 'make check' doesn't run them directly, so they are not included in TEST_PROG.
 # Also build testmeta, which is used for timings test.  It builds quickly,
 # and this lets automake keep all its test programs in one place.
 check_PROGRAMS=$(TEST_PROG) error_test err_compat tcheck_version \
     testmeta accum_swmr_reader atomic_writer atomic_reader external_env \
     links_env filenotclosed del_many_dense_attrs flushrefresh \
-    use_append_chunk use_append_mchunks use_disable_mdc_flushes \
+    use_append_chunk use_append_chunk_mirror use_append_mchunks use_disable_mdc_flushes \
     swmr_generator swmr_start_write swmr_reader swmr_writer swmr_remove_reader \
     swmr_remove_writer swmr_addrem_writer swmr_sparse_reader swmr_sparse_writer \
-    swmr_check_compat_vfd vds_env vds_swmr_gen vds_swmr_reader vds_swmr_writer
+    swmr_check_compat_vfd vds_env vds_swmr_gen vds_swmr_reader vds_swmr_writer \
+    mirror_vfd
 if HAVE_SHARED_CONDITIONAL
   check_PROGRAMS+= filter_plugin vol_plugin
 endif
@@ -144,6 +148,7 @@ LDADD=libh5test.la $(LIBHDF5)
 ttsafe_SOURCES=ttsafe.c ttsafe_dcreate.c ttsafe_error.c ttsafe_cancel.c       \
                ttsafe_acreate.c
 cache_image_SOURCES=cache_image.c genall5.c
+mirror_vfd_SOURCES=mirror_vfd.c genall5.c
 
 VFD_LIST = sec2 stdio core core_paged split multi family
 if DIRECT_VFD_CONDITIONAL
@@ -213,7 +218,8 @@ CHECK_CLEANFILES+=accum.h5 cmpd_dset.h5 compact_dataset.h5 dataset.h5 dset_offse
     flushrefresh_VERIFICATION_DONE atomic_data accum_swmr_big.h5 ohdr_swmr.h5 \
     test_swmr*.h5 cache_logging.h5 cache_logging.out vds_swmr.h5 vds_swmr_src_*.h5 \
     swmr[0-2].h5 swmr_writer.out swmr_writer.log.* swmr_reader.out.* swmr_reader.log.* \
-    tbogus.h5.copy cache_image_test.h5 direct_chunk.h5 native_vol_test.h5
+    tbogus.h5.copy cache_image_test.h5 direct_chunk.h5 native_vol_test.h5 \
+    splitter*.h5 splitter.log
 
 # Sources for testhdf5 executable
 testhdf5_SOURCES=testhdf5.c tarray.c tattr.c tchecksum.c tconfig.c tfile.c \
@@ -223,12 +229,13 @@ testhdf5_SOURCES=testhdf5.c tarray.c tattr.c tchecksum.c tconfig.c tfile.c \
 
 # Sources for Use Cases
 use_append_chunk_SOURCES=use_append_chunk.c use_common.c
+use_append_chunk_mirror_SOURCES=use_append_chunk_mirror.c use_common.c
 use_append_mchunks_SOURCES=use_append_mchunks.c use_common.c
 use_disable_mdc_flushes_SOURCES=use_disable_mdc_flushes.c
 
 # Temporary files.
 DISTCLEANFILES=testerror.sh testlibinfo.sh testcheck_version.sh testlinks_env.sh test_filter_plugin.sh \
-    testexternal_env.sh testswmr.sh testvds_env.sh testvdsswmr.sh test_usecases.sh testflushrefresh.sh testabort_fail.sh \
-	test_vol_plugin.sh
+    testexternal_env.sh testswmr.sh testvds_env.sh testvdsswmr.sh test_usecases.sh testflushrefresh.sh \
+    testabort_fail.sh test_vol_plugin.sh test_mirror.sh
 
 include $(top_srcdir)/config/conclude.am
diff --git a/test/h5test.c b/test/h5test.c
index 03731c6..e7d91b6 100644
--- a/test/h5test.c
+++ b/test/h5test.c
@@ -2043,6 +2043,80 @@ h5_get_version_string(H5F_libver_t libver)
 } /* end of h5_get_version_string */
 
 /*-------------------------------------------------------------------------
+ * Function:    h5_compare_file_bytes()
+ *
+ * Purpose:     Helper function to compare two files byte-for-byte.
+ *
+ * Return:      Success:  0, if files are identical
+ *              Failure: -1, if files differ
+ *
+ * Programmer:  Binh-Minh Ribler
+ *              October, 2018
+ *-------------------------------------------------------------------------
+ */
+int
+h5_compare_file_bytes(char *f1name, char *f2name)
+{
+    FILE    *f1ptr = NULL;  /* two file pointers */
+    FILE    *f2ptr = NULL;
+    hsize_t  f1size = 0; /* size of the files */
+    hsize_t  f2size = 0;
+    char     f1char = 0; /* one char from each file */
+    char     f2char = 0;
+    hsize_t  ii = 0;
+    int      ret_value = 0; /* for error handling */
+
+    /* Open files for reading */
+    f1ptr = fopen(f1name, "r");
+    if (f1ptr == NULL) {
+        HDfprintf(stderr, "Unable to fopen() %s\n", f1name);
+        ret_value = -1;
+        goto done;
+    }
+    f2ptr = fopen(f2name, "r");
+    if (f2ptr == NULL) {
+        HDfprintf(stderr, "Unable to fopen() %s\n", f2name);
+        ret_value = -1;
+        goto done;
+    }
+
+    /* Get the file sizes and verify that they equal */
+    fseek(f1ptr , 0 , SEEK_END);
+    f1size = ftell(f1ptr);
+
+    fseek(f2ptr , 0 , SEEK_END);
+    f2size = ftell(f2ptr);
+
+    if (f1size != f2size) {
+        HDfprintf(stderr, "Files differ in size, %llu vs. %llu\n", f1size, f2size);
+        ret_value = -1;
+        goto done;
+    }
+
+    /* Compare each byte and fail if a difference is found */
+    rewind(f1ptr);
+    rewind(f2ptr);
+    for (ii = 0; ii < f1size; ii++) {
+        fread(&f1char, 1, 1, f1ptr);
+        fread(&f2char, 1, 1, f2ptr);
+        if (f1char != f2char) {
+            HDfprintf(stderr, "Mismatch @ 0x%llX: 0x%X != 0x%X\n", ii, f1char, f2char);
+            ret_value = -1;
+            goto done;
+        }
+    }
+
+done:
+    if (f1ptr) {
+        fclose(f1ptr);
+    }
+    if (f2ptr) {
+        fclose(f2ptr);
+    }
+    return(ret_value);
+} /* end h5_compare_file_bytes() */
+
+/*-------------------------------------------------------------------------
  * Function:    H5_get_srcdir_filename
  *
  * Purpose:     Append the test file name to the srcdir path and return the whole string
@@ -2094,3 +2168,4 @@ const char *H5_get_srcdir(void)
     else
         return(NULL);
 } /* end H5_get_srcdir() */
+
diff --git a/test/h5test.h b/test/h5test.h
index e67e559..39ec9d7 100644
--- a/test/h5test.h
+++ b/test/h5test.h
@@ -150,6 +150,7 @@ H5TEST_DLL herr_t h5_verify_cached_stabs(const char *base_name[], hid_t fapl);
 H5TEST_DLL H5FD_class_t *h5_get_dummy_vfd_class(void);
 H5TEST_DLL H5VL_class_t *h5_get_dummy_vol_class(void);
 H5TEST_DLL const char *h5_get_version_string(H5F_libver_t libver);
+H5TEST_DLL int h5_compare_file_bytes(char *fname1, char *fname2);
 
 /* Functions that will replace components of a FAPL */
 H5TEST_DLL herr_t h5_get_vfd_fapl(hid_t fapl_id);
diff --git a/test/mirror_vfd.c b/test/mirror_vfd.c
new file mode 100644
index 0000000..112d67a
--- /dev/null
+++ b/test/mirror_vfd.c
@@ -0,0 +1,2761 @@
+/* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
+ * Copyright by The HDF Group.                                               *
+ * All rights reserved.                                                      *
+ *                                                                           *
+ * This file is part of HDF5.  The full HDF5 copyright notice, including     *
+ * terms governing use, modification, and redistribution, is contained in    *
+ * the COPYING file, which can be found at the root of the source code       *
+ * distribution tree, or in https://support.hdfgroup.org/ftp/HDF5/releases.  *
+ * If you do not have access to either file, you may request a copy from     *
+ * help@hdfgroup.org.                                                        *
+ * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */
+
+/*
+ * Purpose: Test the Mirror VFD functionality.
+ */
+
+/* WARNING: The use of realpath() is probably system-dependent, as are
+ * other things here such as the socket calls.
+ * Notable to realpath() in particular is the use of "PATH_MAX", which
+ * apparently has some major potential issues if paths are abused.
+ * http://insanecoding.blogspot.com/2007/11/pathmax-simply-isnt.html
+ * so BE CAREFUL about the paths we throw around?
+ */
+
+#include "h5test.h"
+#include "cache_common.h"
+#include "genall5.h"
+
+#ifdef H5_HAVE_MIRROR_VFD
+
+/* For future consideration, IP address and port number might be
+ * environment variables?
+ */
+#define SERVER_IP_ADDRESS "127.0.0.1"
+
+/* Primary listening port on server. */
+#define SERVER_HANDSHAKE_PORT 3000
+
+#define DATABUFFER_SIZE 128
+#define DSET_NAME_LEN   16
+
+/* Parameters for the "large chunked dataset" writing */
+#define MAX_DSET_COUNT 255
+#define DSET_DIM        32
+#define CHUNK_DIM        8
+
+#define CONCURRENT_COUNT 3 /* Number of files in concurrent test */
+
+/* Macro: LOGPRINT()
+ * Prints logging and debugging messages to the output stream based
+ * on the level of verbosity.
+ *     0 : no logging
+ *     1 : errors only
+ *     2 : details
+ *     3 : all
+ */
+#define DEFAULT_VERBOSITY 1
+static unsigned int g_verbosity = DEFAULT_VERBOSITY;
+
+/* Macro for selective debug printing / logging */
+#define LOGPRINT(lvl, ...)                  \
+do {                                        \
+    if ((lvl) <= g_verbosity) {             \
+        fprintf(g_log_stream, __VA_ARGS__); \
+        fflush(g_log_stream);               \
+    }                                       \
+} while (0)
+
+#define MIRROR_RW_DIR "mirror_rw/"
+#define MIRROR_WO_DIR "mirror_wo/"
+
+/* String buffer for error messages */
+#define MIRR_MESG_SIZE 128
+static char mesg[MIRR_MESG_SIZE + 1];
+
+/* Convenience structure for passing file names via helper functions.
+ */
+struct mirrortest_filenames {
+    char rw[H5FD_SPLITTER_PATH_MAX+1];
+    char wo[H5FD_SPLITTER_PATH_MAX+1];
+    char log[H5FD_SPLITTER_PATH_MAX+1];
+};
+
+static FILE *g_log_stream = NULL; /* initialized at runtime */
+
+static herr_t _verify_datasets(unsigned min_dset, unsigned max_dset,
+        hid_t *filespace_id, hid_t *dataset_id, hid_t memspace_id);
+
+static herr_t _create_chunking_ids(hid_t file_id, unsigned min_dset,
+        unsigned max_dset, hsize_t *chunk_dims, hsize_t *dset_dims,
+        hid_t *dataspace_ids, hid_t *filespace_ids, hid_t *dataset_ids,
+        hid_t *memspace_id);
+
+static herr_t _close_chunking_ids(unsigned min_dset, unsigned max_dset,
+        hid_t *dataspace_ids, hid_t *filespace_ids, hid_t *dataset_ids,
+        hid_t *memspace_id);
+
+static herr_t _populate_filepath(const char *dirname, const char *_basename,
+        hid_t fapl_id, char *path_out, hbool_t h5suffix);
+
+static hid_t create_mirroring_split_fapl(const char *_basename,
+        struct mirrortest_filenames *names);
+
+static void mybzero(void *dest, size_t size);
+
+
+/* ----------------------------------------------------------------------------
+ * Function:   mybzero
+ *
+ * Purpose:    Have bzero simplicity and abstraction in (possible) absence of 
+ *             it being available.
+ *
+ * Programmer: Jacob Smith
+ *             2020-03-30
+ * ----------------------------------------------------------------------------
+ */
+static void
+mybzero(void *dest, size_t size)
+{
+    size_t i = 0;
+    char *s = NULL;
+    HDassert(dest != NULL);
+    s = (char *)dest;
+    for (i = 0; i < size; i++) {
+        *(s+i) = 0;
+    }
+} /* end mybzero() */
+
+
+/* ----------------------------------------------------------------------------
+ * Function:   _populate_filepath
+ *
+ * Purpose:    Given a directory name and a base name, concatenate the two and
+ *             run h5fixname() to get the "actual" path to the intented target.
+ *             `h5suffix' should be FALSE to keep the base name unaltered;
+ *             TRUE will append the '.h5' h5suffix to the basename...
+ *             FALSE -> h5fixname_no_suffix(), TRUE -> h5fixname()
+ *             <h5fixname_prefix> / <dirname> / <_basename> <h5prefix?>
+ *
+ * Programmer: Jacob Smith
+ *             2019-08-16
+ * ----------------------------------------------------------------------------
+ */
+static herr_t
+_populate_filepath(const char *dirname, const char *_basename, hid_t fapl_id,
+                   char *path_out, hbool_t h5suffix)
+{
+    char _path[H5FD_SPLITTER_PATH_MAX];
+
+    if ((_basename == NULL) ||
+        (*_basename == 0)   ||
+        (dirname == NULL)   ||
+        (*dirname == 0)     ||
+        (path_out == NULL))
+    {
+        TEST_ERROR;
+    }
+
+    if (HDsnprintf(_path, H5FD_SPLITTER_PATH_MAX, "%s%s%s", dirname,
+            (dirname[strlen(dirname)] == '/') ? "" : "/", /* slash iff needed */
+            _basename)
+        > H5FD_SPLITTER_PATH_MAX)
+    {
+        TEST_ERROR;
+    }
+
+    if (h5suffix == TRUE) {
+        if (h5_fixname(_path, fapl_id, path_out,
+                H5FD_SPLITTER_PATH_MAX)
+            == NULL)
+        {
+            TEST_ERROR;
+        }
+    }
+    else {
+        if (h5_fixname_no_suffix(_path, fapl_id, path_out,
+                H5FD_SPLITTER_PATH_MAX)
+            == NULL)
+        {
+            TEST_ERROR;
+        }
+    }
+
+    return SUCCEED;
+
+error:
+    return FAIL;
+} /* end _populate_filepath() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    build_paths
+ *
+ * Purpose:     Convenience function to create the three file paths used in
+ *              most mirror tests.
+ *
+ * Return:      SUCCEED/FAIL
+ *
+ * Programmer:  Jacob Smith
+ *              2019-08-16
+ * ---------------------------------------------------------------------------
+ */
+static herr_t
+build_paths(
+        const char                  *_basename,
+        H5FD_splitter_vfd_config_t  *splitter_config,
+        struct mirrortest_filenames *names)
+{
+    char baselogname[H5FD_SPLITTER_PATH_MAX];
+
+    if (_populate_filepath(MIRROR_RW_DIR, _basename,
+            splitter_config->rw_fapl_id, names->rw, TRUE)
+        == FAIL)
+    {
+        TEST_ERROR;
+    }
+
+    if (_populate_filepath(MIRROR_WO_DIR, _basename,
+            splitter_config->wo_fapl_id, names->wo, TRUE)
+        == FAIL)
+    {
+        TEST_ERROR;
+    }
+
+    if (_basename == NULL || *_basename == 0)
+        return FAIL;
+    if (HDsnprintf(baselogname, H5FD_SPLITTER_PATH_MAX, "%s_err.log",
+            _basename)
+        > H5FD_SPLITTER_PATH_MAX)
+    {
+        TEST_ERROR;
+    }
+
+    if (_populate_filepath(MIRROR_WO_DIR, baselogname,
+            splitter_config->wo_fapl_id, names->log, FALSE)
+        == FAIL)
+    {
+        TEST_ERROR;
+    }
+
+    return SUCCEED;
+
+error:
+    return FAIL;
+} /* end build_paths() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    test_fapl_configuration
+ *
+ * Purpose:     Test FAPL configuration and examination.
+ *
+ * Return:      Success: 0
+ *              Failure: -1
+ *
+ * Programmer:  Jacob Smith
+ *              2019-03-12
+ * ---------------------------------------------------------------------------
+ */
+static int
+test_fapl_configuration(void)
+{
+    hid_t fapl_id;
+    H5FD_mirror_fapl_t mirror_conf = {
+        H5FD_MIRROR_FAPL_MAGIC,          /* magic */
+        H5FD_MIRROR_CURR_FAPL_T_VERSION, /* version */
+        SERVER_HANDSHAKE_PORT,           /* handhake_port */
+        SERVER_IP_ADDRESS,               /* remote_ip "IP address" */
+    };
+    H5FD_mirror_fapl_t fa_out = {0, 0, 0, ""};
+
+    TESTING("Mirror fapl configuration (set/get)");
+
+    fapl_id = H5Pcreate(H5P_FILE_ACCESS);
+    if (H5I_INVALID_HID == fapl_id) {
+        TEST_ERROR;
+    }
+
+    if (H5Pset_fapl_mirror(fapl_id, &mirror_conf) == FAIL) {
+        TEST_ERROR;
+    }
+
+    if (H5Pget_fapl_mirror(fapl_id, &fa_out) == FAIL) {
+        TEST_ERROR;
+    }
+    if (H5FD_MIRROR_FAPL_MAGIC != fa_out.magic) {
+        TEST_ERROR;
+    }
+    if (H5FD_MIRROR_CURR_FAPL_T_VERSION != fa_out.version) {
+        TEST_ERROR;
+    }
+    if (SERVER_HANDSHAKE_PORT != fa_out.handshake_port) {
+        TEST_ERROR;
+    }
+    if (HDstrncmp(SERVER_IP_ADDRESS, (const char *)fa_out.remote_ip,
+            H5FD_MIRROR_MAX_IP_LEN))
+    {
+        TEST_ERROR;
+    }
+
+    if (H5Pclose(fapl_id) == FAIL) {
+        TEST_ERROR;
+    }
+
+    PASSED();
+    return 0;
+
+error:
+    if (H5I_INVALID_HID != fapl_id) {
+        (void)H5Pclose(fapl_id);
+    }
+    return -1;
+} /* end test_fapl_configuration() */
+
+
+
+#define PRINT_BUFFER_DIFF(act, exp, len) do {                                 \
+    size_t _x = 0;                                                            \
+    while ((act)[_x] == (exp)[_x]) {                                          \
+        _x++;                                                                 \
+    }                                                                         \
+    if (_x != (len)) {                                                        \
+        size_t _y = 0;                                                        \
+        HDprintf("First bytes differ at %zu\n", _x);                          \
+        HDprintf("exp  ");                                                    \
+        for (_y = _x; _y < (len); _y++) {                                     \
+            HDprintf("%02X", (unsigned char)(exp)[_y]);                       \
+        }                                                                     \
+        HDprintf("\nact  ");                                                  \
+        for (_y = _x; _y < (len); _y++) {                                     \
+            HDprintf("%02X", (unsigned char)(act)[_y]);                       \
+        }                                                                     \
+        HDprintf("\n");                                                       \
+    }                                                                         \
+} while (0); /* end PRINT_BUFFER_DIFF */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    test_xmit_encode_decode
+ *
+ * Purpose:     Test byte-encoding operations for network transport.
+ *
+ * Return:      Success: 0
+ *              Failure: -1
+ *
+ * Programmer:  Jacob Smith
+ *              2020-02-02
+ * ---------------------------------------------------------------------------
+ */
+static int
+test_xmit_encode_decode(void)
+{
+    H5FD_mirror_xmit_t xmit_mock; /* re-used header in various xmit tests */
+
+    TESTING("Mirror encode/decode of xmit elements");
+
+    /* Set bogus values matching expected; encoding doesn't care
+     * Use sequential values to easily generate the expected buffer with a
+     * for loop.
+     */
+    xmit_mock.magic = 0x00010203;
+    xmit_mock.version = 0x04;
+    xmit_mock.session_token = 0x05060708;
+    xmit_mock.xmit_count = 0x090A0B0C;
+    xmit_mock.op = 0x0D;
+
+    /* Test uint8_t encode/decode
+     */
+    do {
+        unsigned char buf[8];
+        unsigned char expected[8];
+        const uint8_t v = 200;
+        unsigned char out = 0;
+
+        /* Start of buffer uint8_t
+         */
+        mybzero(buf, 8);
+        mybzero(expected, 8);
+        expected[0] = 200;
+        out = 0;
+        if (H5FD__mirror_xmit_encode_uint8(buf, v) != 1) {
+            TEST_ERROR;
+        }
+        if (HDmemcmp(buf, expected, 8) != 0) {
+            PRINT_BUFFER_DIFF(buf, expected, 8);
+            TEST_ERROR;
+        }
+        if (H5FD__mirror_xmit_decode_uint8(&out, buf) != 1) {
+            TEST_ERROR;
+        }
+        if (v != out) {
+            TEST_ERROR;
+        }
+
+        /* Middle of buffer uint8_t
+         */
+        mybzero(buf, 8);
+        mybzero(expected, 8);
+        expected[3] = v;
+        out = 0;
+        if (H5FD__mirror_xmit_encode_uint8((buf+3), v) != 1) {
+            TEST_ERROR;
+        }
+        if (HDmemcmp(buf, expected, 8) != 0) {
+            PRINT_BUFFER_DIFF(buf, expected, 8);
+            TEST_ERROR;
+        }
+        if (H5FD__mirror_xmit_decode_uint8(&out, (buf+3)) != 1) {
+            TEST_ERROR;
+        }
+        if (v != out) {
+            TEST_ERROR;
+        }
+
+        /* End of buffer uint8_t
+         */
+        mybzero(buf, 8);
+        mybzero(expected, 8);
+        expected[7] = v;
+        out = 0;
+        if (H5FD__mirror_xmit_encode_uint8((buf+7), v) != 1) {
+            TEST_ERROR;
+        }
+        if (HDmemcmp(buf, expected, 8) != 0) {
+            PRINT_BUFFER_DIFF(buf, expected, 8);
+            TEST_ERROR;
+        }
+        if (H5FD__mirror_xmit_decode_uint8(&out, (buf+7)) != 1) {
+            TEST_ERROR;
+        }
+        if (v != out) {
+            TEST_ERROR;
+        }
+
+    } while (0); /* end uint8_t en/decode */
+
+    /* Test uint16_t encode/decode
+     */
+    do {
+        unsigned char  buf[8];
+        unsigned char  expected[8];
+        const uint16_t v = 0x8F02;
+        uint16_t out = 0;
+
+        /* Start of buffer uint16_t
+         */
+        mybzero(buf, 8);
+        mybzero(expected, 8);
+        expected[0] = 0x8F;
+        expected[1] = 0x02;
+        out = 0;
+        if (H5FD__mirror_xmit_encode_uint16(buf, v) != 2) {
+            TEST_ERROR;
+        }
+        if (HDmemcmp(buf, expected, 8) != 0) {
+            PRINT_BUFFER_DIFF(buf, expected, 8);
+            TEST_ERROR;
+        }
+        if (H5FD__mirror_xmit_decode_uint16(&out, buf) != 2) {
+            TEST_ERROR;
+        }
+        if (out != v) {
+            TEST_ERROR;
+        }
+
+        /* Middle of buffer uint16_t
+         */
+        mybzero(buf, 8);
+        mybzero(expected, 8);
+        expected[3] = 0x8F;
+        expected[4] = 0x02;
+        out = 0;
+        if (H5FD__mirror_xmit_encode_uint16((buf+3), v) != 2) {
+            TEST_ERROR;
+        }
+        if (HDmemcmp(buf, expected, 8) != 0) {
+            PRINT_BUFFER_DIFF(buf, expected, 8);
+            TEST_ERROR;
+        }
+        if (H5FD__mirror_xmit_decode_uint16(&out, (buf+3)) != 2) {
+            TEST_ERROR;
+        }
+        if (out != v) {
+            TEST_ERROR;
+        }
+        /* slice */
+        if (H5FD__mirror_xmit_decode_uint16(&out, (buf+4)) != 2) {
+            TEST_ERROR;
+        }
+        if (out != 0x0200) {
+            TEST_ERROR;
+        }
+
+        /* End of buffer uint16_t
+         */
+        mybzero(buf, 8);
+        mybzero(expected, 8);
+        expected[6] = 0x8F;
+        expected[7] = 0x02;
+        out = 0;
+        if (H5FD__mirror_xmit_encode_uint16((buf+6), v) != 2) {
+            TEST_ERROR;
+        }
+        if (HDmemcmp(buf, expected, 8) != 0) {
+            PRINT_BUFFER_DIFF(buf, expected, 8);
+            TEST_ERROR;
+        }
+        if (H5FD__mirror_xmit_decode_uint16(&out, (buf+6)) != 2) {
+            TEST_ERROR;
+        }
+        if (out != v) {
+            TEST_ERROR;
+        }
+
+    } while (0); /* end uint16_t en/decode */
+
+    /* Test uint32_t encode/decode
+     */
+    do {
+        unsigned char  buf[8];
+        unsigned char  expected[8];
+        const uint32_t v = 0x8F020048;
+        uint32_t out = 0;
+
+        /* Start of buffer uint32_t
+         */
+        mybzero(buf, 8);
+        mybzero(expected, 8);
+        expected[0] = 0x8F;
+        expected[1] = 0x02;
+        expected[2] = 0x00;
+        expected[3] = 0x48;
+        out = 0;
+        if (H5FD__mirror_xmit_encode_uint32(buf, v) != 4) {
+            TEST_ERROR;
+        }
+        if (HDmemcmp(buf, expected, 8) != 0) {
+            PRINT_BUFFER_DIFF(buf, expected, 8);
+            TEST_ERROR;
+        }
+        if (H5FD__mirror_xmit_decode_uint32(&out, buf) != 4) {
+            TEST_ERROR;
+        }
+        if (out != v) {
+            TEST_ERROR;
+        }
+
+        /* Middle of buffer uint32_t
+         */
+        mybzero(buf, 8);
+        mybzero(expected, 8);
+        expected[3] = 0x8F;
+        expected[4] = 0x02;
+        expected[5] = 0x00;
+        expected[6] = 0x48;
+        out = 0;
+        if (H5FD__mirror_xmit_encode_uint32((buf+3), v) != 4) {
+            TEST_ERROR;
+        }
+        if (HDmemcmp(buf, expected, 8) != 0) {
+            PRINT_BUFFER_DIFF(buf, expected, 8);
+            TEST_ERROR;
+        }
+        if (H5FD__mirror_xmit_decode_uint32(&out, (buf+3)) != 4) {
+            TEST_ERROR;
+        }
+        if (out != v) {
+            TEST_ERROR;
+        }
+        /* slice */
+        if (H5FD__mirror_xmit_decode_uint32(&out, (buf+4)) != 4) {
+            TEST_ERROR;
+        }
+        if (out != 0x02004800) {
+            TEST_ERROR;
+        }
+
+        /* End of buffer uint32_t
+         */
+        mybzero(buf, 8);
+        mybzero(expected, 8);
+        expected[4] = 0x8F;
+        expected[5] = 0x02;
+        expected[6] = 0x00;
+        expected[7] = 0x48;
+        out = 0;
+        if (H5FD__mirror_xmit_encode_uint32((buf+4), v) != 4) {
+            TEST_ERROR;
+        }
+        if (HDmemcmp(buf, expected, 8) != 0) {
+            PRINT_BUFFER_DIFF(buf, expected, 8);
+            TEST_ERROR;
+        }
+        if (H5FD__mirror_xmit_decode_uint32(&out, (buf+4)) != 4) {
+            TEST_ERROR;
+        }
+        if (out != v) {
+            TEST_ERROR;
+        }
+
+    } while (0); /* end uint32_t en/decode */
+
+    /* Test uint64_t encode/decode
+     */
+    do {
+        unsigned char  buf[16];
+        unsigned char  expected[16];
+        const uint64_t v = 0x90DCBE17939CE4BB;
+        uint64_t out = 0;
+
+        /* Start of buffer uint64_t
+         */
+        mybzero(buf, 16);
+        mybzero(expected, 16);
+        expected[0] = 0x90;
+        expected[1] = 0xDC;
+        expected[2] = 0xBE;
+        expected[3] = 0x17;
+        expected[4] = 0x93;
+        expected[5] = 0x9C;
+        expected[6] = 0xE4;
+        expected[7] = 0xBB;
+        out = 0;
+        if (H5FD__mirror_xmit_encode_uint64(buf, v) != 8) {
+            TEST_ERROR;
+        }
+        if (HDmemcmp(buf, expected, 16) != 0) {
+            PRINT_BUFFER_DIFF(buf, expected, 16);
+            TEST_ERROR;
+        }
+        if (H5FD__mirror_xmit_decode_uint64(&out, buf) != 8) {
+            TEST_ERROR;
+        }
+        if (out != v) {
+            TEST_ERROR;
+        }
+
+        /* Middle of buffer uint64_t
+         */
+        mybzero(buf, 16);
+        mybzero(expected, 16);
+        expected[3]  = 0x90;
+        expected[4]  = 0xDC;
+        expected[5]  = 0xBE;
+        expected[6]  = 0x17;
+        expected[7]  = 0x93;
+        expected[8]  = 0x9C;
+        expected[9]  = 0xE4;
+        expected[10] = 0xBB;
+        out = 0;
+        if (H5FD__mirror_xmit_encode_uint64((buf+3), v) != 8) {
+            TEST_ERROR;
+        }
+        if (HDmemcmp(buf, expected, 16) != 0) {
+            PRINT_BUFFER_DIFF(buf, expected, 16);
+            TEST_ERROR;
+        }
+        if (H5FD__mirror_xmit_decode_uint64(&out, (buf+3)) != 8) {
+            TEST_ERROR;
+        }
+        if (out != v) {
+            TEST_ERROR;
+        }
+        /* slice */
+        if (H5FD__mirror_xmit_decode_uint64(&out, (buf+6)) != 8) {
+            TEST_ERROR;
+        }
+        if (out != 0x17939CE4BB000000) {
+            TEST_ERROR;
+        }
+
+        /* End of buffer uint64_t
+         */
+        mybzero(buf, 16);
+        mybzero(expected, 16);
+        expected[8]  = 0x90;
+        expected[9]  = 0xDC;
+        expected[10] = 0xBE;
+        expected[11] = 0x17;
+        expected[12] = 0x93;
+        expected[13] = 0x9C;
+        expected[14] = 0xE4;
+        expected[15] = 0xBB;
+        out = 0;
+        if (H5FD__mirror_xmit_encode_uint64((buf+8), v) != 8) {
+            TEST_ERROR;
+        }
+        if (HDmemcmp(buf, expected, 16) != 0) {
+            PRINT_BUFFER_DIFF(buf, expected, 16);
+            TEST_ERROR;
+        }
+        if (H5FD__mirror_xmit_decode_uint64(&out, (buf+8)) != 8) {
+            TEST_ERROR;
+        }
+        if (out != v) {
+            TEST_ERROR;
+        }
+
+    } while (0); /* end uint64_t en/decode */
+
+    /* Test xmit header structure encode/decode
+     * Write bogus but easily verifiable data to inside a buffer, and compare.
+     * Then decode the buffer and compare the structure contents.
+     * Then repeat from a different offset in the buffer and compare.
+     */
+    do {
+        unsigned char buf[H5FD_MIRROR_XMIT_HEADER_SIZE+8];
+        unsigned char expected[H5FD_MIRROR_XMIT_HEADER_SIZE+8];
+        H5FD_mirror_xmit_t xmit_out;
+        size_t i = 0;
+
+        /* sanity check */
+        if (14 != H5FD_MIRROR_XMIT_HEADER_SIZE) {
+            FAIL_PUTS_ERROR("Header size definition does not match test\n");
+        }
+
+        /* Populate the expected buffer; expect end padding of 0xFF
+         */
+        HDmemset(expected, 0xFF, H5FD_MIRROR_XMIT_HEADER_SIZE+8);
+        for (i=0; i < H5FD_MIRROR_XMIT_HEADER_SIZE; i++) {
+            expected[i+2] = (unsigned char)i;
+        }
+
+        /* Encode, and compare buffer contents
+         * Initial buffer is filled with 0xFF to match expected padding
+         */
+        HDmemset(buf, 0xFF, H5FD_MIRROR_XMIT_HEADER_SIZE+8);
+        if (H5FD_mirror_xmit_encode_header((buf+2), &xmit_mock)
+            != H5FD_MIRROR_XMIT_HEADER_SIZE)
+        {
+            TEST_ERROR;
+        }
+        if (HDmemcmp(buf, expected, H5FD_MIRROR_XMIT_HEADER_SIZE+8) != 0) {
+            PRINT_BUFFER_DIFF(buf, expected, H5FD_MIRROR_XMIT_HEADER_SIZE+8);
+            TEST_ERROR;
+        }
+
+        /* Decode from buffer
+         */
+        if (H5FD_mirror_xmit_decode_header(&xmit_out, (buf+2))
+            != H5FD_MIRROR_XMIT_HEADER_SIZE)
+        {
+            TEST_ERROR;
+        }
+        if (xmit_out.magic         != xmit_mock.magic)         TEST_ERROR;
+        if (xmit_out.version       != xmit_mock.version)       TEST_ERROR;
+        if (xmit_out.session_token != xmit_mock.session_token) TEST_ERROR;
+        if (xmit_out.xmit_count    != xmit_mock.xmit_count)    TEST_ERROR;
+        if (xmit_out.op            != xmit_mock.op)            TEST_ERROR;
+
+        /* Decode from different offset in buffer
+         * Observe changes when ingesting the padding
+         */
+        if (H5FD_mirror_xmit_decode_header(&xmit_out, (buf))
+            != H5FD_MIRROR_XMIT_HEADER_SIZE)
+        {
+            TEST_ERROR;
+        }
+        if (xmit_out.magic         != 0xFFFF0001) TEST_ERROR;
+        if (xmit_out.version       != 0x02)       TEST_ERROR;
+        if (xmit_out.session_token != 0x03040506) TEST_ERROR;
+        if (xmit_out.xmit_count    != 0x0708090A) TEST_ERROR;
+        if (xmit_out.op            != 0x0B)       TEST_ERROR;
+
+    } while (0); /* end xmit header en/decode */
+
+    /* Test xmit set-eoa structure encode/decode
+     * Write bogus but easily verifiable data to inside a buffer, and compare.
+     * Then decode the buffer and compare the structure contents.
+     * Then repeat from a different offset in the buffer and compare.
+     */
+    do {
+        unsigned char buf[H5FD_MIRROR_XMIT_EOA_SIZE+8];
+        unsigned char expected[H5FD_MIRROR_XMIT_EOA_SIZE+8];
+        H5FD_mirror_xmit_eoa_t xmit_in;
+        H5FD_mirror_xmit_eoa_t xmit_out;
+        size_t i = 0;
+
+        /* sanity check */
+        if ((14+9) != H5FD_MIRROR_XMIT_EOA_SIZE) {
+            FAIL_PUTS_ERROR("Header size definition does not match test\n");
+        }
+        if (xmit_mock.op != 0x0D) {
+            FAIL_PUTS_ERROR("shared header structure is not in expected state");
+        }
+
+        /* Populate the expected buffer; expect end padding of 0xFF
+         */
+        HDmemset(expected, 0xFF, H5FD_MIRROR_XMIT_EOA_SIZE+8);
+        for (i=0; i < H5FD_MIRROR_XMIT_EOA_SIZE; i++) {
+            expected[i+2] = (unsigned char)i;
+        }
+
+        /* Set xmit_in
+         */
+        xmit_in.pub = xmit_mock; /* shared/common */
+        xmit_in.type = 0x0E;
+        xmit_in.eoa_addr = 0x0F10111213141516;
+
+        /* Encode, and compare buffer contents
+         * Initial buffer is filled with 0xFF to match expected padding
+         */
+        HDmemset(buf, 0xFF, H5FD_MIRROR_XMIT_EOA_SIZE+8);
+        if (H5FD_mirror_xmit_encode_set_eoa((buf+2), &xmit_in)
+            != H5FD_MIRROR_XMIT_EOA_SIZE)
+        {
+            TEST_ERROR;
+        }
+        if (HDmemcmp(buf, expected, H5FD_MIRROR_XMIT_EOA_SIZE+8) != 0) {
+            PRINT_BUFFER_DIFF(buf, expected, H5FD_MIRROR_XMIT_EOA_SIZE+8);
+            TEST_ERROR;
+        }
+
+        /* Decode from buffer
+         */
+        if (H5FD_mirror_xmit_decode_set_eoa(&xmit_out, (buf+2))
+            != H5FD_MIRROR_XMIT_EOA_SIZE)
+        {
+            TEST_ERROR;
+        }
+        if (xmit_out.pub.magic         != xmit_mock.magic)         TEST_ERROR;
+        if (xmit_out.pub.version       != xmit_mock.version)       TEST_ERROR;
+        if (xmit_out.pub.session_token != xmit_mock.session_token) TEST_ERROR;
+        if (xmit_out.pub.xmit_count    != xmit_mock.xmit_count)    TEST_ERROR;
+        if (xmit_out.pub.op            != xmit_mock.op)            TEST_ERROR;
+        if (xmit_out.type              != 0x0E)                    TEST_ERROR;
+        if (xmit_out.eoa_addr          != 0x0F10111213141516)      TEST_ERROR;
+
+        /* Decode from different offset in buffer
+         * Observe changes when ingesting the padding
+         */
+        if (H5FD_mirror_xmit_decode_set_eoa(&xmit_out, (buf))
+            != H5FD_MIRROR_XMIT_EOA_SIZE)
+        {
+            TEST_ERROR;
+        }
+        if (xmit_out.pub.magic         != 0xFFFF0001)         TEST_ERROR;
+        if (xmit_out.pub.version       != 0x02)               TEST_ERROR;
+        if (xmit_out.pub.session_token != 0x03040506)         TEST_ERROR;
+        if (xmit_out.pub.xmit_count    != 0x0708090A)         TEST_ERROR;
+        if (xmit_out.pub.op            != 0x0B)               TEST_ERROR;
+        if (xmit_out.type              != 0x0C)               TEST_ERROR;
+        if (xmit_out.eoa_addr          != 0x0D0E0F1011121314) TEST_ERROR;
+
+    } while (0); /* end xmit set-eoa en/decode */
+
+    /* Test xmit lock structure encode/decode
+     * Write bogus but easily verifiable data to inside a buffer, and compare.
+     * Then decode the buffer and compare the structure contents.
+     * Then repeat from a different offset in the buffer and compare.
+     */
+    do {
+        unsigned char buf[H5FD_MIRROR_XMIT_LOCK_SIZE+8];
+        unsigned char expected[H5FD_MIRROR_XMIT_LOCK_SIZE+8];
+        H5FD_mirror_xmit_lock_t xmit_in;
+        H5FD_mirror_xmit_lock_t xmit_out;
+        size_t i = 0;
+
+        /* sanity check */
+        if ((14+8) != H5FD_MIRROR_XMIT_LOCK_SIZE) {
+            FAIL_PUTS_ERROR("Header size definition does not match test\n");
+        }
+        if (xmit_mock.op != 0x0D) {
+            FAIL_PUTS_ERROR("shared header structure is not in expected state");
+        }
+
+        /* Populate the expected buffer; expect end padding of 0xFF
+         */
+        HDmemset(expected, 0xFF, H5FD_MIRROR_XMIT_LOCK_SIZE+8);
+        for (i=0; i < H5FD_MIRROR_XMIT_LOCK_SIZE; i++) {
+            expected[i+2] = (unsigned char)i;
+        }
+
+        /* Set xmit_in
+         */
+        xmit_in.pub = xmit_mock; /* shared/common */
+        xmit_in.rw = 0x0E0F101112131415;
+
+        /* Encode, and compare buffer contents
+         * Initial buffer is filled with 0xFF to match expected padding
+         */
+        HDmemset(buf, 0xFF, H5FD_MIRROR_XMIT_LOCK_SIZE+8);
+        if (H5FD_mirror_xmit_encode_lock((buf+2), &xmit_in)
+            != H5FD_MIRROR_XMIT_LOCK_SIZE)
+        {
+            TEST_ERROR;
+        }
+        if (HDmemcmp(buf, expected, H5FD_MIRROR_XMIT_LOCK_SIZE+8) != 0) {
+            PRINT_BUFFER_DIFF(buf, expected, H5FD_MIRROR_XMIT_LOCK_SIZE+8);
+            TEST_ERROR;
+        }
+
+        /* Decode from buffer
+         */
+        if (H5FD_mirror_xmit_decode_lock(&xmit_out, (buf+2))
+            != H5FD_MIRROR_XMIT_LOCK_SIZE)
+        {
+            TEST_ERROR;
+        }
+        if (xmit_out.pub.magic         != xmit_mock.magic)         TEST_ERROR;
+        if (xmit_out.pub.version       != xmit_mock.version)       TEST_ERROR;
+        if (xmit_out.pub.session_token != xmit_mock.session_token) TEST_ERROR;
+        if (xmit_out.pub.xmit_count    != xmit_mock.xmit_count)    TEST_ERROR;
+        if (xmit_out.pub.op            != xmit_mock.op)            TEST_ERROR;
+        if (xmit_out.rw                != 0x0E0F101112131415)      TEST_ERROR;
+
+        /* Decode from different offset in buffer
+         * Observe changes when ingesting the padding
+         */
+        if (H5FD_mirror_xmit_decode_lock(&xmit_out, (buf))
+            != H5FD_MIRROR_XMIT_LOCK_SIZE)
+        {
+            TEST_ERROR;
+        }
+        if (xmit_out.pub.magic         != 0xFFFF0001)         TEST_ERROR;
+        if (xmit_out.pub.version       != 0x02)               TEST_ERROR;
+        if (xmit_out.pub.session_token != 0x03040506)         TEST_ERROR;
+        if (xmit_out.pub.xmit_count    != 0x0708090A)         TEST_ERROR;
+        if (xmit_out.pub.op            != 0x0B)               TEST_ERROR;
+        if (xmit_out.rw                != 0x0C0D0E0F10111213) TEST_ERROR;
+
+    } while (0); /* end xmit lock en/decode */
+
+    /* Test xmit open structure encode/decode
+     * Write bogus but easily verifiable data to inside a buffer, and compare.
+     * Then decode the buffer and compare the structure contents.
+     * Then repeat from a different offset in the buffer and compare.
+     *
+     * Verifies that the first zero character in the filepath will end the
+     * string, with all following bytes in the encoded buffer being zeroed.
+     */
+    do {
+        unsigned char buf[H5FD_MIRROR_XMIT_OPEN_SIZE+8];
+        unsigned char expected[H5FD_MIRROR_XMIT_OPEN_SIZE+8];
+        H5FD_mirror_xmit_open_t xmit_in;
+        H5FD_mirror_xmit_open_t xmit_out;
+        size_t i = 0;
+
+        /* sanity check */
+        if ((14+20+4097) != H5FD_MIRROR_XMIT_OPEN_SIZE) {
+            FAIL_PUTS_ERROR("Header size definition does not match test\n");
+        }
+        if (xmit_mock.op != 0x0D) {
+            FAIL_PUTS_ERROR("shared header structure is not in expected state");
+        }
+
+        /* Populate the expected buffer; expect end padding of 0xFF
+         */
+        HDmemset(expected, 0xFF, H5FD_MIRROR_XMIT_OPEN_SIZE+8);
+        for (i=0; i < H5FD_MIRROR_XMIT_OPEN_SIZE; i++) {
+            /* 0x100 is "zero" in a byte, so encode will treat it as a NULL-
+             * terminator in the filepath string. Expect all zeroes following.
+             */
+            expected[i+2] = (i > 0xFF) ? 0 : (unsigned char)i;
+        }
+
+        /* Set xmit_in
+         */
+        xmit_in.pub = xmit_mock; /* shared/common */
+        xmit_in.flags = 0x0E0F1011;
+        xmit_in.maxaddr = 0x1213141516171819;
+        xmit_in.size_t_blob = 0x1A1B1C1D1E1F2021;
+        for (i=0x22; i < H5FD_MIRROR_XMIT_FILEPATH_MAX+0x22; i++) {
+            /* nonzero values repeat after 0x100, but will not be encoded */
+            xmit_in.filename[i-0x22] = (char)(i % 0x100);
+        }
+        xmit_in.filename[H5FD_MIRROR_XMIT_FILEPATH_MAX-1] = 0;
+
+        /* Encode, and compare buffer contents
+         * Initial buffer is filled with 0xFF to match expected padding
+         */
+        HDmemset(buf, 0xFF, H5FD_MIRROR_XMIT_OPEN_SIZE+8);
+        if (H5FD_mirror_xmit_encode_open((buf+2), &xmit_in)
+            != H5FD_MIRROR_XMIT_OPEN_SIZE)
+        {
+            TEST_ERROR;
+        }
+        if (HDmemcmp(buf, expected, H5FD_MIRROR_XMIT_OPEN_SIZE+8) != 0) {
+            PRINT_BUFFER_DIFF(buf, expected, H5FD_MIRROR_XMIT_OPEN_SIZE+8);
+            TEST_ERROR;
+        }
+
+        /* Decode from buffer
+         */
+        if (H5FD_mirror_xmit_decode_open(&xmit_out, (buf+2))
+            != H5FD_MIRROR_XMIT_OPEN_SIZE)
+        {
+            TEST_ERROR;
+        }
+        if (xmit_out.pub.magic         != xmit_mock.magic)         TEST_ERROR;
+        if (xmit_out.pub.version       != xmit_mock.version)       TEST_ERROR;
+        if (xmit_out.pub.session_token != xmit_mock.session_token) TEST_ERROR;
+        if (xmit_out.pub.xmit_count    != xmit_mock.xmit_count)    TEST_ERROR;
+        if (xmit_out.pub.op            != xmit_mock.op)            TEST_ERROR;
+        if (xmit_out.flags             != xmit_in.flags)           TEST_ERROR;
+        if (xmit_out.maxaddr           != xmit_in.maxaddr)         TEST_ERROR;
+        if (xmit_out.size_t_blob       != xmit_in.size_t_blob)     TEST_ERROR;
+        if (HDstrncmp(xmit_out.filename, xmit_in.filename,
+                      H5FD_MIRROR_XMIT_FILEPATH_MAX)
+            != 0)
+        {
+            PRINT_BUFFER_DIFF(xmit_out.filename, xmit_in.filename,
+                    H5FD_MIRROR_XMIT_FILEPATH_MAX);
+            TEST_ERROR;
+        }
+
+        /* Decode from different offset in buffer
+         * Observe changes when ingesting the padding
+         */
+        if (H5FD_mirror_xmit_decode_open(&xmit_out, (buf))
+            != H5FD_MIRROR_XMIT_OPEN_SIZE)
+        {
+            TEST_ERROR;
+        }
+        if (xmit_out.pub.magic         != 0xFFFF0001)         TEST_ERROR;
+        if (xmit_out.pub.version       != 0x02)               TEST_ERROR;
+        if (xmit_out.pub.session_token != 0x03040506)         TEST_ERROR;
+        if (xmit_out.pub.xmit_count    != 0x0708090A)         TEST_ERROR;
+        if (xmit_out.pub.op            != 0x0B)               TEST_ERROR;
+        if (xmit_out.flags             != 0x0C0D0E0F)         TEST_ERROR;
+        if (xmit_out.maxaddr           != 0x1011121314151617) TEST_ERROR;
+        if (xmit_out.size_t_blob       != 0x18191A1B1C1D1E1F) TEST_ERROR;
+        /* update expected "filepath" in structure */
+        for (i=0x20; i < H5FD_MIRROR_XMIT_FILEPATH_MAX+0x20; i++) {
+            xmit_in.filename[i-0x20] = (i > 0xFF) ? 0 : (char)i;
+        }
+        if (HDstrncmp(xmit_out.filename, xmit_in.filename,
+                      H5FD_MIRROR_XMIT_FILEPATH_MAX)
+            != 0)
+        {
+            PRINT_BUFFER_DIFF(xmit_out.filename, xmit_in.filename,
+                    H5FD_MIRROR_XMIT_FILEPATH_MAX);
+            TEST_ERROR;
+        }
+
+    } while (0); /* end xmit open en/decode */
+
+    /* Test xmit reply structure encode/decode
+     * Write bogus but easily verifiable data to inside a buffer, and compare.
+     * Then decode the buffer and compare the structure contents.
+     * Then repeat from a different offset in the buffer and compare.
+     *
+     * Verifies that the first zero character in the filepath will end the
+     * string, with all following bytes in the encoded buffer being zeroed.
+     */
+    do {
+        unsigned char buf[H5FD_MIRROR_XMIT_REPLY_SIZE+8];
+        unsigned char expected[H5FD_MIRROR_XMIT_REPLY_SIZE+8];
+        H5FD_mirror_xmit_reply_t xmit_in;
+        H5FD_mirror_xmit_reply_t xmit_out;
+        size_t i = 0;
+
+        /* sanity check */
+        if ((14+4+256) != H5FD_MIRROR_XMIT_REPLY_SIZE) {
+            FAIL_PUTS_ERROR("Header size definition does not match test\n");
+        }
+        if (xmit_mock.op != 0x0D) {
+            FAIL_PUTS_ERROR("shared header structure is not in expected state");
+        }
+
+        /* Populate the expected buffer; expect end padding of 0xFF
+         */
+        HDmemset(expected, 0xFF, H5FD_MIRROR_XMIT_REPLY_SIZE+8);
+        for (i=0; i < H5FD_MIRROR_XMIT_REPLY_SIZE; i++) {
+            /* 0x100 is "zero" in a byte, so encode will treat it as a NULL-
+             * terminator in the filepath string. Expect all zeroes following.
+             */
+            expected[i+2] = (i > 0xFF) ? 0 : (unsigned char)i;
+        }
+
+        /* Set xmit_in
+         */
+        xmit_in.pub = xmit_mock; /* shared/common */
+        xmit_in.status = 0x0E0F1011;
+        for (i=0x12; i < H5FD_MIRROR_STATUS_MESSAGE_MAX+0x12; i++) {
+            /* nonzero values repeat after 0x100, but will not be encoded */
+            xmit_in.message[i-0x12] = (char)(i % 0x100);
+        }
+        xmit_in.message[H5FD_MIRROR_STATUS_MESSAGE_MAX-1] = 0;
+
+        /* Encode, and compare buffer contents
+         * Initial buffer is filled with 0xFF to match expected padding
+         */
+        HDmemset(buf, 0xFF, H5FD_MIRROR_XMIT_REPLY_SIZE+8);
+        if (H5FD_mirror_xmit_encode_reply((buf+2), &xmit_in)
+            != H5FD_MIRROR_XMIT_REPLY_SIZE)
+        {
+            TEST_ERROR;
+        }
+        if (HDmemcmp(buf, expected, H5FD_MIRROR_XMIT_REPLY_SIZE+8) != 0) {
+            PRINT_BUFFER_DIFF(buf, expected, H5FD_MIRROR_XMIT_REPLY_SIZE+8);
+            TEST_ERROR;
+        }
+
+        /* Decode from buffer
+         */
+        if (H5FD_mirror_xmit_decode_reply(&xmit_out, (buf+2))
+            != H5FD_MIRROR_XMIT_REPLY_SIZE)
+        {
+            TEST_ERROR;
+        }
+        if (xmit_out.pub.magic         != xmit_mock.magic)         TEST_ERROR;
+        if (xmit_out.pub.version       != xmit_mock.version)       TEST_ERROR;
+        if (xmit_out.pub.session_token != xmit_mock.session_token) TEST_ERROR;
+        if (xmit_out.pub.xmit_count    != xmit_mock.xmit_count)    TEST_ERROR;
+        if (xmit_out.pub.op            != xmit_mock.op)            TEST_ERROR;
+        if (xmit_out.status            != xmit_in.status)          TEST_ERROR;
+        if (HDstrncmp(xmit_out.message, xmit_in.message,
+                      H5FD_MIRROR_STATUS_MESSAGE_MAX)
+            != 0)
+        {
+            PRINT_BUFFER_DIFF(xmit_out.message, xmit_in.message,
+                    H5FD_MIRROR_STATUS_MESSAGE_MAX);
+            TEST_ERROR;
+        }
+
+        /* Decode from different offset in buffer
+         * Observe changes when ingesting the padding
+         */
+        if (H5FD_mirror_xmit_decode_reply(&xmit_out, (buf))
+            != H5FD_MIRROR_XMIT_REPLY_SIZE)
+        {
+            TEST_ERROR;
+        }
+        if (xmit_out.pub.magic         != 0xFFFF0001)         TEST_ERROR;
+        if (xmit_out.pub.version       != 0x02)               TEST_ERROR;
+        if (xmit_out.pub.session_token != 0x03040506)         TEST_ERROR;
+        if (xmit_out.pub.xmit_count    != 0x0708090A)         TEST_ERROR;
+        if (xmit_out.pub.op            != 0x0B)               TEST_ERROR;
+        if (xmit_out.status            != 0x0C0D0E0F)         TEST_ERROR;
+        /* update expected "message" in structure */
+        for (i=0x10; i < H5FD_MIRROR_STATUS_MESSAGE_MAX+0x10; i++) {
+            xmit_in.message[i-0x10] = (i > 0xFF) ? 0 : (char)i;
+        }
+        if (HDstrncmp(xmit_out.message, xmit_in.message,
+                      H5FD_MIRROR_STATUS_MESSAGE_MAX)
+            != 0)
+        {
+            PRINT_BUFFER_DIFF(xmit_out.message, xmit_in.message,
+                    H5FD_MIRROR_STATUS_MESSAGE_MAX);
+            TEST_ERROR;
+        }
+
+    } while (0); /* end xmit reply en/decode */
+
+    /* Test xmit write structure encode/decode
+     * Write bogus but easily verifiable data to inside a buffer, and compare.
+     * Then decode the buffer and compare the structure contents.
+     * Then repeat from a different offset in the buffer and compare.
+     */
+    do {
+        unsigned char buf[H5FD_MIRROR_XMIT_WRITE_SIZE+8];
+        unsigned char expected[H5FD_MIRROR_XMIT_WRITE_SIZE+8];
+        H5FD_mirror_xmit_write_t xmit_in;
+        H5FD_mirror_xmit_write_t xmit_out;
+        size_t i = 0;
+
+        /* sanity check */
+        if ((14+17) != H5FD_MIRROR_XMIT_WRITE_SIZE) {
+            FAIL_PUTS_ERROR("Header size definition does not match test\n");
+        }
+        if (xmit_mock.op != 0x0D) {
+            FAIL_PUTS_ERROR("shared header structure is not in expected state");
+        }
+
+        /* Populate the expected buffer; expect end padding of 0xFF
+         */
+        HDmemset(expected, 0xFF, H5FD_MIRROR_XMIT_WRITE_SIZE+8);
+        for (i=0; i < H5FD_MIRROR_XMIT_WRITE_SIZE; i++) {
+            expected[i+2] = (unsigned char)i;
+        }
+
+        /* Set xmit_in
+         */
+        xmit_in.pub    = xmit_mock; /* shared/common */
+        xmit_in.type   = 0x0E;
+        xmit_in.offset = 0x0F10111213141516;
+        xmit_in.size   = 0x1718191A1B1C1D1E;
+
+        /* Encode, and compare buffer contents
+         * Initial buffer is filled with 0xFF to match expected padding
+         */
+        HDmemset(buf, 0xFF, H5FD_MIRROR_XMIT_WRITE_SIZE+8);
+        if (H5FD_mirror_xmit_encode_write((buf+2), &xmit_in)
+            != H5FD_MIRROR_XMIT_WRITE_SIZE)
+        {
+            TEST_ERROR;
+        }
+        if (HDmemcmp(buf, expected, H5FD_MIRROR_XMIT_WRITE_SIZE+8) != 0) {
+            PRINT_BUFFER_DIFF(buf, expected, H5FD_MIRROR_XMIT_WRITE_SIZE+8);
+            TEST_ERROR;
+        }
+
+        /* Decode from buffer
+         */
+        if (H5FD_mirror_xmit_decode_write(&xmit_out, (buf+2))
+            != H5FD_MIRROR_XMIT_WRITE_SIZE)
+        {
+            TEST_ERROR;
+        }
+        if (xmit_out.pub.magic         != xmit_mock.magic)         TEST_ERROR;
+        if (xmit_out.pub.version       != xmit_mock.version)       TEST_ERROR;
+        if (xmit_out.pub.session_token != xmit_mock.session_token) TEST_ERROR;
+        if (xmit_out.pub.xmit_count    != xmit_mock.xmit_count)    TEST_ERROR;
+        if (xmit_out.pub.op            != xmit_mock.op)            TEST_ERROR;
+        if (xmit_out.type              != 0x0E)                    TEST_ERROR;
+        if (xmit_out.offset            != 0x0F10111213141516)      TEST_ERROR;
+        if (xmit_out.size              != 0x1718191A1B1C1D1E)      TEST_ERROR;
+
+        /* Decode from different offset in buffer
+         * Observe changes when ingesting the padding
+         */
+        if (H5FD_mirror_xmit_decode_write(&xmit_out, (buf))
+            != H5FD_MIRROR_XMIT_WRITE_SIZE)
+        {
+            TEST_ERROR;
+        }
+        if (xmit_out.pub.magic         != 0xFFFF0001)         TEST_ERROR;
+        if (xmit_out.pub.version       != 0x02)               TEST_ERROR;
+        if (xmit_out.pub.session_token != 0x03040506)         TEST_ERROR;
+        if (xmit_out.pub.xmit_count    != 0x0708090A)         TEST_ERROR;
+        if (xmit_out.pub.op            != 0x0B)               TEST_ERROR;
+        if (xmit_out.type              != 0x0C)               TEST_ERROR;
+        if (xmit_out.offset            != 0x0D0E0F1011121314) TEST_ERROR;
+        if (xmit_out.size              != 0x15161718191A1B1C) TEST_ERROR;
+
+    } while (0); /* end xmit write en/decode */
+
+    PASSED();
+    return 0;
+
+error:
+    return -1;
+} /* end test_xmit_encode_decode */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    create_mirroring_split_fapl
+ *
+ * Purpose:     Create and populate a mirroring FAPL ID.
+ *              Creates target files with the given base name -- ideally the
+ *              test name -- and creates mirroring/split FAPL set to use the
+ *              global mirroring info and a sec2 R/W channel driver.
+ *
+ *              TODO: receive target IP from caller?
+ *
+ * Return:      Success: HID of the top-level (splitter) FAPL, a non-negative
+ *                       value.
+ *              Failure: H5I_INVALID_HID, a negative value.
+ *
+ * Programmer:  Jacob Smith
+ *              2019
+ * ---------------------------------------------------------------------------
+ */
+static hid_t
+create_mirroring_split_fapl(const char                      *_basename,
+                            struct mirrortest_filenames     *names)
+{
+    H5FD_splitter_vfd_config_t  splitter_config;
+    H5FD_mirror_fapl_t          mirror_conf;
+    hid_t                       ret_value = H5I_INVALID_HID;
+
+    if (_basename == NULL || *_basename == '\0') {
+        TEST_ERROR;
+    }
+
+    splitter_config.magic = H5FD_SPLITTER_MAGIC;
+    splitter_config.version = H5FD_CURR_SPLITTER_VFD_CONFIG_VERSION;
+    splitter_config.ignore_wo_errs = FALSE;
+
+    /* Create Splitter R/W channel driver (sec2)
+     */
+    splitter_config.rw_fapl_id = H5Pcreate(H5P_FILE_ACCESS);
+    if (H5I_INVALID_HID == splitter_config.rw_fapl_id) {
+        TEST_ERROR;
+    }
+    if (H5Pset_fapl_sec2(splitter_config.rw_fapl_id) == FAIL) {
+        TEST_ERROR;
+    }
+
+    /* Create Splitter W/O channel driver (mirror)
+     */
+    mirror_conf.magic = H5FD_MIRROR_FAPL_MAGIC;
+    mirror_conf.version = H5FD_MIRROR_CURR_FAPL_T_VERSION;
+    mirror_conf.handshake_port = SERVER_HANDSHAKE_PORT;
+    if (HDstrncpy(mirror_conf.remote_ip, SERVER_IP_ADDRESS,
+            H5FD_MIRROR_MAX_IP_LEN)
+        == NULL)
+    {
+        TEST_ERROR;
+    }
+    splitter_config.wo_fapl_id = H5Pcreate(H5P_FILE_ACCESS);
+    if (H5I_INVALID_HID == splitter_config.wo_fapl_id) {
+        TEST_ERROR;
+    }
+    if (H5Pset_fapl_mirror(splitter_config.wo_fapl_id, &mirror_conf) == FAIL) {
+        TEST_ERROR;
+    }
+
+    /* Build r/w, w/o, and log file paths
+     */
+    if (build_paths(_basename, &splitter_config, names) < 0) {
+        TEST_ERROR;
+    }
+
+    /* Set file paths for w/o and logfile
+     */
+    if (HDstrncpy(splitter_config.wo_path, (const char *)names->wo,
+            H5FD_SPLITTER_PATH_MAX)
+        == NULL)
+    {
+        TEST_ERROR;
+    }
+    if (HDstrncpy(splitter_config.log_file_path, (const char *)names->log,
+            H5FD_SPLITTER_PATH_MAX)
+        == NULL)
+    {
+        TEST_ERROR;
+    }
+
+    /* Create Splitter FAPL
+     */
+    ret_value = H5Pcreate(H5P_FILE_ACCESS);
+    if (H5I_INVALID_HID == ret_value) {
+        TEST_ERROR;
+    }
+    if (H5Pset_fapl_splitter(ret_value, &splitter_config) == FAIL) {
+        TEST_ERROR;
+    }
+
+    /* Close FAPLs created for child channels
+     */
+    if (H5Pclose(splitter_config.rw_fapl_id) < 0) {
+        TEST_ERROR;
+    }
+    splitter_config.rw_fapl_id = H5I_INVALID_HID;
+    if (H5Pclose(splitter_config.wo_fapl_id) < 0) {
+        TEST_ERROR;
+    }
+    splitter_config.wo_fapl_id = H5I_INVALID_HID;
+
+    return ret_value;
+
+error:
+    if (splitter_config.wo_fapl_id >= 0) {
+        (void)H5Pclose(splitter_config.wo_fapl_id);
+    }
+    if (splitter_config.rw_fapl_id >= 0) {
+        (void)H5Pclose(splitter_config.rw_fapl_id);
+    }
+    if (ret_value >= 0) {
+        (void)H5Pclose(ret_value);
+    }
+    return H5I_INVALID_HID;
+} /* end create_mirroring_split_fapl() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    test_create_and_close
+ *
+ * Purpose:     Test/demonstrate a do-nothing file open and close.
+ *
+ *              Verifying file existence and contents is part of other tests.
+ *
+ *              TODO: receive target IP from caller?
+ *
+ * Return:      Success: 0
+ *              Failure: -1
+ *
+ * Programmer:  Jacob Smith
+ *              2019-12-17
+ * ---------------------------------------------------------------------------
+ */
+static int
+test_create_and_close(void)
+{
+    struct mirrortest_filenames names;
+    hid_t file_id = H5I_INVALID_HID;
+    hid_t fapl_id = H5P_DEFAULT;
+
+    TESTING("File creation and immediate close");
+
+    /* Create FAPL for Splitter[sec2|mirror]
+     */
+    fapl_id = create_mirroring_split_fapl("basic_create", &names);
+    if (H5I_INVALID_HID == fapl_id) {
+        TEST_ERROR;
+    }
+
+    /* -------------------- */
+    /* TEST: Create and Close */
+
+    file_id = H5Fcreate(names.rw, H5F_ACC_TRUNC, H5P_DEFAULT, fapl_id);
+    if (H5I_INVALID_HID == file_id) {
+        TEST_ERROR;
+    }
+
+    /* -------------------- */
+    /* Standard cleanup */
+
+    if (H5Fclose(file_id) == FAIL) {
+        TEST_ERROR;
+    }
+    if (fapl_id != H5P_DEFAULT && fapl_id >= 0) {
+        if (H5Pclose(fapl_id) == FAIL) {
+            TEST_ERROR;
+        }
+    }
+
+    PASSED();
+    return 0;
+
+error:
+    H5E_BEGIN_TRY{
+        (void)H5Fclose(file_id);
+        (void)H5Pclose(fapl_id);
+    } H5E_END_TRY;
+    return -1;
+} /* end test_create_and_close() */
+
+
+/* ----------------------------------------------------------------------------
+ * Function:    create_datasets
+ *
+ * Purpose:     Given a file ID and least and greateset dataset indices, create
+ *              populated chunked datasets in the target file from min_dset to
+ *              (and including) max_dset.
+ *              Uses #defined constants to determine chunk and dataset sizes
+ *              and values.
+ *
+ * Return:      SUCCEED/FAIL
+ *
+ * Programmer:  Jacob Smith
+ *              2019-08-14
+ * ----------------------------------------------------------------------------
+ */
+static herr_t
+create_datasets(hid_t    file_id,
+                unsigned min_dset,
+                unsigned max_dset)
+{
+    hid_t       dataspace_ids[MAX_DSET_COUNT + 1];
+    hid_t       dataset_ids[MAX_DSET_COUNT + 1];
+    hid_t       filespace_ids[MAX_DSET_COUNT + 1];
+    int         data_chunk[CHUNK_DIM][CHUNK_DIM];
+    unsigned int        i, j, k, l, m;
+    hsize_t     offset[2];
+    hid_t       memspace_id   = H5I_INVALID_HID;
+    hsize_t     a_size[2]     = {CHUNK_DIM, CHUNK_DIM};
+    hsize_t     chunk_dims[2] = {CHUNK_DIM, CHUNK_DIM};
+    hsize_t     dset_dims[2]  = {DSET_DIM, DSET_DIM};
+
+    HDassert(file_id >= 0);
+    HDassert(min_dset <= max_dset);
+    HDassert(max_dset <= MAX_DSET_COUNT);
+
+    LOGPRINT(2, "create_dataset()\n");
+
+    /* ---------------------------------
+     * "Clear" ID arrays
+     */
+
+    for (i = 0; i < MAX_DSET_COUNT; i++) {
+        LOGPRINT(3, "clearing IDs [%d]\n", i);
+        dataspace_ids[i] = H5I_INVALID_HID;
+        dataset_ids[i] = H5I_INVALID_HID;
+        filespace_ids[i] = H5I_INVALID_HID;
+    }
+
+    /* ---------------------------------
+     * Generate dataspace, dataset, and 'filespace' IDs
+     */
+
+    if (_create_chunking_ids(file_id, min_dset, max_dset, chunk_dims,
+            dset_dims, dataspace_ids, filespace_ids, dataset_ids, &memspace_id)
+        == FAIL)
+    {
+        TEST_ERROR;
+    }
+
+    /* ---------------------------------
+     * Initialize (write) all datasets in a "round robin"...
+     * for a given chunk 'location', write chunk data to each dataset.
+     */
+
+    for (i = 0; i < DSET_DIM; i += CHUNK_DIM)
+    {
+        LOGPRINT(3, "i: %d\n", i);
+        for (j = 0; j < DSET_DIM; j += CHUNK_DIM)
+        {
+            LOGPRINT(3, "  j: %d\n", j);
+            for (m = min_dset; m <= max_dset; m++)
+            {
+                LOGPRINT(3, "    m: %d\n", m);
+                for (k = 0; k < CHUNK_DIM; k++)
+                {
+                    for (l = 0; l < CHUNK_DIM; l++)
+                    {
+                        data_chunk[k][l] = (int)((DSET_DIM * DSET_DIM * m) +
+                                                 (DSET_DIM * (i + k)) + j + l);
+                        LOGPRINT(3, "      data_chunk[%d][%d]: %d\n",
+                                 k, l, data_chunk[k][l]);
+                    }
+                }
+
+                /* select on disk hyperslab */
+                offset[0] = (hsize_t)i;
+                offset[1] = (hsize_t)j;
+                LOGPRINT(3, "    H5Sselect_hyperslab()\n");
+                if (H5Sselect_hyperslab(filespace_ids[m], H5S_SELECT_SET,
+                        offset, NULL, a_size, NULL)
+                    < 0)
+                {
+                    TEST_ERROR;
+                }
+
+                LOGPRINT(3, "    H5Dwrite()\n");
+                if (H5Dwrite(dataset_ids[m], H5T_NATIVE_INT, memspace_id,
+                        filespace_ids[m], H5P_DEFAULT, data_chunk)
+                    < 0)
+                {
+                    TEST_ERROR;
+                }
+
+            }
+        }
+    }
+
+    /* ---------------------------------
+     * Read and verify data from datasets
+     */
+
+    if (_verify_datasets(min_dset, max_dset, filespace_ids, dataset_ids,
+                memspace_id)
+        == FAIL)
+    {
+        TEST_ERROR;
+    }
+
+    /* ---------------------------------
+     * Cleanup
+     */
+
+    if (_close_chunking_ids(min_dset, max_dset, dataspace_ids, filespace_ids,
+                dataset_ids, &memspace_id)
+        == FAIL)
+    {
+        TEST_ERROR;
+    }
+
+    return SUCCEED;
+
+error:
+    (void)_close_chunking_ids(min_dset, max_dset, dataspace_ids,
+                filespace_ids, dataset_ids, &memspace_id);
+    LOGPRINT(1, "create_datasets() FAILED\n");
+    return FAIL;
+} /* end create_datasets() */
+
+
+/* ----------------------------------------------------------------------------
+ * Function:   _create_chunking_ids
+ *
+ * Purpose:    Create new IDs to be used with the associated file.
+ *
+ * Return:     SUCCEED/FAIL
+ *
+ * Programer:  Jacob Smith
+ *             2019
+ * ----------------------------------------------------------------------------
+ */
+static herr_t
+_create_chunking_ids(hid_t     file_id,
+                     unsigned  min_dset,
+                     unsigned  max_dset,
+                     hsize_t  *chunk_dims,
+                     hsize_t  *dset_dims,
+                     hid_t    *dataspace_ids,
+                     hid_t    *filespace_ids,
+                     hid_t    *dataset_ids,
+                     hid_t    *memspace_id)
+{
+    char     dset_name[DSET_NAME_LEN + 1];
+    unsigned m         = 0;
+    hid_t    dcpl_id   = H5I_INVALID_HID;
+
+    LOGPRINT(2, "_create_chunking_ids()\n");
+
+    /* --------------------
+     * Create chunking DCPL
+     */
+
+    dcpl_id = H5Pcreate(H5P_DATASET_CREATE);
+    if (dcpl_id < 0) {
+        TEST_ERROR;
+    }
+    if (H5Pset_chunk(dcpl_id, 2, chunk_dims) == FAIL) {
+        TEST_ERROR;
+    }
+
+    /* --------------------
+     * Create dataspace IDs
+     */
+
+    for (m = min_dset; m <= max_dset; m++) {
+        dataspace_ids[m] = H5Screate_simple(2, dset_dims, NULL);
+        if (dataspace_ids[m] < 0) {
+            HDsnprintf(mesg, MIRR_MESG_SIZE,
+                    "unable to create dataspace ID %d\n", m);
+            FAIL_PUTS_ERROR(mesg);
+        }
+    }
+
+    /* --------------------
+     * Create dataset IDs
+     */
+
+    for (m = min_dset; m <= max_dset; m++) {
+        if (HDsnprintf(dset_name, DSET_NAME_LEN, "/dset%03d", m)
+            > DSET_NAME_LEN)
+        {
+            HDsnprintf(mesg, MIRR_MESG_SIZE,
+                    "unable to compose dset name %d\n", m);
+            FAIL_PUTS_ERROR(mesg);
+        }
+
+        dataset_ids[m] = H5Dcreate(file_id, dset_name, H5T_STD_I32BE,
+                dataspace_ids[m], H5P_DEFAULT, dcpl_id, H5P_DEFAULT);
+        if (dataset_ids[m] < 0) {
+            HDsnprintf(mesg, MIRR_MESG_SIZE,
+                    "unable to create dset ID %d\n", m);
+            FAIL_PUTS_ERROR(mesg);
+        }
+    }
+
+    /* --------------------
+     * Get file space IDs
+     */
+
+    for (m = min_dset; m <= max_dset; m++) {
+        filespace_ids[m] = H5Dget_space(dataset_ids[m]);
+        if (filespace_ids[m] < 0) {
+            HDsnprintf(mesg, MIRR_MESG_SIZE,
+                    "unable to create filespace ID %d\n", m);
+            FAIL_PUTS_ERROR(mesg);
+        }
+    }
+
+    /* --------------------
+     * Create mem space to be used to read and write chunks
+     */
+
+    *memspace_id = H5Screate_simple(2, chunk_dims, NULL);
+    if (*memspace_id < 0) {
+        TEST_ERROR;
+    }
+
+    /* --------------------
+     * Clean up the DCPL, even if there were errors before
+     */
+
+    if (dcpl_id != H5P_DEFAULT && dcpl_id != H5I_INVALID_HID) {
+        if (H5Pclose(dcpl_id) == FAIL) {
+            TEST_ERROR;
+        }
+    }
+
+    return SUCCEED;
+
+error:
+    if (dcpl_id != H5P_DEFAULT && dcpl_id != H5I_INVALID_HID) {
+        (void)H5Pclose(dcpl_id);
+    }
+    LOGPRINT(1, "_create_chunking_ids() FAILED\n");
+    return FAIL;
+} /* end _create_chunking_ids() */
+
+
+/* ----------------------------------------------------------------------------
+ * Function:    _open_chunking_ids
+ *
+ * Purpose:     Open/access IDs from the given file.
+ *
+ * Return:      SUCCEED/FAIL
+ *
+ * Programmer:  Jacob Smith
+ *              2019
+ * ----------------------------------------------------------------------------
+ */
+static herr_t
+_open_chunking_ids(
+        hid_t     file_id,
+        unsigned  min_dset,
+        unsigned  max_dset,
+        hsize_t  *chunk_dims,
+        hid_t    *filespace_ids,
+        hid_t    *dataset_ids,
+        hid_t    *memspace_id)
+{
+    char     dset_name[DSET_NAME_LEN+1];
+    unsigned m = 0;
+
+    LOGPRINT(2, "_open_chunking_ids()\n");
+
+    /* --------------------
+     * Open dataset IDs
+     */
+
+    for (m = min_dset; m <= max_dset; m++) {
+        if (HDsnprintf(dset_name, DSET_NAME_LEN, "/dset%03d", m)
+            > DSET_NAME_LEN)
+        {
+            HDsnprintf(mesg, MIRR_MESG_SIZE,
+                    "unable to compose dset name %d\n", m);
+            FAIL_PUTS_ERROR(mesg);
+        }
+
+        dataset_ids[m] = H5Dopen2(file_id, dset_name, H5P_DEFAULT);
+        if (dataset_ids[m] < 0) {
+            HDsnprintf(mesg, MIRR_MESG_SIZE, "unable to open dset ID %d\n", m);
+            FAIL_PUTS_ERROR(mesg);
+        }
+    }
+
+    /* --------------------
+     * Open filespace IDs
+     */
+
+    for (m = min_dset; m <= max_dset; m++) {
+        filespace_ids[m] = H5Dget_space(dataset_ids[m]);
+        if (filespace_ids[m] < 0) {
+            HDsnprintf(mesg, MIRR_MESG_SIZE,
+                    "unable to get filespace ID %d\n", m);
+            FAIL_PUTS_ERROR(mesg);
+        }
+    }
+
+    /* --------------------
+     * Create mem space to be used to read and write chunks
+     */
+
+    *memspace_id = H5Screate_simple(2, chunk_dims, NULL);
+    if (*memspace_id < 0) {
+        TEST_ERROR;
+    }
+
+    return SUCCEED;
+
+error:
+    LOGPRINT(1, "_open_chunking_ids() FAILED\n");
+    return FAIL;
+} /* end _open_chunking_ids() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    _close_chunking_ids
+ *
+ * Purpose:     Close IDs that were created or opened.
+ *              Pass NULL into `dataspace_ids` when closing items opened with
+ *              _open_chunking_ids(). (as opposed to created IDs)
+ *
+ * Return:      SUCCEED/FAIL
+ *
+ * Programmer:  Jacob Smith
+ *              2019
+ * ---------------------------------------------------------------------------
+ */
+static herr_t
+_close_chunking_ids(unsigned min_dset,
+                    unsigned max_dset,
+                    hid_t *dataspace_ids,
+                    hid_t *filespace_ids,
+                    hid_t *dataset_ids,
+                    hid_t *memspace_id)
+{
+    unsigned m;
+
+    LOGPRINT(2, "_close_chunking_ids()\n");
+
+    for (m = min_dset; m <= max_dset; m++) {
+        LOGPRINT(3, "closing ids[%d]\n", m);
+        if (dataspace_ids) {
+            if (H5Sclose(dataspace_ids[m]) < 0) {
+                HDsnprintf(mesg, MIRR_MESG_SIZE,
+                        "unable to close dataspace_id[%d]\n", m);
+                FAIL_PUTS_ERROR(mesg);
+            }
+        }
+        if (H5Dclose(dataset_ids[m]) < 0) {
+            HDsnprintf(mesg, MIRR_MESG_SIZE,
+                        "unable to close dataset_id[%d]\n", m);
+            FAIL_PUTS_ERROR(mesg);
+        }
+        if (H5Sclose(filespace_ids[m]) < 0) {
+            HDsnprintf(mesg, MIRR_MESG_SIZE,
+                        "unable to close filespace_id[%d]\n", m);
+            FAIL_PUTS_ERROR(mesg);
+        }
+    }
+
+    if ( (*memspace_id != H5I_INVALID_HID) &&
+         (H5Sclose(*memspace_id) < 0) )
+    {
+        TEST_ERROR;
+    }
+
+    return SUCCEED;
+
+error:
+    LOGPRINT(1, "_close_chunking_ids() FAILED\n");
+    return FAIL;
+} /* end _close_chunking_ids() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    _verify_datasets
+ *
+ * Purpose:     Check that each chunk's contents are as expected, as pertaining
+ *              to create_datasets().
+ *
+ * Return:      SUCCEED/FAIL
+ *
+ * Programmer:  Jacob Smith
+ *              2019
+ * ---------------------------------------------------------------------------
+ */
+static herr_t
+_verify_datasets(unsigned  min_dset,
+                 unsigned  max_dset,
+                 hid_t    *filespace_ids,
+                 hid_t    *dataset_ids,
+                 hid_t     memspace_id)
+{
+    unsigned i, j, k, l, m;
+    int      data_chunk[CHUNK_DIM][CHUNK_DIM];
+    hsize_t  offset[2];
+    hsize_t  a_size[2] = {CHUNK_DIM, CHUNK_DIM};
+
+    LOGPRINT(2, "_verify_datasets()\n");
+
+    for (i = 0; i < DSET_DIM; i += CHUNK_DIM)
+    {
+        LOGPRINT(3, "i: %d\n", i);
+        for (j = 0; j < DSET_DIM; j += CHUNK_DIM)
+        {
+            LOGPRINT(3, "  j: %d\n", j);
+            for (m = min_dset; m <= max_dset; m++)
+            {
+                LOGPRINT(3, "    m: %d\n", m);
+
+                /* select on disk hyperslab */
+                offset[0] = (hsize_t)i;
+                offset[1] = (hsize_t)j;
+                if (H5Sselect_hyperslab(filespace_ids[m], H5S_SELECT_SET,
+                        offset, NULL, a_size, NULL)
+                    < 0)
+                {
+                    TEST_ERROR;
+                }
+
+                if (H5Dread(dataset_ids[m], H5T_NATIVE_INT, memspace_id,
+                        filespace_ids[m], H5P_DEFAULT, data_chunk)
+                    < 0)
+                {
+                    HDsnprintf(mesg, MIRR_MESG_SIZE,
+                               "      H5Dread() [%d][%d][%d]\n",
+                               i, j, m);
+                    FAIL_PUTS_ERROR(mesg);
+                }
+
+                for (k = 0; k < CHUNK_DIM; k++) {
+                    for (l = 0; l < CHUNK_DIM; l++) {
+                        if ((unsigned)data_chunk[k][l]
+                            !=
+                            ((DSET_DIM * DSET_DIM * m) +
+                             (DSET_DIM * (i + k)) + j + l))
+                        {
+                            HDsnprintf(mesg, MIRR_MESG_SIZE,
+                                    "      MISMATCH [%d][%d][%d][%d][%d]\n",
+                                    i, j, m, k, l);
+                            FAIL_PUTS_ERROR(mesg);
+                        }
+                    }
+                }
+
+            }
+        }
+    }
+
+    return SUCCEED;
+
+error:
+    LOGPRINT(1, "_verify_datasets() FAILED\n");
+    return FAIL;
+} /* end _verify_datasets() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    verify_datasets
+ *
+ * Purpose:     Inspect the datasets in the file created by create_datasets().
+ *              Wrapper for _verify_datasets() -- this function sets up and
+ *              tears down accessor information.
+ *
+ * Return:      SUCCEED/FAIL
+ *
+ * Programmer:  Jacob Smith
+ *              2019
+ * ---------------------------------------------------------------------------
+ */
+static herr_t
+verify_datasets(hid_t    file_id,
+                unsigned min_dset,
+                unsigned max_dset)
+{
+    hid_t    dataset_ids[MAX_DSET_COUNT + 1];
+    hid_t    filespace_ids[MAX_DSET_COUNT + 1];
+    unsigned i;
+    hid_t    memspace_id   = H5I_INVALID_HID;
+    hsize_t  chunk_dims[2] = {CHUNK_DIM, CHUNK_DIM};
+
+    HDassert(file_id >= 0);
+    HDassert(min_dset <= max_dset);
+    HDassert(max_dset <= MAX_DSET_COUNT);
+
+    LOGPRINT(2, "verify_datasets()\n");
+
+    /* ---------------------------------
+     * "Clear" ID arrays
+     */
+
+    for (i = 0; i < MAX_DSET_COUNT; i++) {
+        LOGPRINT(3, "clearing IDs [%d]\n", i);
+        dataset_ids[i] = H5I_INVALID_HID;
+        filespace_ids[i] = H5I_INVALID_HID;
+    }
+
+    /* ---------------------------------
+     * Generate dataspace, dataset, and 'filespace' IDs
+     */
+
+    if (_open_chunking_ids(file_id, min_dset, max_dset, chunk_dims,
+            filespace_ids, dataset_ids, &memspace_id)
+        == FAIL)
+    {
+        TEST_ERROR;
+    }
+
+    /* ---------------------------------
+     * Read and verify data from datasets
+     */
+
+    if (_verify_datasets(min_dset, max_dset, filespace_ids, dataset_ids,
+            memspace_id)
+        == FAIL)
+    {
+        TEST_ERROR;
+    }
+
+    /* ---------------------------------
+     * Cleanup
+     */
+
+    if (_close_chunking_ids(min_dset, max_dset, NULL, filespace_ids,
+                dataset_ids, &memspace_id)
+        == FAIL)
+    {
+        TEST_ERROR;
+    }
+
+    return SUCCEED;
+
+error:
+    LOGPRINT(1, "verify_datasets() FAILED\n");
+    (void)_close_chunking_ids(min_dset, max_dset, NULL, filespace_ids,
+            dataset_ids, &memspace_id);
+    return FAIL;
+
+} /* end verify_datasets() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    test_basic_dataset_write
+ *
+ * Purpose:     Create and close files; repoen files and write a dataset,
+ *              close; compare files.
+ *
+ *              TODO: receive target IP from caller?
+ *
+ * Return:      Success: 0
+ *              Failure: -1
+ *
+ * Programmer:  Jacob Smith
+ *              2019
+ * ---------------------------------------------------------------------------
+ */
+static int
+test_basic_dataset_write(void)
+{
+    struct mirrortest_filenames names;
+    hid_t       file_id = H5I_INVALID_HID;
+    hid_t       fapl_id = H5P_DEFAULT;
+    hid_t       dset_id = H5I_INVALID_HID;
+    hid_t       dspace_id = H5I_INVALID_HID;
+    hid_t       dtype_id = H5T_NATIVE_INT;
+    hsize_t     dims[2] = { DATABUFFER_SIZE, DATABUFFER_SIZE };
+    int        *buf = NULL;
+    int         i = 0;
+    int         j = 0;
+
+    TESTING("Mirror open and dataset writing");
+
+    /* Create FAPL for Splitter[sec2|mirror]
+     */
+    fapl_id = create_mirroring_split_fapl("basic_write", &names);
+    if (H5I_INVALID_HID == fapl_id) {
+        TEST_ERROR;
+    }
+
+    /* Prepare data to be written
+     */
+    buf = (int *)HDmalloc(DATABUFFER_SIZE * DATABUFFER_SIZE * sizeof(int));
+    if (NULL == buf) {
+        TEST_ERROR;
+    }
+    for (i = 0; i < DATABUFFER_SIZE; i++) {
+        for (j = 0; j < DATABUFFER_SIZE; j++) {
+            int k = i * DATABUFFER_SIZE + j;
+            buf[k] = k;
+        }
+    }
+
+    /* -------------------- */
+    /* TEST: Create and Close */
+
+    file_id = H5Fcreate(names.rw, H5F_ACC_TRUNC, H5P_DEFAULT, fapl_id);
+    if (H5I_INVALID_HID == file_id) {
+        TEST_ERROR;
+    }
+    if (H5Fclose(file_id) == FAIL) {
+        TEST_ERROR;
+    }
+    file_id = H5I_INVALID_HID;
+
+
+    /* -------------------- */
+    /* TEST: Repoen and Write */
+
+    file_id = H5Fopen(names.rw, H5F_ACC_RDWR, fapl_id);
+    if (H5I_INVALID_HID == file_id) {
+        TEST_ERROR;
+    }
+
+    dspace_id = H5Screate_simple(2, dims, NULL);
+    if (H5I_INVALID_HID == dspace_id) {
+        TEST_ERROR;
+    }
+    dset_id = H5Dcreate2(file_id, "dataset", dtype_id, dspace_id, H5P_DEFAULT,
+            H5P_DEFAULT, H5P_DEFAULT);
+    if (H5I_INVALID_HID == dset_id) {
+        TEST_ERROR;
+    }
+
+    if (H5Dwrite(dset_id, dtype_id, H5S_ALL, H5S_ALL, H5P_DEFAULT, buf)
+        == FAIL)
+    {
+        TEST_ERROR;
+    }
+
+    /* -------------------- */
+    /* Standard cleanup */
+
+    HDfree(buf);
+    buf = NULL;
+    if (H5Dclose(dset_id) == FAIL) {
+        TEST_ERROR;
+    }
+    if (H5Sclose(dspace_id) == FAIL) {
+        TEST_ERROR;
+    }
+    if (H5Fclose(file_id) == FAIL) {
+        TEST_ERROR;
+    }
+    if (fapl_id != H5P_DEFAULT && fapl_id > 0) {
+        if (H5Pclose(fapl_id) == FAIL) {
+            TEST_ERROR;
+        }
+    }
+
+    /* -------------------- */
+    /* TEST: Verify that the R/W and W/O files are identical */
+
+    if (h5_compare_file_bytes(names.rw, names.wo) < 0) {
+        TEST_ERROR;
+    }
+
+    PASSED();
+    return 0;
+
+error:
+    H5E_BEGIN_TRY{
+        (void)H5Fclose(file_id);
+        if (buf) {
+            HDfree(buf);
+        }
+        (void)H5Dclose(dset_id);
+        (void)H5Sclose(dspace_id);
+        if (fapl_id != H5P_DEFAULT && fapl_id > 0) {
+            (void)H5Pclose(fapl_id);
+        }
+    } H5E_END_TRY;
+    return -1;
+} /* end test_basic_dataset_write() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    test_chunked_dataset_write
+ *
+ * Purpose:     Create and close files; repoen files and write a dataset,
+ *              close; compare files.
+ *
+ *              TODO: receive target IP from caller?
+ *
+ * Return:      Success: 0
+ *              Failure: -1
+ *
+ * Programmer:  Jacob Smith
+ *              2019
+ * ---------------------------------------------------------------------------
+ */
+static int
+test_chunked_dataset_write(void)
+{
+    struct mirrortest_filenames names;
+    hid_t       file_id = H5I_INVALID_HID;
+    hid_t       fapl_id = H5P_DEFAULT;
+
+    TESTING("Mirror open and dataset writing (chunked)");
+
+    /* Create FAPL for Splitter[sec2|mirror]
+     */
+    fapl_id = create_mirroring_split_fapl("chunked_write", &names);
+    if (H5I_INVALID_HID == fapl_id) {
+        TEST_ERROR;
+    }
+
+    /* -------------------- */
+    /* TEST: Create and Close */
+
+    file_id = H5Fcreate(names.rw, H5F_ACC_TRUNC, H5P_DEFAULT, fapl_id);
+    if (H5I_INVALID_HID == file_id) {
+        TEST_ERROR;
+    }
+    if (H5Fclose(file_id) == FAIL) {
+        TEST_ERROR;
+    }
+    file_id = H5I_INVALID_HID;
+
+    /* -------------------- */
+    /* TEST: Reopen and Write */
+
+    file_id = H5Fopen(names.rw, H5F_ACC_RDWR, fapl_id);
+    if (H5I_INVALID_HID == file_id) {
+        TEST_ERROR;
+    }
+
+    /* Write datasets to file
+     */
+    if (create_datasets(file_id, 0, MAX_DSET_COUNT) == FAIL) {
+        TEST_ERROR;
+    }
+
+    /* Close to 'flush to disk', and reopen file
+     */
+    if (H5Fclose(file_id) == FAIL) {
+        TEST_ERROR;
+    }
+    file_id = H5I_INVALID_HID;
+
+    /* Reopen file
+     */
+    file_id = H5Fopen(names.rw, H5F_ACC_RDWR, fapl_id);
+    if (H5I_INVALID_HID == file_id) {
+        TEST_ERROR;
+    }
+
+    /* Verify written data integrity
+     */
+    if (verify_datasets(file_id, 0, MAX_DSET_COUNT) == FAIL) {
+        TEST_ERROR;
+    }
+
+    /* -------------------- */
+    /* Standard cleanup */
+
+    if (H5Fclose(file_id) == FAIL) {
+        TEST_ERROR;
+    }
+    file_id = H5I_INVALID_HID;
+    if (fapl_id != H5P_DEFAULT && fapl_id > 0) {
+        if (H5Pclose(fapl_id) == FAIL) {
+            TEST_ERROR;
+        }
+        fapl_id = H5I_INVALID_HID;
+    }
+
+    /* -------------------- */
+    /* TEST: Verify that the R/W and W/O files are identical */
+
+    if (h5_compare_file_bytes(names.rw, names.wo) < 0) {
+        TEST_ERROR;
+    }
+
+    PASSED();
+    return 0;
+
+error:
+    H5E_BEGIN_TRY {
+        (void)H5Fclose(file_id);
+        if (fapl_id != H5P_DEFAULT && fapl_id > 0) {
+            (void)H5Pclose(fapl_id);
+        }
+    } H5E_END_TRY;
+    return -1;
+} /* end test_chunked_dataset_write() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    test_on_disk_zoo
+ *
+ * Purpose:     Verify that the mirror can handle the passing of all the
+ *              various on-disk data structures over the wire, as implemented
+ *              in genall5.c:create_zoo().
+ *
+ *              TODO: receive target IP from caller?
+ *
+ * Return:      Success: 0
+ *              Failure: -1
+ *
+ * Programmer:  Jacob Smith
+ *              2019
+ * ---------------------------------------------------------------------------
+ */
+static int
+test_on_disk_zoo(void)
+{
+    const char  grp_name[] = "/only";
+    struct mirrortest_filenames names;
+    hid_t       file_id = H5I_INVALID_HID;
+    hid_t       grp_id  = H5I_INVALID_HID;
+    hid_t       fapl_id = H5P_DEFAULT;
+
+    TESTING("'Zoo' of on-disk structures");
+
+    /* Create FAPL for Splitter[sec2|mirror]
+     */
+    fapl_id = create_mirroring_split_fapl("zoo", &names);
+    if (H5I_INVALID_HID == fapl_id) {
+        TEST_ERROR;
+    }
+
+    /* -------------------- */
+    /* TEST: Create file    */
+    file_id = H5Fcreate(names.rw, H5F_ACC_TRUNC, H5P_DEFAULT, fapl_id);
+    if (H5I_INVALID_HID == file_id) {
+        TEST_ERROR;
+    }
+
+    grp_id = H5Gcreate2(file_id, grp_name, H5P_DEFAULT, H5P_DEFAULT,
+            H5P_DEFAULT);
+    if (grp_id == H5I_INVALID_HID) {
+        TEST_ERROR;
+    }
+
+    /* Create datasets in file, close (flush) and reopen, validate.
+     * Use of ( pass ) a conceit required for using create_ and validate_zoo()
+     * from cache_common and/or genall5.
+     */
+
+    if ( pass ) {
+        create_zoo(file_id, grp_name, 0);
+    }
+    if ( pass ) {
+        if (H5Fclose(file_id) == FAIL) {
+            TEST_ERROR;
+        }
+        file_id = H5Fopen(names.rw, H5F_ACC_RDWR, fapl_id);
+        if (H5I_INVALID_HID == file_id) {
+            TEST_ERROR;
+        }
+    }
+    if ( pass ) {
+        validate_zoo(file_id, grp_name, 0); /* sanity-check */
+    }
+    if ( !pass ) {
+        HDprintf(failure_mssg);
+        TEST_ERROR;
+    }
+
+    /* -------------------- */
+    /* Standard cleanup */
+
+    if (fapl_id != H5P_DEFAULT && fapl_id >= 0) {
+        if (H5Pclose(fapl_id) == FAIL) {
+            TEST_ERROR;
+        }
+    }
+    if (H5Gclose(grp_id) == FAIL) {
+        TEST_ERROR;
+    }
+    if (H5Fclose(file_id) == FAIL) {
+        TEST_ERROR;
+    }
+
+    /* -------------------- */
+    /* TEST: Verify that the R/W and W/O files are identical */
+
+    if (h5_compare_file_bytes(names.rw, names.wo) < 0) {
+        TEST_ERROR;
+    }
+
+    PASSED();
+    return 0;
+
+error:
+    H5E_BEGIN_TRY {
+        (void)H5Fclose(file_id);
+        (void)H5Gclose(grp_id);
+        if (fapl_id != H5P_DEFAULT && fapl_id > 0) {
+            (void)H5Pclose(fapl_id);
+        }
+    } H5E_END_TRY;
+    return -1;
+} /* end test_on_disk_zoo() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    test_vanishing_datasets
+ *
+ * Purpose:     Verify behavior when writing to a file where data is deleted.
+ *
+ *              Each dataset is populated with the value of its suffix
+ *              (dset5 is all fives).
+ *
+ *              Opens 0..15 create one new dataset each, '/dset[i]'.
+ *              Opens 3..18 delete '/dset[1-3]'
+ *
+ *              Should end with no data in file.
+ *
+ * Return:      Success:  0
+ *              Failure: -1
+ *
+ * Programmer:  Jacob Smith
+ *              2019
+ * ---------------------------------------------------------------------------
+ */
+static int
+test_vanishing_datasets(void)
+{
+    struct mirrortest_filenames names;
+    hid_t       file_id = H5I_INVALID_HID;
+    hid_t       fapl_id = H5I_INVALID_HID;
+    hid_t       dset_id = H5I_INVALID_HID;
+    hid_t       dspace_id = H5I_INVALID_HID;
+    hid_t       mirror_fapl_id = H5I_INVALID_HID;
+    hsize_t     dims[2] = {DATABUFFER_SIZE, DATABUFFER_SIZE};
+    uint32_t    buf[DATABUFFER_SIZE][DATABUFFER_SIZE]; /* consider malloc? */
+    H5G_info_t  group_info;
+    unsigned int        i, j, k;
+    const unsigned int  max_loops = 20;
+    const unsigned int  max_at_one_time = 3;
+
+    TESTING("Vanishing Datasets");
+
+    /* -------------------- */
+    /* Set up recurrent data (FAPL, dataspace) */
+
+    /* Create FAPL for Splitter[sec2|mirror]
+     */
+    fapl_id = create_mirroring_split_fapl("vanishing", &names);
+    if (H5I_INVALID_HID == fapl_id) {
+        TEST_ERROR;
+    }
+
+    dspace_id = H5Screate_simple(2, dims, NULL);
+    if (dspace_id < 0) {
+        TEST_ERROR;
+    }
+
+    /* create file */
+    file_id = H5Fcreate(names.rw, H5F_ACC_TRUNC, H5P_DEFAULT, fapl_id);
+    if (H5I_INVALID_HID == file_id) {
+        TEST_ERROR;
+    }
+
+    for (i=0; i < max_loops; i++) {
+        char namebuf[DSET_NAME_LEN + 1];
+
+        /* deleting datasets */
+        if (i >= max_at_one_time) {
+            if (HDsnprintf(namebuf, DSET_NAME_LEN, "/dset%02d",
+                    (i - max_at_one_time) )
+                > DSET_NAME_LEN)
+            {
+                TEST_ERROR;
+            }
+            if (H5Ldelete(file_id, namebuf, H5P_DEFAULT) < 0) {
+                TEST_ERROR;
+            }
+        } /* end if deleting a dataset */
+
+        /* writing datasets */
+        if (i < (max_loops - max_at_one_time)) {
+            if (HDsnprintf(namebuf, DSET_NAME_LEN, "/dset%02d", i)
+                > DSET_NAME_LEN)
+            {
+                TEST_ERROR;
+            }
+            dset_id = H5Dcreate2(file_id, namebuf, H5T_STD_U32LE, dspace_id,
+                    H5P_DEFAULT, H5P_DEFAULT, H5P_DEFAULT);
+            if (H5I_INVALID_HID == dset_id) {
+                TEST_ERROR;
+            }
+
+            for (j=0; j < DATABUFFER_SIZE; j++) {
+                for (k=0; k < DATABUFFER_SIZE; k++) {
+                    buf[j][k] = (uint32_t)i;
+                }
+            }
+
+            if (H5Dwrite(dset_id, H5T_STD_U32LE, H5S_ALL, H5S_ALL, H5P_DEFAULT,
+                    buf)
+                < 0)
+            {
+                TEST_ERROR;
+            }
+
+            if (H5Dclose(dset_id) < 0) {
+                TEST_ERROR;
+            }
+            dset_id = H5I_INVALID_HID;
+        } /* end if writing a dataset */
+
+    } /* end for dataset create-destroy cycles */
+
+    if (H5Fclose(file_id) < 0) {
+        TEST_ERROR;
+    }
+    file_id = H5I_INVALID_HID;
+
+    /* verify there are no datasets in file */
+    file_id = H5Fopen(names.rw, H5F_ACC_RDONLY, H5P_DEFAULT);
+    if (file_id < 0) {
+        TEST_ERROR;
+    }
+    if (H5Gget_info(file_id, &group_info) < 0) {
+        TEST_ERROR;
+    }
+    if (group_info.nlinks > 0) {
+        HDfprintf(stderr, "links in rw file: %d\n", group_info.nlinks);
+        HDfflush(stderr);
+        TEST_ERROR;
+    }
+    if (H5Fclose(file_id) < 0) {
+        TEST_ERROR;
+    }
+    file_id = H5Fopen(names.wo, H5F_ACC_RDONLY, H5P_DEFAULT);
+    if (file_id < 0) {
+        TEST_ERROR;
+    }
+    if (H5Gget_info(file_id, &group_info) < 0) {
+        TEST_ERROR;
+    }
+    if (group_info.nlinks > 0) {
+        HDfprintf(stderr, "links in wo file: %d\n", group_info.nlinks);
+        HDfflush(stderr);
+        TEST_ERROR;
+    }
+    if (H5Fclose(file_id) < 0) {
+        TEST_ERROR;
+    }
+    file_id = H5I_INVALID_HID;
+
+    if (h5_compare_file_bytes(names.rw, names.wo) < 0)
+        TEST_ERROR;
+
+    /* -------------------- */
+    /* Teardown */
+
+    if (H5Sclose(dspace_id) < 0) {
+        TEST_ERROR;
+    }
+    if (H5Pclose(fapl_id) < 0) {
+        TEST_ERROR;
+    }
+
+    PASSED();
+    return 0;
+
+error:
+    H5E_BEGIN_TRY {
+        if (mirror_fapl_id != H5I_INVALID_HID) {
+            H5Pclose(mirror_fapl_id);
+        }
+        if (fapl_id != H5I_INVALID_HID) {
+            H5Pclose(fapl_id);
+        }
+        if (file_id != H5I_INVALID_HID) {
+            H5Fclose(file_id);
+        }
+        if (dset_id != H5I_INVALID_HID) {
+            H5Dclose(dset_id);
+        }
+        if (dspace_id != H5I_INVALID_HID) {
+            H5Sclose(dspace_id);
+        }
+    } H5E_END_TRY;
+    return -1;
+} /* test_vanishing_datasets() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    test_concurrent_access
+ *
+ * Purpose:     Verify that more than one file may be opened at a time.
+ *
+ *              TODO: receive target IP from caller?
+ *
+ * Return:      Success: 0
+ *              Failure: -1
+ *
+ * Programmer:  Jacob Smith
+ *              2020-03-09
+ * ---------------------------------------------------------------------------
+ */
+static int
+test_concurrent_access(void)
+{
+    struct file_bundle {
+        struct mirrortest_filenames names;
+        hid_t dset_id;
+        hid_t fapl_id;
+        hid_t file_id;
+    } bundle[CONCURRENT_COUNT];
+    hid_t       dspace_id = H5I_INVALID_HID;
+    hid_t       dtype_id = H5T_NATIVE_INT;
+    hsize_t     dims[2] = { DATABUFFER_SIZE, DATABUFFER_SIZE };
+    int        *buf = NULL;
+    int         i = 0;
+    int         j = 0;
+
+    TESTING("Concurrent opened mirrored files");
+
+    /* blank bundle */
+    for (i = 0; i < CONCURRENT_COUNT; i++) {
+        bundle[i].dset_id = H5I_INVALID_HID;
+        bundle[i].fapl_id = H5I_INVALID_HID;
+        bundle[i].file_id = H5I_INVALID_HID;
+        *bundle[i].names.rw = '\0';
+        *bundle[i].names.wo = '\0';
+        *bundle[i].names.log = '\0';
+    }
+
+    /* Create FAPL for Splitter[sec2|mirror]
+     */
+    for (i = 0; i < CONCURRENT_COUNT; i++) {
+        char _name[16] = "";
+        hid_t _fapl_id = H5I_INVALID_HID;
+        HDsnprintf(_name, 15, "concurrent%d", i);
+        _fapl_id = create_mirroring_split_fapl(_name, &bundle[i].names);
+        if (H5I_INVALID_HID == _fapl_id) {
+            TEST_ERROR;
+        }
+        bundle[i].fapl_id = _fapl_id;
+    }
+
+    /* Prepare data to be written
+     */
+    buf = (int *)HDmalloc(DATABUFFER_SIZE * DATABUFFER_SIZE * sizeof(int));
+    if (NULL == buf) {
+        TEST_ERROR;
+    }
+    for (i = 0; i < DATABUFFER_SIZE; i++) {
+        for (j = 0; j < DATABUFFER_SIZE; j++) {
+            int k = i * DATABUFFER_SIZE + j;
+            buf[k] = k;
+        }
+    }
+
+    /* Prepare generic dataspace
+     */
+    dspace_id = H5Screate_simple(2, dims, NULL);
+    if (H5I_INVALID_HID == dspace_id) {
+        TEST_ERROR;
+    }
+
+    /* -------------------- */
+    /* TEST: Create file and open elements */
+
+    for (i = 0; i < CONCURRENT_COUNT; i++) {
+        hid_t _file_id = H5I_INVALID_HID;
+        hid_t _dset_id = H5I_INVALID_HID;
+
+        _file_id = H5Fcreate(bundle[i].names.rw, H5F_ACC_TRUNC, H5P_DEFAULT,
+                bundle[i].fapl_id);
+        if (H5I_INVALID_HID == _file_id) {
+            TEST_ERROR;
+        }
+
+        bundle[i].file_id  = _file_id;
+
+        _dset_id = H5Dcreate2(_file_id, "dataset", dtype_id, dspace_id,
+                H5P_DEFAULT, H5P_DEFAULT, H5P_DEFAULT);
+        if (H5I_INVALID_HID == _dset_id) {
+            TEST_ERROR;
+        }
+        bundle[i].dset_id = _dset_id;
+    }
+
+    /* -------------------- */
+    /* TEST: Write to files */
+
+    for (i = 0; i < CONCURRENT_COUNT; i++) {
+        if (H5Dwrite(bundle[i].dset_id, dtype_id, H5S_ALL, H5S_ALL,
+                H5P_DEFAULT, buf)
+            == FAIL)
+        {
+            TEST_ERROR;
+        }
+    }
+
+    /* -------------------- */
+    /* TEST: Close elements  */
+
+    for (i = 0; i < CONCURRENT_COUNT; i++) {
+        if (H5Dclose(bundle[i].dset_id) == FAIL) {
+            TEST_ERROR;
+        }
+        if (H5Fclose(bundle[i].file_id) == FAIL) {
+            TEST_ERROR;
+        }
+        if (H5Pclose(bundle[i].fapl_id) == FAIL) {
+            TEST_ERROR;
+        }
+    }
+
+    /* -------------------- */
+    /* Standard cleanup */
+
+    HDfree(buf);
+    buf = NULL;
+    if (H5Sclose(dspace_id) == FAIL) {
+        TEST_ERROR;
+    }
+
+    /* -------------------- */
+    /* TEST: Verify that the R/W and W/O files are identical */
+
+    for (i = 0; i < CONCURRENT_COUNT; i++) {
+        if (h5_compare_file_bytes(bundle[i].names.rw, bundle[i].names.wo) < 0) {
+            TEST_ERROR;
+        }
+    }
+
+    PASSED();
+    return 0;
+
+error:
+    H5E_BEGIN_TRY{
+        if (buf) {
+            HDfree(buf);
+        }
+        (void)H5Sclose(dspace_id);
+        for (i = 0; i < CONCURRENT_COUNT; i++) {
+            (void)H5Dclose(bundle[i].dset_id);
+            (void)H5Fclose(bundle[i].file_id);
+            (void)H5Pclose(bundle[i].fapl_id);
+        }
+    } H5E_END_TRY;
+    return -1;
+} /* end test_concurrent_access() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    main
+ *
+ * Purpose:     Run tests.
+ *
+ * Return:      Success: 0
+ *              Failure: 1
+ *
+ * Programmer:  Jacob Smith
+ *              2019
+ * ---------------------------------------------------------------------------
+ */
+int
+main(void)
+{
+    int nerrors = 0;
+
+    h5_reset();
+
+    g_log_stream = stdout; /* default debug/logging output stream */
+
+    HDprintf("Testing Mirror VFD functionality.\n");
+
+    /* -------------------- */
+    /* SETUP */
+
+    /* Create directories for test-generated .h5 files
+     */
+    if (nerrors == 0) {
+        if ((HDmkdir(MIRROR_RW_DIR, (mode_t)0755) < 0) && (errno != EEXIST)) {
+            nerrors++;
+        }
+    }
+    if (nerrors == 0) {
+        if ((HDmkdir(MIRROR_WO_DIR, (mode_t)0755) < 0) && (errno != EEXIST)) {
+            nerrors++;
+        }
+    }
+
+    /* -------------------- */
+    /* TESTS */
+    /* Tests return negative values; `-=' increments nerrors count */
+
+    if (nerrors == 0) {
+        nerrors -= test_fapl_configuration();
+        nerrors -= test_xmit_encode_decode();
+        nerrors -= test_create_and_close();
+        nerrors -= test_basic_dataset_write();
+        nerrors -= test_chunked_dataset_write();
+        nerrors -= test_on_disk_zoo();
+        nerrors -= test_vanishing_datasets();
+        nerrors -= test_concurrent_access();
+    }
+
+    if (nerrors) {
+        HDprintf("***** %d Mirror VFD TEST%s FAILED! *****\n",
+                nerrors, nerrors > 1 ? "S" : "");
+        return EXIT_FAILURE;
+    }
+
+    HDprintf("All Mirror Virtual File Driver tests passed.\n");
+    return EXIT_SUCCESS;
+} /* end main() */
+
+#else /* H5_HAVE_MIRROR_VFD */
+
+int
+main(void)
+{
+    h5_reset();
+    HDprintf("Testing Mirror VFD functionality.\n");
+    HDprintf("SKIPPED - Mirror VFD not built.\n");
+    return EXIT_SUCCESS;
+}
+
+#endif /* H5_HAVE_MIRROR_VFD */
+
+
diff --git a/test/test_mirror.sh.in b/test/test_mirror.sh.in
new file mode 100644
index 0000000..9635c38
--- /dev/null
+++ b/test/test_mirror.sh.in
@@ -0,0 +1,100 @@
+#! /bin/bash
+#
+# Copyright by The HDF Group.
+# All rights reserved.
+#
+# This file is part of HDF5.  The full HDF5 copyright notice, including
+# terms governing use, modification, and redistribution, is contained in
+# the COPYING file, which can be found at the root of the source code
+# distribution tree, or in https://support.hdfgroup.org/ftp/HDF5/releases.
+# If you do not have access to either file, you may request a copy from
+# help@hdfgroup.org.
+#
+# Tests for the Mirror VFD feature.
+#
+# Created:
+#   Jacob Smith, 2019-12-30
+
+###############################################################################
+## test parameters
+###############################################################################
+
+nerrors=0
+
+SERVER_VERBOSITY="--verbosity=1"
+SERVER_PORT="--port=3000"
+
+
+###############################################################################
+## Main
+###############################################################################
+
+## TODO: arguments for main port, port range, verbosity?
+# Parse options (none accepted at this time)
+while [ $# -gt 0 ]; do
+    case "$1" in
+    *)  # unknown option
+        echo "$0: Unknown option ($1)"
+        exit 1
+        ;;
+    esac
+done
+
+
+
+RUN_DIR=mirror_vfd_test
+MIRROR_UTILS=../utils/mirror_vfd # TODO: presupposes from test/
+
+mkdir $RUN_DIR
+
+# Copy program files into dedicated test directory
+for FILE in $MIRROR_UTILS/mirror_* ; do
+    case "$FILE" in
+        *.o) continue ;; # Don't copy .o files
+    esac
+    cp $FILE $RUN_DIR
+done
+cp mirror_vfd $RUN_DIR
+
+# With the --disable-shared option, program files are built in their main
+# directories; otherwise they are built in dir/.libs with a corresponding
+# wrapper script. Copy these libs builds if appropriate.
+if [ -f $MIRROR_UTILS/.libs/mirror_server ] ; then
+    RUN_LIBS=$RUN_DIR/.libs
+    mkdir $RUN_LIBS
+    for FILE in $MIRROR_UTILS/.libs/mirror_* ; do
+        case "$FILE" in
+            *.o) continue ;; # Don't copy .o files
+        esac
+        cp $FILE $RUN_LIBS
+    done
+    cp .libs/mirror_vfd $RUN_LIBS
+fi
+
+cd $RUN_DIR
+
+echo "Launching Mirror Server"
+SERVER_ARGS="$SERVER_PORT $SERVER_VERBOSITY"
+./mirror_server $SERVER_ARGS &
+
+./mirror_vfd
+nerrors=$?
+
+echo "Stopping Mirror Server"
+./mirror_server_stop $SERVER_PORT
+
+###############################################################################
+## Report and exit
+###############################################################################
+cd ..
+if test $nerrors -eq 0 ; then
+    echo "Mirror VFD tests passed."
+    if test -z "$HDF5_NOCLEANUP" ; then
+        rm -rf $RUN_DIR
+    fi
+    exit 0
+else
+    echo "Mirror VFD tests FAILED."
+    exit 1
+fi
+
diff --git a/test/use.h b/test/use.h
index 54d9b26..e5eced2 100644
--- a/test/use.h
+++ b/test/use.h
@@ -16,48 +16,46 @@
 #include "h5test.h"
 
 /* Macro definitions */
-#define Hgoto_error(val)	{ret_value=val; goto done;}
-#define Hgoto_done		{goto done;}
-#define Chunksize_DFT		256	/* chunksize default */
-#define ErrorReportMax		10	/* max number of errors reported */
+#define Hgoto_error(val)        {ret_value=val; goto done;}
+#define Hgoto_done              {goto done;}
+#define Chunksize_DFT           256     /* chunksize default */
+#define ErrorReportMax          10      /* max number of errors reported */
 /* these two definitions must match each other */
-#define UC_DATATYPE		H5T_NATIVE_SHORT    /* use case HDF5 data type */
-#define UC_CTYPE		short		    /* use case C data type */
-#define UC_RANK			3		    /* use case dataset rank */
+#define UC_DATATYPE             H5T_NATIVE_SHORT  /* use case HDF5 data type */
+#define UC_CTYPE                short   /* use case C data type */
+#define UC_RANK                 3       /* use case dataset rank */
 
 /* Name of message file that is sent by the writer */
 #define WRITER_MESSAGE          "USE_WRITER_MESSAGE"
 
 /* type declarations */
 typedef enum part_t {
-    UC_READWRITE	=0,	/* both writer and reader */
-    UC_WRITER,			/* writer only */
-    UC_READER			/* reader only */
+    UC_READWRITE = 0,           /* both writer and reader */
+    UC_WRITER,                  /* writer only */
+    UC_READER                   /* reader only */
 } part_t;
 
 typedef struct options_t {
-    hsize_t chunksize;		/* chunks are chunksize^2 planes	*/
-    hsize_t chunkplanes;	/* number of planes per chunk, default 1 */
+    hsize_t chunksize;          /* chunks are chunksize^2 planes         */
+    hsize_t chunkplanes;        /* number of planes per chunk, default 1 */
     hsize_t chunkdims[UC_RANK]; /* chunk dims is (chunkplan, chunksize, chunksize) */
     hsize_t dims[UC_RANK];      /* dataset initial dims */
     hsize_t max_dims[UC_RANK];  /* dataset max dims */
-    hsize_t nplanes;		/* number of planes to write, default proportional to chunksize */
-    char *filename;		/* use case data filename		*/
-    part_t launch;		/* launch writer, reader or both	*/
-    hbool_t use_swmr;           /* use swmr open (1) or not 		*/
-    int iterations;		/* iterations, default 1		*/
+    hsize_t nplanes;            /* number of planes to write, default proportional to chunksize */
+    char *filename;             /* use case data filename               */
+    part_t launch;              /* launch writer, reader or both        */
+    hbool_t use_swmr;           /* use swmr open (1) or not             */
+    int iterations;             /* iterations, default 1                */
+    hid_t fapl_id;              /* instance-specific FAPL ID            */
+    char *progname;             /* Program name (used in usage and dset name) */
 } options_t;
 
-/* global variables declarations */
-extern options_t UC_opts;	/* Use Case Options */
-extern const char *progname_g;	/* Program name */
-
 /* prototype declarations */
-int parse_option(int argc, char * const argv[]);
-int setup_parameters(int argc, char * const argv[]);
-void show_parameters(void);
+int parse_option(int argc, char * const argv[], options_t * opts);
+int setup_parameters(int argc, char * const argv[], options_t * opts);
+void show_parameters(options_t * opts);
 void usage(const char *prog);
-int create_uc_file(void);
-int write_uc_file(hbool_t tosend, hid_t fid);
-int read_uc_file(hbool_t towait);
+int create_uc_file(options_t * opts);
+int write_uc_file(hbool_t tosend, hid_t file_id, options_t * opts);
+int read_uc_file(hbool_t towait, options_t * opts);
 
diff --git a/test/use_append_chunk.c b/test/use_append_chunk.c
index 6b34f1e..35594cf 100644
--- a/test/use_append_chunk.c
+++ b/test/use_append_chunk.c
@@ -11,7 +11,7 @@
  * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */
 
 /* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
- * Use Case 1.7	Appending a single chunk
+ * Use Case 1.7 Appending a single chunk
  * Description:
  *     Appending a single chunk of raw data to a dataset along an unlimited
  *     dimension within a pre-created file and reading the new data back.
@@ -24,35 +24,36 @@
  * Level:
  *     User Level
  * Guarantees:
- *     o	Readers will see the modified dimension sizes after the Writer
- * 	finishes HDF5 metadata updates and issues H5Fflush or H5Oflush calls.
- *     o	Readers will see newly appended data after the Writer finishes
- * 	the flush operation.
- * 
+ *     o Readers will see the modified dimension sizes after the Writer
+ *       finishes HDF5 metadata updates and issues H5Fflush or H5Oflush calls.
+ *     o Readers will see newly appended data after the Writer finishes
+ *       the flush operation.
+ *
  * Preconditions:
- *     o	Readers are not allowed to modify the file.  o	 All datasets
- * 	that are modified by the Writer exist when the Writer opens the file.
- *     o	All datasets that are modified by the Writer exist when a Reader
- * 	opens the file.  o	 Data is written by a hyperslab contained in
- * 	one chunk.
- * 
+ *     o Readers are not allowed to modify the file.
+ *     o All datasets that are modified by the Writer exist when the Writer
+ *       opens the file.
+ *     o All datasets that are modified by the Writer exist when a Reader
+ *       opens the file.
+ *     o Data is written by a hyperslab contained in one chunk.
+ *
  * Main Success Scenario:
- *     1.	An application creates a file with required objects (groups,
- * 	datasets, and attributes).
- *     2.	The Writer application opens the file and datasets in the file
- * 	and starts adding data along the unlimited dimension using a hyperslab
- * 	selection that corresponds to an HDF5 chunk.
- *     3.	A Reader opens the file and a dataset in a file, and queries
- * 	the sizes of the dataset; if the extent of the dataset has changed,
- * 	reads the appended data back.
- * 
+ *     1. An application creates a file with required objects (groups,
+ *        datasets, and attributes).
+ *     2. The Writer application opens the file and datasets in the file
+ *        and starts adding data along the unlimited dimension using a hyperslab
+ *        selection that corresponds to an HDF5 chunk.
+ *     3. A Reader opens the file and a dataset in a file, and queries
+ *        the sizes of the dataset; if the extent of the dataset has changed,
+ *        reads the appended data back.
+ *
  * Discussion points:
- *     1.	Since the new data is written to the file, and metadata update
- * 	operation of adding pointer to the newly written chunk is atomic and
- * 	happens after the chunk is on the disk, only two things may happen
- * 	to the Reader:
- * 	    o	The Reader will not see new data.
- * 	    o	The Reader will see all new data written by Writer.
+ *     1. Since the new data is written to the file, and metadata update
+ *        operation of adding pointer to the newly written chunk is atomic and
+ *        happens after the chunk is on the disk, only two things may happen
+ *        to the Reader:
+ *         o The Reader will not see new data.
+ *         o The Reader will see all new data written by Writer.
  * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */
 
 /* Created: Albert Cheng, 2013/5/28 */
@@ -68,46 +69,43 @@
 
 #include "use.h"
 
-/* Global Variable definitions */
-options_t UC_opts;	/* Use Case Options */
-const char *progname_g="use_append_chunk";	/* program name */
+#define USE_APPEND_CHUNK_PROGNAME "use_append_chunk"
+
+static options_t UC_opts; /* Use Case Options */
 
 /* Setup parameters for the use case.
  * Return: 0 succeed; -1 fail.
  */
-int setup_parameters(int argc, char * const argv[])
+int
+setup_parameters(int argc, char * const argv[], options_t * opts)
 {
     /* use case defaults */
-    HDmemset(&UC_opts, 0, sizeof(options_t));
-    UC_opts.chunksize = Chunksize_DFT;
-    UC_opts.use_swmr = TRUE;	/* use swmr open */
-    UC_opts.iterations = 1;
-    UC_opts.chunkplanes = 1;
-
-    /* parse options */
-    if (parse_option(argc, argv) < 0)
-	return(-1);
-
-    /* set chunk dims */
-    UC_opts.chunkdims[0] = UC_opts.chunkplanes;
-    UC_opts.chunkdims[1] = UC_opts.chunkdims[2] = UC_opts.chunksize;
-
-    /* set dataset initial and max dims */
-    UC_opts.dims[0] = 0;
-    UC_opts.max_dims[0] = H5S_UNLIMITED;
-    UC_opts.dims[1] = UC_opts.dims[2] = UC_opts.max_dims[1] = UC_opts.max_dims[2] = UC_opts.chunksize;
-
-    /* set nplanes */
-    if (UC_opts.nplanes == 0)
-        UC_opts.nplanes = (hsize_t)UC_opts.chunksize;
-
-    /* show parameters and return */
-    show_parameters();
+    HDmemset(opts, 0, sizeof(options_t));
+    opts->chunksize = Chunksize_DFT;
+    opts->use_swmr = TRUE; /* use swmr open */
+    opts->iterations = 1;
+    opts->chunkplanes = 1;
+    opts->progname = USE_APPEND_CHUNK_PROGNAME;
+
+    if (parse_option(argc, argv, opts) < 0)
+        return(-1);
+
+    opts->chunkdims[0] = opts->chunkplanes;
+    opts->chunkdims[1] = opts->chunkdims[2] = opts->chunksize;
+
+    opts->dims[0] = 0;
+    opts->max_dims[0] = H5S_UNLIMITED;
+    opts->dims[1] = opts->dims[2] = opts->max_dims[1] = opts->max_dims[2] = opts->chunksize;
+
+    if (opts->nplanes == 0)
+        opts->nplanes = (hsize_t)opts->chunksize;
+
+    show_parameters(opts);
     return(0);
-}
+} /* setup_parameters() */
 
 
-/* Overall Algorithm: 
+/* Overall Algorithm:
  * Parse options from user;
  * Generate/pre-created test files needed and close it;
  * fork: child process becomes the reader process;
@@ -119,22 +117,20 @@ main(int argc, char *argv[])
 {
     pid_t childpid=0;
     pid_t mypid, tmppid;
-    int	child_status;
+    int child_status;
     int child_wait_option=0;
     int ret_value = 0;
     int child_ret_value;
     hbool_t send_wait = FALSE;
     hid_t fapl = -1;    /* File access property list */
     hid_t fid = -1;     /* File ID */
-    char *name;         /* Test file name */
 
-    /* initialization */
-    if (setup_parameters(argc, argv) < 0){
+    if (setup_parameters(argc, argv, &UC_opts) < 0) {
         Hgoto_error(1);
     }
 
     /* Determine the need to send/wait message file*/
-    if(UC_opts.launch == UC_READWRITE) {
+    if (UC_opts.launch == UC_READWRITE) {
         HDunlink(WRITER_MESSAGE);
         send_wait = TRUE;
     }
@@ -144,38 +140,63 @@ main(int argc, char *argv[])
     /* UC_WRITER:    create datafile, skip reader, launch writer.    */
     /* UC_READER:    skip create, launch reader, exit.               */
     /* ==============================================================*/
-    /* ============*/
+    /* =========== */
     /* Create file */
-    /* ============*/
-    if (UC_opts.launch != UC_READER){
+    /* =========== */
+    if (UC_opts.launch != UC_READER) {
         HDprintf("Creating skeleton data file for test...\n");
-        if (create_uc_file() < 0){
+        if ((UC_opts.fapl_id = h5_fileaccess()) < 0) {
+            HDfprintf(stderr, "can't create creation FAPL\n");
+            Hgoto_error(1);
+        }   
+        if (H5Pset_libver_bounds(UC_opts.fapl_id, H5F_LIBVER_LATEST, H5F_LIBVER_LATEST) < 0) {
+            HDfprintf(stderr, "can't set creation FAPL libver bounds\n");
+            Hgoto_error(1);
+        }
+        if (create_uc_file(&UC_opts) < 0) {
             HDfprintf(stderr, "***encounter error\n");
             Hgoto_error(1);
-        }else
+        } else {
             HDprintf("File created.\n");
+        }
+        /* Close FAPL to prevent issues with forking later */ 
+        if (H5Pclose(UC_opts.fapl_id) < 0) {
+            HDfprintf(stderr, "can't close creation FAPL\n");
+            Hgoto_error(1);
+        }
+        UC_opts.fapl_id = H5I_INVALID_HID;
     }
 
-    if (UC_opts.launch==UC_READWRITE){
-        /* fork process */
-        if((childpid = HDfork()) < 0) {
+    /* ============ */
+    /* Fork process */
+    /* ============ */
+    if (UC_opts.launch == UC_READWRITE) {
+        if ((childpid = HDfork()) < 0) {
             HDperror("fork");
             Hgoto_error(1);
-        };
-    };
+        }
+    }
     mypid = HDgetpid();
 
     /* ============= */
     /* launch reader */
     /* ============= */
-    if (UC_opts.launch != UC_WRITER){
+    if (UC_opts.launch != UC_WRITER) {
         /* child process launch the reader */
-        if(0 == childpid) {
+        if (0 == childpid) {
             HDprintf("%d: launch reader process\n", mypid);
-            if (read_uc_file(send_wait) < 0){
+            if ((UC_opts.fapl_id = h5_fileaccess()) < 0) {
+                HDfprintf(stderr, "can't create read FAPL\n");
+                HDexit(EXIT_FAILURE);
+            }
+            if (read_uc_file(send_wait, &UC_opts) < 0) {
                 HDfprintf(stderr, "read_uc_file encountered error\n");
                 HDexit(EXIT_FAILURE);
             }
+            if (H5Pclose(UC_opts.fapl_id) < 0) {
+                HDfprintf(stderr, "can't close read FAPL\n");
+                HDexit(EXIT_FAILURE);
+            }
             HDexit(EXIT_SUCCESS);
         }
     }
@@ -186,65 +207,63 @@ main(int argc, char *argv[])
     /* this process continues to launch the writer */
     HDprintf("%d: continue as the writer process\n", mypid);
 
-    name = UC_opts.filename;
-
-    /* Set file access proeprty list */
-    if((fapl = h5_fileaccess()) < 0)
+    if ((fapl = h5_fileaccess()) < 0) {
+        HDfprintf(stderr, "can't create write FAPL\n");
         Hgoto_error(1);
+    }
 
-    if(UC_opts.use_swmr)
-        if(H5Pset_libver_bounds(fapl, H5F_LIBVER_LATEST, H5F_LIBVER_LATEST) < 0)
+    if (UC_opts.use_swmr) {
+        if (H5Pset_libver_bounds(fapl, H5F_LIBVER_LATEST, H5F_LIBVER_LATEST) < 0) {
+            HDfprintf(stderr, "can't set write FAPL libver bounds\n");
             Hgoto_error(1);
+        }
+    }
 
-    /* Open the file */
-    if((fid = H5Fopen(name, H5F_ACC_RDWR | (UC_opts.use_swmr ? H5F_ACC_SWMR_WRITE : 0), fapl)) < 0) {
+    if ((fid = H5Fopen(UC_opts.filename, H5F_ACC_RDWR | (UC_opts.use_swmr ? H5F_ACC_SWMR_WRITE : 0), fapl)) < 0) {
         HDfprintf(stderr, "H5Fopen failed\n");
         Hgoto_error(1);
     }
 
-    if(write_uc_file(send_wait, fid) < 0) {
+    if (write_uc_file(send_wait, fid, &UC_opts) < 0) {
         HDfprintf(stderr, "write_uc_file encountered error\n");
         Hgoto_error(1);
     }
 
+    if (H5Fclose(fid) < 0) {
+        HDfprintf(stderr, "Failed to close write\n");
+        Hgoto_error(1);
+    }
+
+    if (H5Pclose(fapl) < 0) {
+        HDfprintf(stderr, "can't close write FAPL\n");
+        Hgoto_error(1);
+    }
+
     /* ================================================ */
     /* If readwrite, collect exit code of child process */
     /* ================================================ */
-    if (UC_opts.launch == UC_READWRITE){
-        if ((tmppid = HDwaitpid(childpid, &child_status, child_wait_option)) < 0){
+    if (UC_opts.launch == UC_READWRITE) {
+        if ((tmppid = HDwaitpid(childpid, &child_status, child_wait_option)) < 0) {
             HDperror("waitpid");
             Hgoto_error(1);
         }
 
-        /* Close the file */
-        if(H5Fclose(fid) < 0) {
-            HDfprintf(stderr, "Failed to close file id\n");
-            Hgoto_error(1);
-        }
-
-        /* Close the property list */
-        if(H5Pclose(fapl) < 0) {
-            HDfprintf(stderr, "Failed to close the property list\n");
-            Hgoto_error(1);
-        }
-
-        if (WIFEXITED(child_status)){
-            if ((child_ret_value=WEXITSTATUS(child_status)) != 0){
+        if (WIFEXITED(child_status)) {
+            if ((child_ret_value = WEXITSTATUS(child_status)) != 0) {
                 HDprintf("%d: child process exited with non-zero code (%d)\n",
                         mypid, child_ret_value);
                 Hgoto_error(2);
             }
-         } else {
-                HDprintf("%d: child process terminated abnormally\n", mypid);
-                Hgoto_error(2);
-         }
+        } else {
+            HDprintf("%d: child process terminated abnormally\n", mypid);
+            Hgoto_error(2);
+        }
     }
-    
+
 done:
-    /* Print result and exit */
-    if (ret_value != 0){
+    if (ret_value != 0) {
         HDprintf("Error(s) encountered\n");
-    }else{
+    } else {
         HDprintf("All passed\n");
     }
 
diff --git a/test/use_append_chunk_mirror.c b/test/use_append_chunk_mirror.c
new file mode 100644
index 0000000..6ee01c0
--- /dev/null
+++ b/test/use_append_chunk_mirror.c
@@ -0,0 +1,403 @@
+/* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
+ * Copyright by The HDF Group.                                               *
+ * All rights reserved.                                                      *
+ *                                                                           *
+ * This file is part of HDF5.  The full HDF5 copyright notice, including     *
+ * terms governing use, modification, and redistribution, is contained in    *
+ * the COPYING file, which can be found at the root of the source code       *
+ * distribution tree, or in https://support.hdfgroup.org/ftp/HDF5/releases.  *
+ * If you do not have access to either file, you may request a copy from     *
+ * help@hdfgroup.org.                                                        *
+ * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */
+
+/* HACKED VERSION
+ * Demonstrate SWMR with a mirrored file.
+ *
+ * Must be built with SERVER_IP as the IP address of the target system
+ * with a running mirror server, and SERVER_PORT as the primary server port.
+ *
+ * In addition to the local file, 'shinano.h5' will be created on the remote
+ * system, mirroring the local file. The file location will be local to
+ * Server's/Writer's invocation directory.
+ *
+ * Template for demonstration purposes:
+ *
+ * # Launch mirror server on remote machine (in foreground to easily stop)
+ * REMOTE(1)$ ./mirror_server /path/to/mirror_worker
+ *
+ * # Launch chunk writer with plenty of chunks.
+ * LOCAL(1)$ ./use_append_chunk_mirror -l w -n 10000
+ *
+ * # Wait one second for files to be created.
+ *
+ * # Launch chunk readers on both files.
+ * LOCAL(2)$ ./use_append_chunk_mirror -l r -n 10000
+ * REMOTE(2)$ ./use_append_chunk_mirror -l r -n 10000 -f shinano.h5
+ *
+ * # Hard-stop the server.
+ * REMOTE(1)$ ^C
+ * # alt, softer shutdown using echo and nc
+ * echo "SHUTDOWN" | nc localhost 3000
+ */
+
+/* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
+ * Use Case 1.7 Appending a single chunk
+ * Description:
+ *     Appending a single chunk of raw data to a dataset along an unlimited
+ *     dimension within a pre-created file and reading the new data back.
+ * Goal:
+ *     Read data appended by the Writer to a pre-existing dataset in a
+ *     file. The dataset has one or more unlimited dimensions. The data is
+ *     appended by a hyperslab that is contained in one chunk (for example,
+ *     appending 2-dim planes along the slowest changing dimension in the
+ *     3-dim dataset).
+ * Level:
+ *     User Level
+ * Guarantees:
+ *     o Readers will see the modified dimension sizes after the Writer
+ *       finishes HDF5 metadata updates and issues H5Fflush or H5Oflush calls.
+ *     o Readers will see newly appended data after the Writer finishes
+ *       the flush operation.
+ *
+ * Preconditions:
+ *     o Readers are not allowed to modify the file.
+ *     o All datasets that are modified by the Writer exist when the Writer
+ *       opens the file.
+ *     o All datasets that are modified by the Writer exist when a Reader
+ *       opens the file.
+ *     o Data is written by a hyperslab contained in one chunk.
+ *
+ * Main Success Scenario:
+ *     1. An application creates a file with required objects (groups,
+ *        datasets, and attributes).
+ *     2. The Writer application opens the file and datasets in the file
+ *        and starts adding data along the unlimited dimension using a hyperslab
+ *        selection that corresponds to an HDF5 chunk.
+ *     3. A Reader opens the file and a dataset in a file, and queries
+ *        the sizes of the dataset; if the extent of the dataset has changed,
+ *        reads the appended data back.
+ *
+ * Discussion points:
+ *     1. Since the new data is written to the file, and metadata update
+ *        operation of adding pointer to the newly written chunk is atomic and
+ *        happens after the chunk is on the disk, only two things may happen
+ *        to the Reader:
+ *         o The Reader will not see new data.
+ *         o The Reader will see all new data written by Writer.
+ * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */
+
+/* Created: Jacob Smith, 2019 */
+
+#include "use.h"
+
+/* This test uses many POSIX things that are not available on
+ * Windows. We're using a check for fork(2) here as a proxy for
+ * all POSIX/Unix/Linux things until this test can be made
+ * more platform-independent.
+ */
+#ifdef H5_HAVE_FORK
+
+#ifdef H5_HAVE_MIRROR_VFD
+
+#define THIS_PROGNAME "use_append_chunk_mirror"
+
+#define CONNECT_WITH_JELLY 0
+
+#if CONNECT_WITH_JELLY
+#define SERVER_IP "10.10.10.248" /* hard-coded IP address */
+#else
+#define SERVER_IP "127.0.0.1"    /* localhost */
+#endif /* CONNECT_WITH_JELLY */
+#define SERVER_PORT 3000         /* hard-coded port number */
+#define MIRROR_FILE_NAME "shinano.h5" /* hard-coded duplicate/mirror filename */
+
+static options_t UC_opts; /* Use Case Options */
+
+/* Setup parameters for the use case.
+ * Return: 0 succeed; -1 fail.
+ */
+int
+setup_parameters(int argc, char * const argv[], options_t * opts)
+{
+    /* use case defaults */
+    HDmemset(opts, 0, sizeof(options_t));
+    opts->chunksize = Chunksize_DFT;
+    opts->use_swmr = TRUE;
+    opts->iterations = 1;
+    opts->chunkplanes = 1;
+    opts->progname = THIS_PROGNAME;
+
+    if (parse_option(argc, argv, opts) < 0)
+        return(-1);
+
+    opts->chunkdims[0] = opts->chunkplanes;
+    opts->chunkdims[1] = opts->chunkdims[2] = opts->chunksize;
+
+    opts->dims[0] = 0;
+    opts->max_dims[0] = H5S_UNLIMITED;
+    opts->dims[1] = opts->dims[2] = opts->max_dims[1] = opts->max_dims[2] = opts->chunksize;
+
+    if (opts->nplanes == 0)
+        opts->nplanes = (hsize_t)opts->chunksize;
+
+    show_parameters(opts);
+    return(0);
+} /* setup_parameters() */
+
+
+/* Overall Algorithm:
+ * Parse options from user;
+ * Generate/pre-created test files needed and close it;
+ * fork: child process becomes the reader process;
+ *       while parent process continues as the writer process;
+ * both run till ending conditions are met.
+ */
+int
+main(int argc, char *argv[])
+{
+    pid_t childpid=0;
+    pid_t mypid, tmppid;
+    int child_status;
+    int child_wait_option=0;
+    int ret_value = 0;
+    int child_ret_value;
+    hbool_t send_wait = FALSE;
+    hid_t fid = -1;     /* File ID */
+    H5FD_mirror_fapl_t mirr_fa;
+    H5FD_splitter_vfd_config_t split_fa;
+    hid_t mirr_fapl_id = H5I_INVALID_HID;
+
+    if (setup_parameters(argc, argv, &UC_opts) < 0) {
+        Hgoto_error(1);
+    }
+
+    mirr_fa.magic          = H5FD_MIRROR_FAPL_MAGIC;
+    mirr_fa.version        = H5FD_MIRROR_CURR_FAPL_T_VERSION;
+    mirr_fa.handshake_port = SERVER_PORT;
+    HDstrncpy(mirr_fa.remote_ip, SERVER_IP, H5FD_MIRROR_MAX_IP_LEN);
+
+
+    split_fa.wo_fapl_id       = H5I_INVALID_HID;
+    split_fa.rw_fapl_id       = H5I_INVALID_HID;
+    split_fa.magic            = H5FD_SPLITTER_MAGIC;
+    split_fa.version          = H5FD_CURR_SPLITTER_VFD_CONFIG_VERSION;
+    split_fa.log_file_path[0] = '\0'; /* none */
+    split_fa.ignore_wo_errs   = FALSE;
+    HDstrncpy(split_fa.wo_path, MIRROR_FILE_NAME, H5FD_SPLITTER_PATH_MAX);
+
+    /* Determine the need to send/wait message file*/
+    if (UC_opts.launch == UC_READWRITE) {
+        HDunlink(WRITER_MESSAGE);
+        send_wait = TRUE;
+    }
+
+    /* ==============================================================*/
+    /* UC_READWRITE: create datafile, launch both reader and writer. */
+    /* UC_WRITER:    create datafile, skip reader, launch writer.    */
+    /* UC_READER:    skip create, launch reader, exit.               */
+    /* ==============================================================*/
+    /* =========== */
+    /* Create file */
+    /* =========== */
+    if (UC_opts.launch != UC_READER) {
+        HDprintf("Creating skeleton data file for test...\n");
+
+        /* Prepare mirror child driver */
+        mirr_fapl_id = H5Pcreate(H5P_FILE_ACCESS);
+        if (mirr_fapl_id == H5I_INVALID_HID) {
+            HDfprintf(stderr, "can't create creation mirror FAPL\n");
+            Hgoto_error(1);
+        }
+        if (H5Pset_fapl_mirror(mirr_fapl_id, &mirr_fa) < 0) {
+            HDfprintf(stderr, "can't set creation mirror FAPL\n");
+            H5Eprint2(H5E_DEFAULT, stdout);
+            Hgoto_error(1);
+        }
+
+        /* Prepare parent "splitter" driver in UC_opts */
+        split_fa.wo_fapl_id       = mirr_fapl_id;
+        split_fa.rw_fapl_id       = H5P_DEFAULT;
+        UC_opts.fapl_id = H5Pcreate(H5P_FILE_ACCESS);
+        if (UC_opts.fapl_id == H5I_INVALID_HID) {
+            HDfprintf(stderr, "can't create creation FAPL\n");
+            Hgoto_error(1);
+        }
+        if (H5Pset_fapl_splitter(UC_opts.fapl_id, &split_fa) < 0) {
+            HDfprintf(stderr, "can't set creation FAPL\n");
+            H5Eprint2(H5E_DEFAULT, stdout);
+            Hgoto_error(1);
+        }
+
+        if (H5Pset_libver_bounds(UC_opts.fapl_id, H5F_LIBVER_LATEST, H5F_LIBVER_LATEST) < 0) {
+            HDfprintf(stderr, "can't set creation FAPL libver bounds\n");
+            Hgoto_error(1);
+        }
+
+        /* Create file */
+        if (create_uc_file(&UC_opts) < 0) {
+            HDfprintf(stderr, "***encounter error\n");
+            Hgoto_error(1);
+        } else {
+            HDprintf("File created.\n");
+        }
+
+        /* Close FAPLs to prevent issues with forking later */
+        if (H5Pclose(UC_opts.fapl_id) < 0) {
+            HDfprintf(stderr, "can't close creation FAPL\n");
+            Hgoto_error(1);
+        }
+        UC_opts.fapl_id = H5I_INVALID_HID;
+        if (H5Pclose(mirr_fapl_id) < 0) {
+            HDfprintf(stderr, "can't close creation mirror FAPL\n");
+            Hgoto_error(1);
+        }
+        mirr_fapl_id = H5I_INVALID_HID;
+    }
+
+    /* ============ */
+    /* Fork process */
+    /* ============ */
+    if (UC_opts.launch == UC_READWRITE) {
+        if ((childpid = HDfork()) < 0) {
+            HDperror("fork");
+            Hgoto_error(1);
+        }
+    }
+    mypid = HDgetpid();
+
+    /* ============= */
+    /* launch reader */
+    /* ============= */
+    if (UC_opts.launch != UC_WRITER) {
+        /* child process -- launch the reader */
+        /* reader only opens the one file -- separate reader needed for mirrored file 'shinano.h5' */
+        if (0 == childpid) {
+            HDprintf("%d: launch reader process\n", mypid);
+
+            UC_opts.fapl_id = H5P_DEFAULT;
+            if (read_uc_file(send_wait, &UC_opts) < 0) {
+                HDfprintf(stderr, "read_uc_file encountered error (%d)\n", mypid);
+                HDexit(1);
+            }
+
+            HDexit(0);
+        }
+    }
+
+    /* ============= */
+    /* launch writer */
+    /* ============= */
+    /* this process continues to launch the writer */
+    HDprintf("%d: continue as the writer process\n", mypid);
+
+    /* Prepare mirror child driver */
+    mirr_fapl_id = H5Pcreate(H5P_FILE_ACCESS);
+    if (mirr_fapl_id == H5I_INVALID_HID) {
+        HDfprintf(stderr, "can't create creation mirror FAPL\n");
+        Hgoto_error(1);
+    }
+    if (H5Pset_fapl_mirror(mirr_fapl_id, &mirr_fa) < 0) {
+        HDfprintf(stderr, "can't set creation mirror FAPL\n");
+        H5Eprint2(H5E_DEFAULT, stdout);
+        Hgoto_error(1);
+    }
+
+    /* Prepare parent "splitter" driver in UC_opts */
+    split_fa.wo_fapl_id       = mirr_fapl_id;
+    split_fa.rw_fapl_id       = H5P_DEFAULT;
+    UC_opts.fapl_id = H5Pcreate(H5P_FILE_ACCESS);
+    if (UC_opts.fapl_id == H5I_INVALID_HID) {
+        HDfprintf(stderr, "can't create creation FAPL\n");
+        Hgoto_error(1);
+    }
+    if (H5Pset_fapl_splitter(UC_opts.fapl_id, &split_fa) < 0) {
+        HDfprintf(stderr, "can't set creation FAPL\n");
+        H5Eprint2(H5E_DEFAULT, stdout);
+        Hgoto_error(1);
+    }
+
+    if (UC_opts.use_swmr) {
+        if (H5Pset_libver_bounds(UC_opts.fapl_id, H5F_LIBVER_LATEST, H5F_LIBVER_LATEST) < 0) {
+            HDfprintf(stderr, "can't set write FAPL libver bounds\n");
+            Hgoto_error(1);
+        }
+    }
+
+    if ((fid = H5Fopen(UC_opts.filename, H5F_ACC_RDWR | (UC_opts.use_swmr ? H5F_ACC_SWMR_WRITE : 0), UC_opts.fapl_id)) < 0) {
+        HDfprintf(stderr, "H5Fopen failed\n");
+        Hgoto_error(1);
+    }
+
+    if (write_uc_file(send_wait, fid, &UC_opts) < 0) {
+        HDfprintf(stderr, "write_uc_file encountered error\n");
+        Hgoto_error(1);
+    }
+
+    if (H5Fclose(fid) < 0) {
+        HDfprintf(stderr, "Failed to close write\n");
+        Hgoto_error(1);
+    }
+
+    if (H5Pclose(UC_opts.fapl_id) < 0) {
+        HDfprintf(stderr, "can't close write FAPL\n");
+        Hgoto_error(1);
+    }
+
+    if (H5Pclose(mirr_fapl_id) < 0) {
+        HDfprintf(stderr, "can't close write mirror FAPL\n");
+        Hgoto_error(1);
+    }
+
+    /* ================================================ */
+    /* If readwrite, collect exit code of child process */
+    /* ================================================ */
+    if (UC_opts.launch == UC_READWRITE) {
+        if ((tmppid = HDwaitpid(childpid, &child_status, child_wait_option)) < 0) {
+            HDperror("waitpid");
+            Hgoto_error(1);
+        }
+
+        if (WIFEXITED(child_status)) {
+            if ((child_ret_value = WEXITSTATUS(child_status)) != 0) {
+                HDprintf("%d: child process exited with non-zero code (%d)\n",
+                        mypid, child_ret_value);
+                Hgoto_error(2);
+            }
+        } else {
+            HDprintf("%d: child process terminated abnormally\n", mypid);
+            Hgoto_error(2);
+        }
+    }
+
+done:
+    if (ret_value != 0) {
+        HDprintf("Error(s) encountered\n");
+    } else {
+        HDprintf("All passed\n");
+    }
+
+    return(ret_value);
+}
+
+#else /* H5_HAVE_MIRROR_VFD */
+
+int
+main(void)
+{
+    HDfprintf(stderr, "Mirror VFD is not built. Skipping.\n");
+    return EXIT_SUCCESS;
+} /* end main() */
+
+#endif /* H5_HAVE_MIRROR_VFD */
+
+#else /* H5_HAVE_FORK */
+
+int
+main(void)
+{
+    HDfprintf(stderr, "Non-POSIX platform. Skipping.\n");
+    return EXIT_SUCCESS;
+} /* end main() */
+
+#endif /* H5_HAVE_FORK */
+
diff --git a/test/use_append_mchunks.c b/test/use_append_mchunks.c
index b7d45a4..2eb5a1d 100644
--- a/test/use_append_mchunks.c
+++ b/test/use_append_mchunks.c
@@ -29,14 +29,14 @@
  *     finishes HDF5 metadata updates and issues H5Fflush or H5Oflush calls.
  *   o Readers will see newly appended data after the Writer finishes
  *     the flush operation.
- * 
+ *
  * Preconditions:
  *   o Readers are not allowed to modify the file.
  *   o All datasets that are modified by the Writer exist when the
  *     Writer opens the file.
  *   o All datasets that are modified by the Writer exist when a Reader
  *     opens the file.
- * 
+ *
  * Main Success Scenario:
  *  1. An application creates a file with required objects (groups,
  *     datasets, and attributes).
@@ -45,7 +45,7 @@
  *     spans several chunks.
  *  3. A Reader opens the file and a dataset in a file; if the size of
  *     the unlimited dimension has changed, reads the appended data back.
- * 
+ *
  * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */
 
 /* Created: Albert Cheng, 2013/6/1 */
@@ -61,46 +61,45 @@
 
 #include "use.h"
 
-/* Global Variable definitions */
-options_t UC_opts;	/* Use Case Options */
-const char *progname_g="use_append_mchunks";	/* program name */
+#define USE_APPEND_MCHUNKS_PROGNAME "use_append_mchunks"
+
+static options_t UC_opts; /* Use Case Options */
 
 /* Setup parameters for the use case.
  * Return: 0 succeed; -1 fail.
  */
-int setup_parameters(int argc, char * const argv[])
+int
+setup_parameters(int argc, char * const argv[], options_t * opts)
 {
     /* use case defaults */
-    HDmemset(&UC_opts, 0, sizeof(options_t));
-    UC_opts.chunksize = Chunksize_DFT;
-    UC_opts.use_swmr = 1;	/* use swmr open */
-    UC_opts.iterations = 1;
-    UC_opts.chunkplanes = 1;
-
-    /* parse options */
-    if (parse_option(argc, argv) < 0){
-	return(-1);
+    HDmemset(opts, 0, sizeof(options_t));
+    opts->chunksize = Chunksize_DFT;
+    opts->use_swmr = 1; /* use swmr open */
+    opts->iterations = 1;
+    opts->chunkplanes = 1;
+    opts->progname = USE_APPEND_MCHUNKS_PROGNAME;
+    opts->fapl_id = H5I_INVALID_HID;
+
+    if (parse_option(argc, argv, opts) < 0) {
+        return(-1);
     }
-    /* set chunk dims */
-    UC_opts.chunkdims[0] = (hsize_t)UC_opts.chunkplanes;
-    UC_opts.chunkdims[1] = UC_opts.chunkdims[2] = (hsize_t)UC_opts.chunksize;
 
-    /* set dataset initial and max dims */
-    UC_opts.dims[0] = 0;
-    UC_opts.max_dims[0] = H5S_UNLIMITED;
-    UC_opts.dims[1] = UC_opts.dims[2] = UC_opts.max_dims[1] = UC_opts.max_dims[2] = 2 * (hsize_t)UC_opts.chunksize;
+    opts->chunkdims[0] = (hsize_t)opts->chunkplanes;
+    opts->chunkdims[1] = opts->chunkdims[2] = (hsize_t)opts->chunksize;
+
+    opts->dims[0] = 0;
+    opts->max_dims[0] = H5S_UNLIMITED;
+    opts->dims[1] = opts->dims[2] = opts->max_dims[1] = opts->max_dims[2] = 2 * (hsize_t)opts->chunksize;
 
-    /* set nplanes */
-    if (UC_opts.nplanes == 0)
-        UC_opts.nplanes = 2 * (hsize_t)UC_opts.chunksize;
+    if (opts->nplanes == 0)
+        opts->nplanes = 2 * (hsize_t)opts->chunksize;
 
-    /* show parameters and return */
-    show_parameters();
+    show_parameters(opts);
     return(0);
-}
+} /* end setup_parameters() */
 
 
-/* Overall Algorithm: 
+/* Overall Algorithm:
  * Parse options from user;
  * Generate/pre-created test files needed and close it;
  * fork: child process becomes the reader process;
@@ -112,22 +111,20 @@ main(int argc, char *argv[])
 {
     pid_t childpid=0;
     pid_t mypid, tmppid;
-    int	child_status;
+    int child_status;
     int child_wait_option=0;
     int ret_value = 0;
     int child_ret_value;
     hbool_t send_wait = 0;
     hid_t fapl = -1;        /* File access property list */
     hid_t fid = -1;         /* File ID */
-    char *name;             /* Test file name */
 
-    /* initialization */
-    if (setup_parameters(argc, argv) < 0){
+    if (setup_parameters(argc, argv, &UC_opts) < 0) {
         Hgoto_error(1);
     }
 
     /* Determine the need to send/wait message file*/
-    if(UC_opts.launch == UC_READWRITE) {
+    if (UC_opts.launch == UC_READWRITE) {
         HDunlink(WRITER_MESSAGE);
         send_wait = 1;
     }
@@ -137,38 +134,63 @@ main(int argc, char *argv[])
     /* UC_WRITER:    create datafile, skip reader, launch writer.    */
     /* UC_READER:    skip create, launch reader, exit.               */
     /* ==============================================================*/
-    /* ============*/
+    /* =========== */
     /* Create file */
-    /* ============*/
-    if (UC_opts.launch != UC_READER){
+    /* =========== */
+    if (UC_opts.launch != UC_READER) {
         HDprintf("Creating skeleton data file for test...\n");
-	if (create_uc_file() < 0){
-	    HDfprintf(stderr, "***encounter error\n");
-	    Hgoto_error(1);
-	}else
-	    HDprintf("File created.\n");
+        if ((UC_opts.fapl_id = h5_fileaccess()) < 0) {
+            HDfprintf(stderr, "can't create creation FAPL\n");
+            Hgoto_error(1);
+        }
+        if (H5Pset_libver_bounds(UC_opts.fapl_id, H5F_LIBVER_LATEST, H5F_LIBVER_LATEST) < 0) {
+            HDfprintf(stderr, "can't set creation FAPL libver bounds\n");
+            Hgoto_error(1);
+        }
+        if (create_uc_file(&UC_opts) < 0) {
+            HDfprintf(stderr, "***encounter error\n");
+            Hgoto_error(1);
+        } else {
+            HDprintf("File created.\n");
+        }
+        /* Close FAPL to prevent issues with forking later */
+        if (H5Pclose(UC_opts.fapl_id) < 0) {
+            HDfprintf(stderr, "can't close creation FAPL\n");
+            Hgoto_error(1);
+        }
+        UC_opts.fapl_id = H5I_INVALID_HID;
     }
 
-    if (UC_opts.launch==UC_READWRITE){
-        /* fork process */
-        if((childpid = fork()) < 0) {
+    /* ============ */
+    /* Fork process */
+    /* ============ */
+    if (UC_opts.launch==UC_READWRITE) {
+        if ((childpid = fork()) < 0) {
             perror("fork");
             Hgoto_error(1);
-        };
-    };
+        }
+    }
     mypid = getpid();
 
     /* ============= */
     /* launch reader */
     /* ============= */
-    if (UC_opts.launch != UC_WRITER){
+    if (UC_opts.launch != UC_WRITER) {
         /* child process launch the reader */
-        if(0 == childpid) {
+        if (0 == childpid) {
             HDprintf("%d: launch reader process\n", mypid);
-            if (read_uc_file(send_wait) < 0){
+            if ((UC_opts.fapl_id = h5_fileaccess()) < 0) {
+                HDfprintf(stderr, "can't create read FAPL\n");
+                HDexit(EXIT_FAILURE);
+            }
+            if (read_uc_file(send_wait, &UC_opts) < 0) {
                 HDfprintf(stderr, "read_uc_file encountered error\n");
                 HDexit(EXIT_FAILURE);
             }
+            if (H5Pclose(UC_opts.fapl_id) < 0) {
+                HDfprintf(stderr, "can't close read FAPL\n");
+                HDexit(EXIT_FAILURE);
+            }
             HDexit(EXIT_SUCCESS);
         }
     }
@@ -179,51 +201,50 @@ main(int argc, char *argv[])
     /* this process continues to launch the writer */
     HDprintf("%d: continue as the writer process\n", mypid);
 
-    name = UC_opts.filename;
-
     /* Set the file access property list */
-    if((fapl = h5_fileaccess()) < 0)
+    if ((fapl = h5_fileaccess()) < 0) {
+        HDfprintf(stderr, "can't get write FAPL\n");
         Hgoto_error(1);
+    }
 
-    if(UC_opts.use_swmr)
-        if(H5Pset_libver_bounds(fapl, H5F_LIBVER_LATEST, H5F_LIBVER_LATEST) < 0)
+    if (UC_opts.use_swmr) {
+        if (H5Pset_libver_bounds(fapl, H5F_LIBVER_LATEST, H5F_LIBVER_LATEST) < 0) {
+            HDfprintf(stderr, "can't set write FAPL libver bounds\n");
             Hgoto_error(1);
+        }
+    }
 
-    /* Open the file */
-    if((fid = H5Fopen(name, H5F_ACC_RDWR | (UC_opts.use_swmr ? H5F_ACC_SWMR_WRITE : 0), fapl)) < 0) {
+    if ((fid = H5Fopen(UC_opts.filename, H5F_ACC_RDWR | (UC_opts.use_swmr ? H5F_ACC_SWMR_WRITE : 0), fapl)) < 0) {
         HDfprintf(stderr, "H5Fopen failed\n");
         Hgoto_error(1);
     }
 
-    if(write_uc_file(send_wait, fid) < 0) {
+    if (write_uc_file(send_wait, fid, &UC_opts) < 0) {
         HDfprintf(stderr, "write_uc_file encountered error\n");
         Hgoto_error(1);
     }
 
+    if (H5Fclose(fid) < 0) {
+        HDfprintf(stderr, "Failed to close file id\n");
+        Hgoto_error(1);
+    }
+
+    if (H5Pclose(fapl) < 0) {
+        HDfprintf(stderr, "can't close write FAPL\n");
+        Hgoto_error(1);
+    }
 
     /* ================================================ */
     /* If readwrite, collect exit code of child process */
     /* ================================================ */
-    if (UC_opts.launch == UC_READWRITE){
-        if ((tmppid = waitpid(childpid, &child_status, child_wait_option)) < 0){
+    if (UC_opts.launch == UC_READWRITE) {
+        if ((tmppid = waitpid(childpid, &child_status, child_wait_option)) < 0) {
             perror("waitpid");
             Hgoto_error(1);
         }
 
-        /* Close the file  */
-        if(H5Fclose(fid) < 0) {
-            HDfprintf(stderr, "Failed to close file id\n");
-            Hgoto_error(1);
-        }
-
-        /* Close the property list */
-        if(H5Pclose(fapl) < 0) {
-            HDfprintf(stderr, "Failed to close the property list\n");
-            Hgoto_error(1);
-        }
-
-        if (WIFEXITED(child_status)){
-            if ((child_ret_value=WEXITSTATUS(child_status)) != 0){
+        if (WIFEXITED(child_status)) {
+            if ((child_ret_value=WEXITSTATUS(child_status)) != 0) {
                 HDprintf("%d: child process exited with non-zero code (%d)\n",
                     mypid, child_ret_value);
                 Hgoto_error(1);
@@ -233,17 +254,16 @@ main(int argc, char *argv[])
             Hgoto_error(2);
         }
     }
-    
+
 done:
-    /* Print result and exit */
-    if (ret_value != 0){
+    if (ret_value != 0) {
         HDprintf("Error(s) encountered\n");
-    }else{
+    } else {
         HDprintf("All passed\n");
     }
 
     return(ret_value);
-}
+} /* end main() */
 
 #else /* H5_HAVE_FORK */
 
diff --git a/test/use_common.c b/test/use_common.c
index 4111b24..bf29936 100644
--- a/test/use_common.c
+++ b/test/use_common.c
@@ -21,10 +21,11 @@
 
 #include "use.h"
 
-#define H5D_FRIEND        /*suppress error about including H5Dpkg      */
-#define H5D_TESTING
-#include "H5Dpkg.h"
-
+/* ----------------------------------------------------------------------------
+ * Print a common/shared usage message.
+ * Receives program name to show default test file name (<program_name>.h5).
+ * ----------------------------------------------------------------------------
+ */
 void
 usage(const char *prog)
 {
@@ -41,10 +42,13 @@ usage(const char *prog)
     HDfprintf(stderr, "\n");
 } /* end usage() */
 
-/* Setup Use Case parameters by parsing command line options.
-* Setup default values if not set by options. */
+/* ----------------------------------------------------------------------------
+ * Setup Use Case parameters by parsing command line options.
+ * Includes default values for unspecified options.
+ * ----------------------------------------------------------------------------
+ */
 int
-parse_option(int argc, char * const argv[])
+parse_option(int argc, char * const argv[], options_t * opts)
 {
     int ret_value=0;
     int c;
@@ -56,302 +60,307 @@ parse_option(int argc, char * const argv[])
     /* suppress getopt from printing error */
     opterr = 0;
 
-    while (1){
-    c = getopt (argc, argv, nagg_options);
-    if (-1 == c)
-        break;
-    switch (c) {
-    case 'h':
-        usage(progname_g);
-        HDexit(EXIT_SUCCESS);
-        break;
-    case 'f':    /* usecase data file name */
-        UC_opts.filename = optarg;
-        break;
-    case 'i':    /* iterations */
-        if ((UC_opts.iterations = HDatoi(optarg)) <= 0) {
-            HDfprintf(stderr, "bad iterations number %s, must be a positive integer\n", optarg);
-        usage(progname_g);
-        Hgoto_error(-1);
-        };
-        break;
-    case 'l':    /* launch reader or writer only */
-        switch (*optarg) {
-        case 'r':    /* reader only */
-        UC_opts.launch = UC_READER;
-        break;
-        case 'w': /* writer only */
-        UC_opts.launch = UC_WRITER;
-        break;
+    while (1) {
+        c = getopt(argc, argv, nagg_options);
+        if (-1 == c)
+            break;
+        switch (c) {
+        case 'h':
+            usage(opts->progname);
+            exit(EXIT_SUCCESS);
+            break;
+        case 'f':     /* usecase data file name */
+            opts->filename = optarg;
+            break;
+        case 'i':     /* iterations */
+            if ((opts->iterations = HDatoi(optarg)) <= 0) {
+                HDfprintf(stderr, "bad iterations number %s, must be a positive integer\n", optarg);
+                usage(opts->progname);
+                Hgoto_error(-1);
+            }
+            break;
+        case 'l':     /* launch reader or writer only */
+            switch (*optarg) {
+            case 'r': /* reader only */
+                opts->launch = UC_READER;
+                break;
+            case 'w': /* writer only */
+                opts->launch = UC_WRITER;
+                break;
+            default:
+                HDfprintf(stderr, "launch value(%c) should be w or r only.\n", *optarg);
+                usage(opts->progname);
+                Hgoto_error(-1);
+                break;
+            } /* end switch (reader/writer-only mode toggle) */
+            break;
+        case 'n':     /* number of planes to write/read */
+            if ((opts->nplanes = HDstrtoul(optarg, NULL, 0)) <= 0) {
+                HDfprintf(stderr, "bad number of planes %s, must be a positive integer\n", optarg);
+                usage(opts->progname);
+                Hgoto_error(-1);
+            }
+            break;
+        case 's':     /* use swmr file open mode */
+            use_swmr = HDatoi(optarg);
+            if (use_swmr != 0 && use_swmr != 1) {
+                HDfprintf(stderr, "swmr value should be 0(no) or 1(yes)\n");
+                usage(opts->progname);
+                Hgoto_error(-1);
+            }
+            opts->use_swmr = (hbool_t)use_swmr;
+            break;
+        case 'y':     /* Number of planes per chunk */
+            if ((opts->chunkplanes = HDstrtoul(optarg, NULL, 0)) <= 0) {
+                HDfprintf(stderr, "bad number of planes per chunk %s, must be a positive integer\n", optarg);
+                usage(opts->progname);
+                Hgoto_error(-1);
+            }
+            break;
+        case 'z':     /* size of chunk=(z,z) */
+            if ((opts->chunksize = HDstrtoull(optarg, NULL, 0)) <= 0) {
+                HDfprintf(stderr, "bad chunksize %s, must be a positive integer\n", optarg);
+                usage(opts->progname);
+                Hgoto_error(-1);
+            }
+            break;
+        case '?':
+            HDfprintf(stderr, "getopt returned '%c'.\n", c);
+            Hgoto_error(-1);
         default:
-            HDfprintf(stderr, "launch value(%c) should be w or r only.\n", *optarg);
-        usage(progname_g);
-        Hgoto_error(-1);
-        break;
-        }
-        break;
-    case 'n':    /* number of planes to write/read */
-        if ((UC_opts.nplanes = HDstrtoul(optarg, NULL, 0)) <= 0) {
-            HDfprintf(stderr, "bad number of planes %s, must be a positive integer\n", optarg);
-        usage(progname_g);
-        Hgoto_error(-1);
-        };
-        break;
-    case 's':    /* use swmr file open mode */
-        use_swmr = HDatoi(optarg);
-        if (use_swmr != 0 && use_swmr != 1) {
-            HDfprintf(stderr, "swmr value should be 0(no) or 1(yes)\n");
-            usage(progname_g);
+            HDfprintf(stderr, "getopt returned unexpected value.\n");
+            HDfprintf(stderr, "Unexpected value is %d\n", c);
             Hgoto_error(-1);
-        }
-        UC_opts.use_swmr = (hbool_t)use_swmr;
-        break;
-    case 'y':    /* Number of planes per chunk */
-        if ((UC_opts.chunkplanes = HDstrtoul(optarg, NULL, 0)) <= 0) {
-            HDfprintf(stderr, "bad number of planes per chunk %s, must be a positive integer\n", optarg);
-        usage(progname_g);
-        Hgoto_error(-1);
-        };
-        break;
-    case 'z':    /* size of chunk=(z,z) */
-        if ((UC_opts.chunksize = HDstrtoull(optarg, NULL, 0)) <= 0) {
-            HDfprintf(stderr, "bad chunksize %s, must be a positive integer\n", optarg);
-        usage(progname_g);
-        Hgoto_error(-1);
-        };
-        break;
-    case '?':
-        HDfprintf(stderr, "getopt returned '%c'.\n", c);
-        Hgoto_error(-1);
-    default:
-        HDfprintf(stderr, "getopt returned unexpected value.\n");
-        HDfprintf(stderr, "Unexpected value is %d\n", c);
-        Hgoto_error(-1);
-    }
-    }
+        } /* end switch (argument symbol) */
+    } /* end while (there are still arguments) */
 
     /* set test file name if not given */
-    if (!UC_opts.filename){
-    /* default data file name is <progname>.h5 */
-    if ((UC_opts.filename=(char*)HDmalloc(HDstrlen(progname_g)+4))==NULL) {
-        HDfprintf(stderr, "malloc: failed\n");
-        Hgoto_error(-1);
-    };
-    HDstrcpy(UC_opts.filename, progname_g);
-    HDstrcat(UC_opts.filename, ".h5");
+    if (!opts->filename) {
+        /* default data file name is <progname>.h5 */
+        if ((opts->filename=(char*)HDmalloc(HDstrlen(opts->progname)+4))==NULL) {
+            HDfprintf(stderr, "malloc: failed\n");
+            Hgoto_error(-1);
+        }
+        HDstrcpy(opts->filename, opts->progname);
+        HDstrcat(opts->filename, ".h5");
     }
 
 done:
-    /* All done. */
     return(ret_value);
-}
+} /* end parse_option() */
 
-/* Show parameters used for this use case */
-void show_parameters(void){
+/* ----------------------------------------------------------------------------
+ * Show parameters used for this use case.
+ * ----------------------------------------------------------------------------
+ */
+void
+show_parameters(options_t * opts)
+{
     HDprintf("===Parameters used:===\n");
-    printf("chunk dims=(%llu, %llu, %llu)\n", (unsigned long long)UC_opts.chunkdims[0],
-        (unsigned long long)UC_opts.chunkdims[1], (unsigned long long)UC_opts.chunkdims[2]);
-    printf("dataset max dims=(%llu, %llu, %llu)\n", (unsigned long long)UC_opts.max_dims[0],
-        (unsigned long long)UC_opts.max_dims[1], (unsigned long long)UC_opts.max_dims[2]);
-    HDprintf("number of planes to write=%llu\n", (unsigned long long)UC_opts.nplanes);
-    HDprintf("using SWMR mode=%s\n", UC_opts.use_swmr ? "yes(1)" : "no(0)");
-    HDprintf("data filename=%s\n", UC_opts.filename);
+    HDprintf("chunk dims=(%llu, %llu, %llu)\n", (unsigned long long)opts->chunkdims[0], 
+        (unsigned long long)opts->chunkdims[1], (unsigned long long)opts->chunkdims[2]);
+    HDprintf("dataset max dims=(%llu, %llu, %llu)\n", (unsigned long long)opts->max_dims[0], 
+        (unsigned long long)opts->max_dims[1], (unsigned long long)opts->max_dims[2]);
+    HDprintf("number of planes to write=%llu\n", (unsigned long long)opts->nplanes);
+    HDprintf("using SWMR mode=%s\n", opts->use_swmr ? "yes(1)" : "no(0)");
+    HDprintf("data filename=%s\n", opts->filename);
     HDprintf("launch part=");
-    switch (UC_opts.launch){
-    case UC_READWRITE:
-        printf("Reader/Writer\n");
-        break;
-    case UC_WRITER:
-        printf("Writer\n");
-        break;
-    case UC_READER:
-        printf("Reader\n");
-        break;
-    default:
-        /* should not happen */
-        printf("Illegal part(%d)\n", UC_opts.launch);
-    };
-    HDprintf("number of iterations=%d (not used yet)\n", UC_opts.iterations);
+        switch (opts->launch) {
+        case UC_READWRITE:
+            HDprintf("Reader/Writer\n");
+            break;
+        case UC_WRITER:
+            HDprintf("Writer\n");
+            break;
+        case UC_READER:
+            HDprintf("Reader\n");
+            break;
+        default:
+            /* should not happen */
+            HDprintf("Illegal part(%d)\n", opts->launch);
+        }
+    HDprintf("number of iterations=%d (not used yet)\n", opts->iterations);
     HDprintf("===Parameters shown===\n");
-}
+} /* end show_parameters() */
 
-/* Create the skeleton use case file for testing.
+/* ----------------------------------------------------------------------------
+ * Create the skeleton use case file for testing.
  * It has one 3d dataset using chunked storage.
  * The dataset is (unlimited, chunksize, chunksize).
  * Dataset type is 2 bytes integer.
  * It starts out "empty", i.e., first dimension is 0.
  *
  * Return: 0 succeed; -1 fail.
+ * ----------------------------------------------------------------------------
  */
-int create_uc_file(void)
+int
+create_uc_file(options_t * opts)
 {
-    hsize_t dims[3];        /* Dataset starting dimensions */
-    hid_t fid;          /* File ID for new HDF5 file */
-    hid_t dcpl;         /* Dataset creation property list */
-    hid_t sid;          /* Dataspace ID */
-    hid_t dsid;         /* Dataset ID */
-    hid_t fapl;         /* File access property list */
+    hsize_t dims[3];            /* Dataset starting dimensions */
+    hid_t fid;                  /* File ID for new HDF5 file */
+    hid_t dcpl;                 /* Dataset creation property list */
+    hid_t sid;                  /* Dataspace ID */
+    hid_t dsid;                 /* Dataset ID */
     H5D_chunk_index_t idx_type; /* Chunk index type */
 
-    /* Create the file */
-    if((fapl = h5_fileaccess()) < 0)
-        return -1;
-    if(H5Pset_libver_bounds(fapl, H5F_LIBVER_LATEST, H5F_LIBVER_LATEST) < 0)
-        return -1;
-    if((fid = H5Fcreate(UC_opts.filename, H5F_ACC_TRUNC, H5P_DEFAULT, fapl)) < 0)
+    if ((fid = H5Fcreate(opts->filename, H5F_ACC_TRUNC, H5P_DEFAULT, opts->fapl_id)) < 0)
         return -1;
 
     /* Set up dimension sizes */
     dims[0] = 0;
-    dims[1] = dims[2] = UC_opts.max_dims[1];
+    dims[1] = dims[2] = opts->max_dims[1];
 
     /* Create dataspace for creating datasets */
-    if((sid = H5Screate_simple(3, dims, UC_opts.max_dims)) < 0)
+    if ((sid = H5Screate_simple(3, dims, opts->max_dims)) < 0)
         return -1;
 
     /* Create dataset creation property list */
-    if((dcpl = H5Pcreate(H5P_DATASET_CREATE)) < 0)
+    if ((dcpl = H5Pcreate(H5P_DATASET_CREATE)) < 0)
         return -1;
-    if(H5Pset_chunk(dcpl, 3, UC_opts.chunkdims) < 0)
+    if (H5Pset_chunk(dcpl, 3, opts->chunkdims) < 0)
         return -1;
 
     /* create dataset of progname */
-    if((dsid = H5Dcreate2(fid, progname_g, UC_DATATYPE, sid, H5P_DEFAULT, dcpl, H5P_DEFAULT)) < 0)
-    return -1;
+    if ((dsid = H5Dcreate2(fid, opts->progname, UC_DATATYPE, sid, H5P_DEFAULT, dcpl, H5P_DEFAULT)) < 0)
+        return -1;
 
     /* Check that the chunk index type is not version 1 B-tree.
      * Version 1 B-trees are not supported under SWMR.
      */
-    if(H5D__layout_idx_type_test(dsid, &idx_type) < 0)
+    if (H5Dget_chunk_index_type(dsid, &idx_type) < 0)
         return -1;
-    if(idx_type == H5D_CHUNK_IDX_BTREE) {
+    if (idx_type == H5D_CHUNK_IDX_BTREE) {
         HDfprintf(stderr, "ERROR: Chunk index is version 1 B-tree: aborting.\n");
         return -1;
     }
 
     /* Close everything */
-    if(H5Dclose(dsid) < 0)
-    return -1;
-    if(H5Pclose(fapl) < 0)
+    if (H5Dclose(dsid) < 0)
         return -1;
-    if(H5Pclose(dcpl) < 0)
+    if (H5Pclose(dcpl) < 0)
         return -1;
-    if(H5Sclose(sid) < 0)
+    if (H5Sclose(sid) < 0)
         return -1;
-    if(H5Fclose(fid) < 0)
+    if (H5Fclose(fid) < 0)
         return -1;
 
     return 0;
-}
+} /* end create_uc_file() */
 
-/* Append planes, each of (1,2*chunksize,2*chunksize) to the dataset.
+/* ----------------------------------------------------------------------------
+ * Append planes, each of (1,2*chunksize,2*chunksize) to the dataset.
  * In other words, 4 chunks are appended to the dataset at a time.
  * Fill each plan with the plane number and then write it at the nth plane.
  * Increase the plane number and repeat till the end of dataset, when it
  * reaches chunksize long. End product is a (2*chunksize)^3 cube.
  *
  * Return: 0 succeed; -1 fail.
+ * ----------------------------------------------------------------------------
  */
-int write_uc_file(hbool_t tosend, hid_t fid)
+int
+write_uc_file(hbool_t tosend, hid_t file_id, options_t * opts)
 {
-    hid_t    dsid;         /* dataset ID */
-    hid_t    dcpl;         /* Dataset creation property list */
-    UC_CTYPE    *buffer, *bufptr;    /* data buffer */
-    hsize_t    cz=UC_opts.chunksize;        /* Chunk size */
-    hid_t    f_sid;        /* dataset file space id */
-    hid_t    m_sid;        /* memory space id */
-    int        rank;        /* rank */
-    hsize_t     chunk_dims[3];    /* Chunk dimensions */
-    hsize_t    dims[3];    /* Dataspace dimensions */
-    hsize_t    memdims[3]; /* Memory space dimensions */
-    hsize_t    start[3] = {0,0,0}, count[3];    /* Hyperslab selection values */
-    hsize_t     i, j, k;
-
-    if(tosend)
+    hid_t     dsid;                         /* dataset ID */
+    hid_t     dcpl;                         /* Dataset creation property list */
+    UC_CTYPE *buffer, *bufptr;              /* data buffer */
+    hsize_t   cz=opts->chunksize;           /* Chunk size */
+    hid_t     f_sid;                        /* dataset file space id */
+    hid_t     m_sid;                        /* memory space id */
+    int       rank;                         /* rank */
+    hsize_t   chunk_dims[3];                /* Chunk dimensions */
+    hsize_t   dims[3];                      /* Dataspace dimensions */
+    hsize_t   memdims[3];                   /* Memory space dimensions */
+    hsize_t   start[3] = {0,0,0}, count[3]; /* Hyperslab selection values */
+    hsize_t   i, j, k;
+
+    if (TRUE == tosend) {
         /* Send a message that H5Fopen is complete--releasing the file lock */
         h5_send_message(WRITER_MESSAGE, NULL, NULL);
+    }
 
     /* Open the dataset of the program name */
-    if((dsid = H5Dopen2(fid, progname_g, H5P_DEFAULT)) < 0){
+    if ((dsid = H5Dopen2(file_id, opts->progname, H5P_DEFAULT)) < 0) {
         HDfprintf(stderr, "H5Dopen2 failed\n");
         return -1;
     }
 
     /* Find chunksize used */
-    if ((dcpl = H5Dget_create_plist(dsid)) < 0){
+    if ((dcpl = H5Dget_create_plist(dsid)) < 0) {
         HDfprintf(stderr, "H5Dget_create_plist failed\n");
         return -1;
     }
-    if (H5D_CHUNKED != H5Pget_layout(dcpl)){
+    if (H5D_CHUNKED != H5Pget_layout(dcpl)) {
         HDfprintf(stderr, "storage layout is not chunked\n");
         return -1;
     }
-    if ((rank = H5Pget_chunk(dcpl, 3, chunk_dims)) != 3){
+    if ((rank = H5Pget_chunk(dcpl, 3, chunk_dims)) != 3) {
         HDfprintf(stderr, "storage rank is not 3\n");
         return -1;
     }
 
     /* verify chunk_dims against set paramenters */
-    if (chunk_dims[0]!=UC_opts.chunkdims[0] || chunk_dims[1] != cz || chunk_dims[2] != cz){
+    if (chunk_dims[0]!=opts->chunkdims[0] || chunk_dims[1] != cz || chunk_dims[2] != cz) {
         HDfprintf(stderr, "chunk size is not as expected. Got dims=(%llu,%llu,%llu)\n",
-        (unsigned long long)chunk_dims[0], (unsigned long long)chunk_dims[1],
+            (unsigned long long)chunk_dims[0], (unsigned long long)chunk_dims[1], 
             (unsigned long long)chunk_dims[2]);
         return -1;
     }
 
     /* allocate space for data buffer 1 X dims[1] X dims[2] of UC_CTYPE */
     memdims[0]=1;
-    memdims[1] = UC_opts.dims[1];
-    memdims[2] = UC_opts.dims[2];
+    memdims[1] = opts->dims[1];
+    memdims[2] = opts->dims[2];
     if ((buffer=(UC_CTYPE*)HDmalloc((size_t)memdims[1]*(size_t)memdims[2]*sizeof(UC_CTYPE)))==NULL) {
         HDfprintf(stderr, "malloc: failed\n");
         return -1;
-    };
+    }
 
     /*
      * Get dataset rank and dimension.
      */
     f_sid = H5Dget_space(dsid);    /* Get filespace handle first. */
     rank  = H5Sget_simple_extent_ndims(f_sid);
-    if (rank != UC_RANK){
+    if (rank != UC_RANK) {
         HDfprintf(stderr, "rank(%d) of dataset does not match\n", rank);
         return -1;
     }
-    if (H5Sget_simple_extent_dims(f_sid, dims, NULL) < 0){
+    if (H5Sget_simple_extent_dims(f_sid, dims, NULL) < 0) {
         HDfprintf(stderr, "H5Sget_simple_extent_dims got error\n");
         return -1;
     }
     HDprintf("dataset rank %d, dimensions %llu x %llu x %llu\n",
-    rank, (unsigned long long)(dims[0]), (unsigned long long)(dims[1]),
+           rank, (unsigned long long)(dims[0]), (unsigned long long)(dims[1]), 
            (unsigned long long)(dims[2]));
     /* verify that file space dims are as expected and are consistent with memory space dims */
-    if (dims[0] != 0 || dims[1] != memdims[1] || dims[2] != memdims[2]){
+    if (dims[0] != 0 || dims[1] != memdims[1] || dims[2] != memdims[2]) {
         HDfprintf(stderr, "dataset is not empty. Got dims=(%llu,%llu,%llu)\n",
-        (unsigned long long)dims[0], (unsigned long long)dims[1],
+            (unsigned long long)dims[0], (unsigned long long)dims[1], 
             (unsigned long long)dims[2]);
         return -1;
     }
 
     /* setup mem-space for buffer */
-    if ((m_sid=H5Screate_simple(rank, memdims, NULL))<0){
+    if ((m_sid=H5Screate_simple(rank, memdims, NULL))<0) {
         HDfprintf(stderr, "H5Screate_simple for memory failed\n");
         return -1;
-    };
+    }
 
     /* write planes */
     count[0]=1;
     count[1]=dims[1];
     count[2]=dims[2];
-    for (i=0; i<UC_opts.nplanes; i++){
+    for (i=0; i < opts->nplanes; i++) {
         /* fill buffer with value i+1 */
         bufptr = buffer;
-        for (j=0; j<dims[1]; j++)
-            for (k=0; k<dims[2]; k++)
+        for (j=0; j < dims[1]; j++) {
+            for (k=0; k < dims[2]; k++) {
                 *bufptr++ = (UC_CTYPE)i;
+            }
+        }
 
         /* Cork the dataset's metadata in the cache, if SWMR is enabled */
-        if(UC_opts.use_swmr) {
-            if(H5Odisable_mdc_flushes(dsid) < 0) {
+        if (opts->use_swmr) {
+            if (H5Odisable_mdc_flushes(dsid) < 0) {
                 HDfprintf(stderr, "H5Odisable_mdc_flushes failed\n");
                 return -1;
             }
@@ -359,64 +368,65 @@ int write_uc_file(hbool_t tosend, hid_t fid)
 
         /* extend the dataset by one for new plane */
         dims[0]=i+1;
-        if(H5Dset_extent(dsid, dims) < 0){
+        if (H5Dset_extent(dsid, dims) < 0) {
             HDfprintf(stderr, "H5Dset_extent failed\n");
             return -1;
         }
 
         /* Get the dataset's dataspace */
-        if((f_sid = H5Dget_space(dsid)) < 0){
+        if ((f_sid = H5Dget_space(dsid)) < 0) {
             HDfprintf(stderr, "H5Dset_extent failed\n");
             return -1;
         }
 
         start[0]=i;
         /* Choose the next plane to write */
-        if(H5Sselect_hyperslab(f_sid, H5S_SELECT_SET, start, NULL, count, NULL) < 0){
+        if (H5Sselect_hyperslab(f_sid, H5S_SELECT_SET, start, NULL, count, NULL) < 0) {
             HDfprintf(stderr, "Failed H5Sselect_hyperslab\n");
             return -1;
         }
 
         /* Write plane to the dataset */
-        if(H5Dwrite(dsid, UC_DATATYPE, m_sid, f_sid, H5P_DEFAULT, buffer) < 0){
+        if (H5Dwrite(dsid, UC_DATATYPE, m_sid, f_sid, H5P_DEFAULT, buffer) < 0) {
             HDfprintf(stderr, "Failed H5Dwrite\n");
             return -1;
         }
 
         /* Uncork the dataset's metadata from the cache, if SWMR is enabled */
-        if(UC_opts.use_swmr)
-            if(H5Oenable_mdc_flushes(dsid) < 0) {
+        if (opts->use_swmr) {
+            if (H5Oenable_mdc_flushes(dsid) < 0) {
                 HDfprintf(stderr, "H5Oenable_mdc_flushes failed\n");
                 return -1;
             }
+        }
 
         /* flush file to make the just written plane available. */
-        if(H5Dflush(dsid) < 0) {
+        if (H5Dflush(dsid) < 0) {
             HDfprintf(stderr, "Failed to H5Fflush file\n");
             return -1;
         }
-    }
+    } /* end for each plane to write */
 
     /* Done writing. Free/Close all resources including data file */
     HDfree(buffer);
-    if (H5Dclose(dsid) < 0){
+    if (H5Dclose(dsid) < 0) {
         HDfprintf(stderr, "Failed to close datasete\n");
         return -1;
     }
-    if (H5Sclose(m_sid) < 0){
+    if (H5Sclose(m_sid) < 0) {
         HDfprintf(stderr, "Failed to close memory space\n");
         return -1;
     }
-    if (H5Sclose(f_sid) < 0){
+    if (H5Sclose(f_sid) < 0) {
         HDfprintf(stderr, "Failed to close file space\n");
         return -1;
     }
 
     return 0;
-}
+} /* end write_uc_file() */
 
-
-/* Read planes from the dataset.
+/* ----------------------------------------------------------------------------
+ * Read planes from the dataset.
  * It expects the dataset is being changed (growing).
  * It checks the unlimited dimension (1st one). When it increases,
  * it will read in the new planes, one by one, and verify the data correctness.
@@ -425,61 +435,51 @@ int write_uc_file(hbool_t tosend, hid_t fid)
  * that is the expected end of data, the reader exits.
  *
  * Return: 0 succeed; -1 fail.
+ * ----------------------------------------------------------------------------
  */
-int read_uc_file(hbool_t towait)
+int
+read_uc_file(hbool_t towait, options_t * opts)
 {
-    hid_t   fapl;         /* file access property list ID */
-    hid_t    fid;          /* File ID for new HDF5 file */
-    hid_t    dsid;         /* dataset ID */
-    char    *name;
-    UC_CTYPE    *buffer, *bufptr;    /* read data buffer */
-    hid_t    f_sid;        /* dataset file space id */
-    hid_t    m_sid;        /* memory space id */
-    int        rank;        /* rank */
-    hsize_t    dims[3];    /* Dataspace dimensions */
-    hsize_t    memdims[3]; /* Memory space dimensions */
-    hsize_t     nplane=0, nplane_old=0;    /* nth plane, last nth plane */
-    hsize_t    start[3] = {0,0,0}, count[3];    /* Hyperslab selection values */
-    hsize_t    j, k;
-    int        nreadererr=0;
-    int        nerrs;
-    int        nonewplane;
+    hid_t     fid;                          /* File ID for new HDF5 file */
+    hid_t     dsid;                         /* dataset ID */
+    UC_CTYPE *buffer, *bufptr;              /* read data buffer */
+    hid_t     f_sid;                        /* dataset file space id */
+    hid_t     m_sid;                        /* memory space id */
+    int       rank;                         /* rank */
+    hsize_t   dims[3];                      /* Dataspace dimensions */
+    hsize_t   memdims[3];                   /* Memory space dimensions */
+    hsize_t   nplane=0, nplanes_seen=0;     /* nth plane, last nth plane */
+    hsize_t   start[3] = {0,0,0}, count[3]; /* Hyperslab selection values */
+    hsize_t   j, k;
+    int       nreadererr=0;
+    int       nerrs;
+    int       loops_waiting_for_plane;
 
     /* Before reading, wait for the message that H5Fopen is complete--file lock is released */
-    if(towait && h5_wait_message(WRITER_MESSAGE) < 0) {
+    if (towait && h5_wait_message(WRITER_MESSAGE) < 0) {
         HDfprintf(stderr, "Cannot find writer message file...failed\n");
         return -1;
     }
 
-    name = UC_opts.filename;
-
-    /* Open the file */
-    if((fapl = h5_fileaccess()) < 0)
-        return -1;
-    if((fid = H5Fopen(name, H5F_ACC_RDONLY | (UC_opts.use_swmr ? H5F_ACC_SWMR_READ : 0), fapl)) < 0){
+    HDfprintf(stderr, "Opening to read %s\n", opts->filename);
+    if ((fid = H5Fopen(opts->filename, H5F_ACC_RDONLY | (opts->use_swmr ? H5F_ACC_SWMR_READ : 0), opts->fapl_id)) < 0) {
         HDfprintf(stderr, "H5Fopen failed\n");
         return -1;
     }
-    if (H5Pclose(fapl) < 0){
-        HDfprintf(stderr, "Failed to property list\n");
-        return -1;
-    }
 
-
-    /* Open the dataset of the program name */
-    if((dsid = H5Dopen2(fid, progname_g, H5P_DEFAULT)) < 0){
+    if ((dsid = H5Dopen2(fid, opts->progname, H5P_DEFAULT)) < 0) {
         HDfprintf(stderr, "H5Dopen2 failed\n");
         return -1;
     }
 
-    /* allocate space for data buffer 1 X dims[1] X dims[2] of UC_CTYPE */
-    memdims[0]=1;
-    memdims[1] = UC_opts.dims[1];
-    memdims[2] = UC_opts.dims[2];
+    /* Allocate space for data buffer 1 X dims[1] X dims[2] of UC_CTYPE */
+    memdims[0] = 1;
+    memdims[1] = opts->dims[1];
+    memdims[2] = opts->dims[2];
     if ((buffer=(UC_CTYPE*)HDmalloc((size_t)memdims[1]*(size_t)memdims[2]*sizeof(UC_CTYPE)))==NULL) {
         HDfprintf(stderr, "malloc: failed\n");
         return -1;
-    };
+    }
 
     /*
      * Get dataset rank and dimension.
@@ -487,11 +487,11 @@ int read_uc_file(hbool_t towait)
      */
     f_sid = H5Dget_space(dsid);    /* Get filespace handle first. */
     rank  = H5Sget_simple_extent_ndims(f_sid);
-    if (rank != UC_RANK){
+    if (rank != UC_RANK) {
         HDfprintf(stderr, "rank(%d) of dataset does not match\n", rank);
         return -1;
     }
-    if (H5Sget_simple_extent_dims(f_sid, dims, NULL) < 0){
+    if (H5Sget_simple_extent_dims(f_sid, dims, NULL) < 0) {
         HDfprintf(stderr, "H5Sget_simple_extent_dims got error\n");
         return -1;
     }
@@ -499,7 +499,7 @@ int read_uc_file(hbool_t towait)
             rank, (unsigned long long)(dims[0]), (unsigned long long)(dims[1]),
             (unsigned long long)(dims[2]));
     /* verify that file space dims are as expected and are consistent with memory space dims */
-    if (dims[1] != memdims[1] || dims[2] != memdims[2]){
+    if (dims[1] != memdims[1] || dims[2] != memdims[2]) {
         HDfprintf(stderr, "dataset dimension is not as expected. Got dims=(%llu,%llu,%llu)\n",
             (unsigned long long)dims[0], (unsigned long long)dims[1],
             (unsigned long long)dims[2]);
@@ -509,120 +509,101 @@ int read_uc_file(hbool_t towait)
         return -1;
     }
 
-    /* setup mem-space for buffer */
-    if ((m_sid=H5Screate_simple(rank, memdims, NULL))<0){
+    /* Setup mem-space for buffer */
+    if ((m_sid=H5Screate_simple(rank, memdims, NULL)) < 0) {
         HDfprintf(stderr, "H5Screate_simple for memory failed\n");
         return -1;
-    };
+    }
 
-    /* Read 1 plane at a time whenever the dataset grows larger
-     * (along dim[0]) */
-    count[0]=1;
-    count[1]=dims[1];
-    count[2]=dims[2];
+    /* Read 1 plane at a time whenever the dataset grows larger (along dim[0]) */
+    count[0] = 1;
+    count[1] = dims[1];
+    count[2] = dims[2];
     /* quit when all nplanes  have been read */
-    nonewplane=0;
-    while (nplane_old < UC_opts.nplanes ){
-    /* print progress message according to if new planes are availalbe */
-    if (nplane_old < dims[0]) {
-        if (nonewplane){
-            /* end the previous message */
-            HDprintf("\n");
-            nonewplane=0;
-        }
-        HDprintf("reading planes %llu to %llu\n", (unsigned long long)nplane_old,
+    loops_waiting_for_plane=0;
+    while (nplanes_seen < opts->nplanes) {
+        /* print progress message according to if new planes are availalbe */
+        if (nplanes_seen < dims[0]) {
+            if (loops_waiting_for_plane) {
+                /* end the previous message */
+                HDprintf("\n");
+                loops_waiting_for_plane=0;
+            }
+            HDprintf("reading planes %llu to %llu\n", (unsigned long long)nplanes_seen, 
                 (unsigned long long)dims[0]);
-    }else{
-        if (nonewplane){
-            HDprintf(".");
-        if (nonewplane>=30){
-            HDfprintf(stderr, "waited too long for new plane, quit.\n");
-            return -1;
-        }
-        }else{
-            /* print mesg only the first time; dots still no new plane */
-            HDprintf("no new planes to read ");
         }
-        nonewplane++;
-        /* pause for a second */
-        HDsleep(1);
-    }
-    for (nplane=nplane_old; nplane < dims[0]; nplane++){
-        /* read planes between last old nplanes and current extent */
-        /* Get the dataset's dataspace */
-        if((f_sid = H5Dget_space(dsid)) < 0){
-            HDfprintf(stderr, "H5Dget_space failed\n");
-            return -1;
+        else {
+            if (loops_waiting_for_plane) {
+                HDprintf(".");
+                if (loops_waiting_for_plane>=30) {
+                    HDfprintf(stderr, "waited too long for new plane, quit.\n");
+                    return -1;
+                }
+            }
+            else {
+                /* print mesg only the first time; dots still no new plane */
+                HDprintf("waiting for new planes to read ");
+            }
+            loops_waiting_for_plane++;
+            /* pause for a second */
+            HDsleep(1);
         }
 
-        start[0]=nplane;
-        /* Choose the next plane to read */
-        if(H5Sselect_hyperslab(f_sid, H5S_SELECT_SET, start, NULL, count, NULL) < 0){
-            HDfprintf(stderr, "H5Sselect_hyperslab failed\n");
-            return -1;
-        }
+        for (nplane=nplanes_seen; nplane < dims[0]; nplane++) {
+            /* read planes between last old nplanes and current extent */
+            /* Get the dataset's dataspace */
+            if ((f_sid = H5Dget_space(dsid)) < 0) {
+                HDfprintf(stderr, "H5Dget_space failed\n");
+                return -1;
+            }
 
-        /* Read the plane from the dataset */
-        if(H5Dread(dsid, UC_DATATYPE, m_sid, f_sid, H5P_DEFAULT, buffer) < 0){
-            HDfprintf(stderr, "H5Dread failed\n");
-            return -1;
-        }
+            start[0]=nplane;
+            /* Choose the next plane to read */
+            if (H5Sselect_hyperslab(f_sid, H5S_SELECT_SET, start, NULL, count, NULL) < 0) {
+                HDfprintf(stderr, "H5Sselect_hyperslab failed\n");
+                return -1;
+            }
 
-        /* compare read data with expected data value which is nplane */
-        bufptr = buffer;
-        nerrs=0;
-        for (j=0; j<dims[1]; j++){
-            for (k=0; k<dims[2]; k++){
-                if ((hsize_t)*bufptr++ != nplane){
-                    if (++nerrs < ErrorReportMax){
-                        HDfprintf(stderr,
-                            "found error %llu plane(%llu,%llu), expected %llu, got %d\n",
-                            (unsigned long long)nplane, (unsigned long long)j,
-                            (unsigned long long)k, (unsigned long long)nplane, (int)*(bufptr-1));
-                    }
-                }
+            /* Read the plane from the dataset */
+            if (H5Dread(dsid, UC_DATATYPE, m_sid, f_sid, H5P_DEFAULT, buffer) < 0) {
+                HDfprintf(stderr, "H5Dread failed\n");
+                return -1;
             }
-        }
-        if (nerrs){
-            nreadererr++;
-            HDfprintf(stderr, "found %d unexpected values in plane %llu\n", nerrs,
+
+            /* compare read data with expected data value which is nplane */
+            bufptr = buffer;
+            nerrs=0;
+            for (j=0; j < dims[1]; j++) {
+                for (k=0; k < dims[2]; k++) {
+                    if ((hsize_t)*bufptr++ != nplane) {
+                        if (++nerrs < ErrorReportMax) {
+                            HDfprintf(stderr,
+                                "found error %llu plane(%llu,%llu), expected %llu, got %d\n",
+                                (unsigned long long)nplane, (unsigned long long)j, 
+                                (unsigned long long)k, (unsigned long long)nplane, (int)*(bufptr-1));
+                        } /* end if should print error */
+                    } /* end if value mismatch */
+                } /* end for plane second dimension */
+            } /* end for plane first dimension */
+            if (nerrs) {
+                nreadererr++;
+                HDfprintf(stderr, "found %d unexpected values in plane %llu\n", nerrs, 
                     (unsigned long long)nplane);
-        }
-    }
-    /* Have read all current planes */
-    nplane_old=dims[0];
+            }
+        } /* end for each plane added since last read */
 
-    /* check if dataset has grown since last time */
-#if 0
-    /* close dsid and file, then reopen them */
-    if (H5Dclose(dsid) < 0){
-        HDfprintf(stderr, "H5Dclose failed\n");
-        return -1;
-    }
-    if (H5Fclose(fid) < 0){
-        HDfprintf(stderr, "H5Fclose failed\n");
-        return -1;
-    }
-    if((fid = H5Fopen(name, H5F_ACC_RDONLY | (UC_opts.use_swmr ? H5F_ACC_SWMR_READ : 0), H5P_DEFAULT)) < 0){
-        HDfprintf(stderr, "H5Fopen failed\n");
-        return -1;
-    }
-    if((dsid = H5Dopen2(fid, progname_g, H5P_DEFAULT)) < 0){
-        HDfprintf(stderr, "H5Dopen2 failed\n");
-        return -1;
-    }
-#else
-    H5Drefresh(dsid);
-#endif
-    f_sid = H5Dget_space(dsid);    /* Get filespace handle first. */
-    if (H5Sget_simple_extent_dims(f_sid, dims, NULL) < 0){
-        HDfprintf(stderr, "H5Sget_simple_extent_dims got error\n");
-        return -1;
-    }
-    }
+        nplanes_seen=dims[0];
+
+        /* check if dataset has grown since last time (update dims) */
+        H5Drefresh(dsid);
+        f_sid = H5Dget_space(dsid);    /* Get filespace handle first. */
+        if (H5Sget_simple_extent_dims(f_sid, dims, NULL) < 0) {
+            HDfprintf(stderr, "H5Sget_simple_extent_dims got error\n");
+            return -1;
+        }
+    } /* end while (expecting more planes to read) */
 
-    /* Close the file */
-    if(H5Fclose(fid) < 0) {
+    if (H5Fclose(fid) < 0) {
         HDfprintf(stderr, "H5Fclose failed\n");
         return -1;
     }
@@ -631,7 +612,7 @@ int read_uc_file(hbool_t towait)
         return -1;
     else
         return 0;
-} /* read_uc_file() */
+} /* end read_uc_file() */
 
 #endif /* H5_HAVE_FORK */
 
diff --git a/test/vfd.c b/test/vfd.c
index e88d9bb..97995d2 100644
--- a/test/vfd.c
+++ b/test/vfd.c
@@ -25,6 +25,7 @@
 #define FAMILY_SIZE     (1*KB)
 #define FAMILY_SIZE2    (5*KB)
 #define MULTI_SIZE      128
+#define SPLITTER_SIZE   8 /* dimensions of a dataset */
 
 #define CORE_INCREMENT  (4*KB)
 #define CORE_PAGE_SIZE  (1024*KB)
@@ -59,6 +60,9 @@ const char *FILENAME[] = {
     "windows_file",      /*8*/
     "new_multi_file_v16",/*9*/
     "ro_s3_file",        /*10*/
+    "splitter_rw_file",  /*11*/
+    "splitter_wo_file",  /*12*/
+    "splitter.log",      /*13*/
     NULL
 };
 
@@ -66,8 +70,47 @@ const char *FILENAME[] = {
 
 #define COMPAT_BASENAME "family_v16_"
 #define MULTI_COMPAT_BASENAME "multi_file_v16"
+#define SPLITTER_DATASET_NAME "dataset"
+
+/* Macro: HEXPRINT()
+ * Helper macro to pretty-print hexadecimal output of a buffer of known size.
+ * Each line has the address of the first printed byte, and four columns of
+ * four-byte data.
+ */
+static int __k;
+#define HEXPRINT(size, buf)                                                   \
+for (__k = 0; __k < (size); __k++) {                                          \
+    if (__k % 16 == 0) {                                                      \
+        HDprintf("\n%04x", __k);                                              \
+    }                                                                         \
+    HDprintf((__k%4 == 0) ? "  %02X" : " %02X", (unsigned char)(buf)[__k]);   \
+} /* end #define HEXPRINT() */
 
 
+/* Helper structure to pass around dataset information.
+ */
+struct splitter_dataset_def {
+    void *buf;             /* contents of dataset */
+    const char *dset_name; /* dataset name, always added to root group */
+    hid_t mem_type_id;     /* datatype */
+    const hsize_t *dims;   /* dimensions */
+    int n_dims;            /* rank */
+};
+
+static int splitter_prepare_file_paths(H5FD_splitter_vfd_config_t *vfd_config,
+        char *filename_rw_out);
+static int splitter_create_single_file_at(const char *filename, hid_t fapl_id,
+        const struct splitter_dataset_def *data);
+static int splitter_compare_expected_data(hid_t file_id,
+        const struct splitter_dataset_def *data);
+static int run_splitter_test(const struct splitter_dataset_def *data,
+        hbool_t ignore_wo_errors, hbool_t provide_logfile_path,
+        hid_t sub_fapl_ids[2]);
+static int splitter_RO_test(const struct splitter_dataset_def *data,
+        hid_t child_fapl_id);
+static int splitter_tentative_open_test(hid_t child_fapl_id);
+static int file_exists(const char *filename, hid_t fapl_id);
+
 /*-------------------------------------------------------------------------
  * Function:    test_sec2
  *
@@ -859,7 +902,7 @@ test_family(void)
     hid_t       driver_id = -1;             /* ID for this VFD              */
     unsigned long driver_flags = 0;         /* VFD feature flags            */
     char        filename[1024];
-    char        dname[]="dataset";
+    char        dname[] = "dataset";
     unsigned int i, j;
     int         *fhandle=NULL, *fhandle2=NULL;
     int         **buf = NULL;
@@ -1263,7 +1306,7 @@ error:
     return FAIL;
 } /* end test_family_member_fapl() */
 
-
+
 /*-------------------------------------------------------------------------
  * Function:    test_multi_opens
  *
@@ -2179,6 +2222,1052 @@ error:
 #endif /* H5_HAVE_ROS3_VFD */
 } /* end test_ros3() */
 
+
+/* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+ * Macro: SPLITTER_TEST_FAULT()
+ *
+ * utility macro, helps create stack-like backtrace on error.
+ * requires defined in the calling function:
+ *    * variable `int ret_value` (return -1 on error)`
+ *    * label `done` for exit on fault
+ * - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+ */
+#define SPLITTER_TEST_FAULT(mesg) { \
+    H5_FAILED();                    \
+    AT();                           \
+    HDfprintf(stderr, mesg);        \
+    H5Eprint2(H5E_DEFAULT, stderr); \
+    fflush(stderr);                 \
+    ret_value = -1;                 \
+    goto done;                      \
+}
+
+
+/*-------------------------------------------------------------------------
+ * Function:    compare_splitter_config_info
+ *
+ * Purpose:     Helper function to compare configuration info found in a
+ *              FAPL against a canonical structure.
+ *
+ * Return:      Success:  0, if config info in FAPL matches info structure.
+ *              Failure: -1, if difference detected.
+ *
+ *-------------------------------------------------------------------------
+ */
+static int
+compare_splitter_config_info(hid_t fapl_id, H5FD_splitter_vfd_config_t *info)
+{
+    int                         ret_value = 0;
+    H5FD_splitter_vfd_config_t  fetched_info;
+
+    fetched_info.magic = H5FD_SPLITTER_MAGIC;
+    fetched_info.version = H5FD_CURR_SPLITTER_VFD_CONFIG_VERSION;
+    fetched_info.rw_fapl_id = H5I_INVALID_HID;
+    fetched_info.wo_fapl_id = H5I_INVALID_HID;
+
+    if (H5Pget_fapl_splitter(fapl_id, &fetched_info) < 0) {
+        SPLITTER_TEST_FAULT("can't get splitter info\n");
+    }
+    if (info->rw_fapl_id == H5P_DEFAULT) {
+        if (H5Pget_driver(fetched_info.rw_fapl_id) != H5Pget_driver(H5P_FILE_ACCESS_DEFAULT)) {
+            SPLITTER_TEST_FAULT("Read-Write driver mismatch (default)\n");
+        }
+    }
+    else {
+        if (H5Pget_driver(fetched_info.rw_fapl_id) != H5Pget_driver(info->rw_fapl_id)) {
+            SPLITTER_TEST_FAULT("Read-Write driver mismatch\n");
+        }
+    }
+    if (info->wo_fapl_id == H5P_DEFAULT) {
+        if (H5Pget_driver(fetched_info.wo_fapl_id) != H5Pget_driver(H5P_FILE_ACCESS_DEFAULT)) {
+            SPLITTER_TEST_FAULT("Write-Only driver mismatch (default)\n");
+        }
+    }
+    else {
+        if (H5Pget_driver(fetched_info.wo_fapl_id) != H5Pget_driver(info->wo_fapl_id)) {
+            SPLITTER_TEST_FAULT("Write-Only driver mismatch\n");
+        }
+    }
+    if ( (HDstrlen(info->wo_path) != HDstrlen(fetched_info.wo_path)) ||
+         HDstrncmp(info->wo_path, fetched_info.wo_path, H5FD_SPLITTER_PATH_MAX))
+    {
+        HDfprintf(stderr, "MISMATCH: '%s' :: '%s'\n", info->wo_path, fetched_info.wo_path);
+        HEXPRINT(H5FD_SPLITTER_PATH_MAX, info->wo_path);
+        HEXPRINT(H5FD_SPLITTER_PATH_MAX, fetched_info.wo_path);
+        SPLITTER_TEST_FAULT("Write-Only file path mismatch\n");
+    }
+
+done:
+    return ret_value;
+} /* end compare_splitter_config_info() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    run_splitter_test
+ *
+ * Purpose:     Auxiliary function for test_splitter().
+ *
+ * Return:      Success:        0
+ *              Failure:        -1
+ *
+ * Description:
+ *              Perform basic open-write-close with the Splitter VFD.
+ *              Prior to operations, removes files from a previous run,
+ *              if they exist.
+ *              After writing, compares read-write and write-only files.
+ *              Includes FAPL sanity testing.
+ *
+ *-------------------------------------------------------------------------
+ */
+static int
+run_splitter_test(const struct splitter_dataset_def      *data,
+                  hbool_t ignore_wo_errors,
+                  hbool_t provide_logfile_path,
+                  hid_t   sub_fapl_ids[2])
+{
+    hid_t                       file_id     = H5I_INVALID_HID;
+    hid_t                       fapl_id     = H5I_INVALID_HID;
+    hid_t                       dset_id     = H5I_INVALID_HID;
+    hid_t                       space_id    = H5I_INVALID_HID;
+    hid_t                       fapl_id_out = H5I_INVALID_HID;
+    hid_t                       fapl_id_cpy = H5I_INVALID_HID;
+    H5FD_splitter_vfd_config_t  vfd_config;
+    char                        filename_rw[H5FD_SPLITTER_PATH_MAX + 1];
+    FILE                       *logfile     = NULL;
+    int                         ret_value   = 0;
+
+    vfd_config.magic = H5FD_SPLITTER_MAGIC;
+    vfd_config.version = H5FD_CURR_SPLITTER_VFD_CONFIG_VERSION;
+    vfd_config.ignore_wo_errs = ignore_wo_errors;
+    vfd_config.rw_fapl_id = sub_fapl_ids[0];
+    vfd_config.wo_fapl_id = sub_fapl_ids[1];
+
+    if (splitter_prepare_file_paths(&vfd_config, filename_rw) < 0) {
+        SPLITTER_TEST_FAULT("can't prepare file paths\n");
+    }
+
+    if (provide_logfile_path == FALSE) {
+        *vfd_config.log_file_path = '\0'; /* reset as empty string */
+    }
+
+    /* Create a new fapl to use the SPLITTER file driver */
+    if ((fapl_id = H5Pcreate(H5P_FILE_ACCESS)) == H5I_INVALID_HID) {
+        SPLITTER_TEST_FAULT("can't create FAPL ID\n");
+    }
+    if (H5Pset_fapl_splitter(fapl_id, &vfd_config) < 0) {
+        SPLITTER_TEST_FAULT("can't set splitter FAPL\n");
+    }
+    if (H5Pget_driver(fapl_id) != H5FD_SPLITTER) {
+        SPLITTER_TEST_FAULT("set FAPL not SPLITTER\n");
+    }
+
+    if (compare_splitter_config_info(fapl_id, &vfd_config) < 0) {
+        SPLITTER_TEST_FAULT("information mismatch\n");
+    }
+
+    /*
+     * Copy property list, light compare, and close the copy.
+     * Helps test driver-implemented FAPL-copying and library ID management.
+     */
+
+    fapl_id_cpy = H5Pcopy(fapl_id);
+    if (H5I_INVALID_HID == fapl_id_cpy) {
+        SPLITTER_TEST_FAULT("can't copy FAPL\n");
+    }
+    if (compare_splitter_config_info(fapl_id_cpy, &vfd_config) < 0) {
+        SPLITTER_TEST_FAULT("information mismatch\n");
+    }
+    if (H5Pclose(fapl_id_cpy) < 0) {
+        SPLITTER_TEST_FAULT("can't close fapl copy\n");
+    }
+
+    /*
+     * Proceed with test. Create file.
+     */
+    file_id = H5Fcreate(filename_rw, H5F_ACC_TRUNC, H5P_DEFAULT, fapl_id);
+    if (file_id < 0) {
+        SPLITTER_TEST_FAULT("can't create file\n");
+    }
+
+    /*
+     * Check driver from file
+     */
+
+    fapl_id_out = H5Fget_access_plist(file_id);
+    if (H5I_INVALID_HID == fapl_id_out) {
+        SPLITTER_TEST_FAULT("can't get file's FAPL\n");
+
+    }
+    if (H5Pget_driver(fapl_id_out) != H5FD_SPLITTER) {
+        SPLITTER_TEST_FAULT("wrong file FAPL driver\n");
+    }
+    if (compare_splitter_config_info(fapl_id_out, &vfd_config) < 0) {
+        SPLITTER_TEST_FAULT("information mismatch\n");
+    }
+    if (H5Pclose(fapl_id_out) < 0) {
+        SPLITTER_TEST_FAULT("can't close file's FAPL\n");
+    }
+
+    /*
+     * Create and write the dataset
+     */
+
+    space_id = H5Screate_simple(data->n_dims, data->dims, NULL);
+    if (space_id < 0) {
+        SPLITTER_TEST_FAULT("can't create dataspace\n");
+    }
+    dset_id = H5Dcreate2(file_id, data->dset_name, data->mem_type_id, space_id, H5P_DEFAULT, H5P_DEFAULT, H5P_DEFAULT);
+    if (dset_id < 0) {
+        SPLITTER_TEST_FAULT("can't create dataset\n");
+    }
+    if (H5Dwrite(dset_id, data->mem_type_id, H5S_ALL, H5S_ALL, H5P_DEFAULT, data->buf) < 0) {
+        SPLITTER_TEST_FAULT("can't write data to dataset\n");
+    }
+
+    /* Close everything */
+    if (H5Dclose(dset_id) < 0) {
+        SPLITTER_TEST_FAULT("can't close dset\n");
+    }
+    if (H5Sclose(space_id) < 0) {
+        SPLITTER_TEST_FAULT("can't close space\n");
+    }
+    if (H5Pclose(fapl_id) < 0) {
+        SPLITTER_TEST_FAULT("can't close fapl\n");
+    }
+    if (H5Fclose(file_id) < 0) {
+        SPLITTER_TEST_FAULT("can't close file\n");
+    }
+
+    /* Verify that the R/W and W/O files are identical */
+    if (h5_compare_file_bytes(filename_rw, vfd_config.wo_path) < 0) {
+        SPLITTER_TEST_FAULT("files are not byte-for-byte equivalent\n");
+    }
+
+    /* Verify existence of logfile iff appropriate */
+    logfile = fopen(vfd_config.log_file_path, "r");
+    if ( (TRUE  == provide_logfile_path && NULL == logfile) ||
+         (FALSE == provide_logfile_path && NULL != logfile) )
+    {
+        SPLITTER_TEST_FAULT("no logfile when one was expected\n");
+    }
+
+done:
+    if (ret_value < 0) {
+        H5E_BEGIN_TRY {
+            (void)H5Dclose(dset_id);
+            (void)H5Sclose(space_id);
+            (void)H5Pclose(fapl_id_out);
+            (void)H5Pclose(fapl_id_cpy);
+            (void)H5Pclose(fapl_id);
+            (void)H5Fclose(file_id);
+        } H5E_END_TRY;
+    }
+    if (logfile != NULL) {
+        fclose(logfile);
+    }
+    return ret_value;
+
+} /* end run_splitter_test() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    driver_is_splitter_compatible
+ *
+ * Purpose:     Determine whether the driver set in the FAPL ID is compatible
+ *              with the Splitter VFD -- specificially, Write-Only channel.
+ *
+ * Return:      Success:        0
+ *              Failure:        -1
+ *
+ * Description: Attempts to put the given FAPL ID as the W/O channel.
+ *              Uses driver's own mechanisms to generate error, and catches
+ *              error.
+ *
+ *-------------------------------------------------------------------------
+ */
+static int
+driver_is_splitter_compatible(hid_t fapl_id)
+{
+    H5FD_splitter_vfd_config_t  vfd_config;
+    hid_t split_fapl_id = H5I_INVALID_HID;
+    herr_t ret;
+    int ret_value = 0;
+
+    split_fapl_id = H5Pcreate(H5P_FILE_ACCESS);
+    if (H5I_INVALID_HID == split_fapl_id) {
+        FAIL_PUTS_ERROR("Can't create contained FAPL");
+    }
+    vfd_config.magic = H5FD_SPLITTER_MAGIC;
+    vfd_config.version = H5FD_CURR_SPLITTER_VFD_CONFIG_VERSION;
+    vfd_config.ignore_wo_errs = FALSE;
+    vfd_config.rw_fapl_id = H5P_DEFAULT;
+    vfd_config.wo_fapl_id = fapl_id;
+    HDstrncpy(vfd_config.wo_path, "nonesuch", H5FD_SPLITTER_PATH_MAX);
+
+    H5E_BEGIN_TRY {
+        ret = H5Pset_fapl_splitter(split_fapl_id, &vfd_config);
+    } H5E_END_TRY;
+    if (SUCCEED == ret) {
+        ret_value = -1;
+    }
+
+    if (H5Pclose(split_fapl_id) < 0) {
+        FAIL_PUTS_ERROR("Can't close contained FAPL")
+    }
+    split_fapl_id = H5I_INVALID_HID;
+
+    return ret_value;
+
+error:
+    H5E_BEGIN_TRY {
+        (void)H5Pclose(split_fapl_id);
+    } H5E_END_TRY;
+    return -1;
+} /* end driver_is_splitter_compatible() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    splitter_RO_test
+ *
+ * Purpose:     Verify Splitter VFD with the Read-Only access flag.
+ *
+ * Return:      Success:        0
+ *              Failure:        -1
+ *
+ * Description: Attempt read-only opening of files with different
+ *              permutations of files already existing on-disk.
+ *
+ *-------------------------------------------------------------------------
+ */
+static int
+splitter_RO_test(
+        const struct splitter_dataset_def *data,
+        hid_t                              child_fapl_id)
+{
+    char        filename_rw[H5FD_SPLITTER_PATH_MAX + 1];
+    H5FD_splitter_vfd_config_t  vfd_config;
+    hid_t       fapl_id = H5I_INVALID_HID;
+    int         ret_value = 0;
+    hid_t       file_id = H5I_INVALID_HID;
+
+    vfd_config.magic = H5FD_SPLITTER_MAGIC;
+    vfd_config.version = H5FD_CURR_SPLITTER_VFD_CONFIG_VERSION;
+    vfd_config.ignore_wo_errs = FALSE;
+    vfd_config.rw_fapl_id = child_fapl_id;
+    vfd_config.wo_fapl_id = child_fapl_id;
+
+    if (splitter_prepare_file_paths(&vfd_config, filename_rw) < 0) {
+        SPLITTER_TEST_FAULT("can't prepare splitter file paths\n");
+    }
+
+    /* Create a new fapl to use the SPLITTER file driver */
+    fapl_id = H5Pcreate(H5P_FILE_ACCESS);
+    if (H5I_INVALID_HID == fapl_id) {
+        SPLITTER_TEST_FAULT("can't create FAPL ID\n");
+    }
+    if (H5Pset_fapl_splitter(fapl_id, &vfd_config) < 0) {
+        SPLITTER_TEST_FAULT("can't set splitter FAPL\n");
+    }
+    if (H5Pget_driver(fapl_id) != H5FD_SPLITTER) {
+        SPLITTER_TEST_FAULT("set FAPL not SPLITTER\n");
+    }
+
+    /* Attempt R/O open when both files are nonexistent
+     * Should fail.
+     */
+
+    H5E_BEGIN_TRY {
+        file_id = H5Fopen(filename_rw, H5F_ACC_RDONLY, fapl_id);
+    } H5E_END_TRY;
+    if (file_id >= 0) {
+        SPLITTER_TEST_FAULT("R/O open on nonexistent files unexpectedly successful\n");
+    }
+
+    /* Attempt R/O open when only W/O file exists
+     * Should fail.
+     */
+
+    if (splitter_create_single_file_at(vfd_config.wo_path, vfd_config.wo_fapl_id, data) < 0) {
+        SPLITTER_TEST_FAULT("can't write W/O file\n");
+    }
+    H5E_BEGIN_TRY {
+        file_id = H5Fopen(filename_rw, H5F_ACC_RDONLY, fapl_id);
+    } H5E_END_TRY;
+    if (file_id >= 0) {
+        SPLITTER_TEST_FAULT("R/O open with extant W/O file unexpectedly successful\n");
+    }
+    HDremove(vfd_config.wo_path);
+
+    /* Attempt R/O open when only R/W file exists
+     * Should fail.
+     */
+
+    if (splitter_create_single_file_at(filename_rw, vfd_config.rw_fapl_id, data) < 0) {
+        SPLITTER_TEST_FAULT("can't create R/W file\n");
+    }
+    H5E_BEGIN_TRY {
+        file_id = H5Fopen(filename_rw, H5F_ACC_RDONLY, fapl_id);
+    } H5E_END_TRY;
+    if (file_id >= 0) {
+        SPLITTER_TEST_FAULT("R/O open with extant R/W file unexpectedly successful\n");
+    }
+
+    /* Attempt R/O open when both R/W and W/O files exist
+     */
+
+    if (splitter_create_single_file_at(vfd_config.wo_path, vfd_config.wo_fapl_id, data) < 0) {
+        SPLITTER_TEST_FAULT("can't create W/O file\n");
+    }
+    file_id = H5Fopen(filename_rw, H5F_ACC_RDONLY, fapl_id);
+    if (file_id < 0) {
+        SPLITTER_TEST_FAULT("R/O open on two extant files failed\n");
+    }
+    if (splitter_compare_expected_data(file_id, data) < 0) {
+        SPLITTER_TEST_FAULT("data mismatch in R/W file\n");
+    }
+    if (H5Fclose(file_id) < 0) {
+        SPLITTER_TEST_FAULT("can't close file(s)\n");
+    }
+    file_id = H5I_INVALID_HID;
+
+    /* Cleanup
+     */
+
+    if (H5Pclose(fapl_id) < 0) {
+        SPLITTER_TEST_FAULT("can't close FAPL ID\n");
+    }
+    fapl_id = H5I_INVALID_HID;
+
+done:
+    if (ret_value < 0) {
+        H5E_BEGIN_TRY {
+            (void)H5Pclose(fapl_id);
+            (void)H5Fclose(file_id);
+        } H5E_END_TRY;
+    } /* end if error */
+    return ret_value;
+} /* end splitter_RO_test() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    splitter_prepare_file_paths
+ *
+ * Purpose:     Get file paths ready for use by the Splitter VFD tests.
+ *
+ * Return:      Success:        0
+ *              Failure:        -1
+ *
+ * Description:
+ *              Use h5_fixname to adjust the splitter-relevant file paths
+ *              from those given in FILENAMES.
+ *
+ *              REMOVES EXISTING FILES AT THE PATH LOCATIONS PRIOR TO RETURN.
+ *
+ *              The relevant file paths will be set in filename_rw_out and
+ *              inside the config structure (wo_path, log_file_path).
+ *
+ *              `filename_rw_out` must be at least H5FD_SPLITTER_PATH_MAX+1
+ *              characters long.
+ *
+ *              `vfd_config` must have its child FAPL IDs preset.
+ *
+ *-------------------------------------------------------------------------
+ */
+static int
+splitter_prepare_file_paths(H5FD_splitter_vfd_config_t *vfd_config, char *filename_rw_out)
+{
+    int ret_value = 0;
+
+    if (vfd_config == NULL || vfd_config->magic != H5FD_SPLITTER_MAGIC) {
+        SPLITTER_TEST_FAULT("invalid splitter config pointer\n");
+    }
+    if (filename_rw_out == NULL) {
+        SPLITTER_TEST_FAULT("NULL filename_rw pointer\n");
+    }
+
+    /* TODO: sanity-check fapl IDs? */
+
+    /* Build the r/w file, w/o file, and the log file paths.
+     * Output is stored in the associated string pointers.
+     */
+    h5_fixname(FILENAME[11], vfd_config->rw_fapl_id, filename_rw_out, H5FD_SPLITTER_PATH_MAX);
+    h5_fixname(FILENAME[12], vfd_config->wo_fapl_id, vfd_config->wo_path, H5FD_SPLITTER_PATH_MAX);
+    h5_fixname_no_suffix(FILENAME[13], vfd_config->wo_fapl_id, vfd_config->log_file_path, H5FD_SPLITTER_PATH_MAX);
+
+    /* Delete any existing files on disk.
+     */
+    HDremove(filename_rw_out);
+    HDremove(vfd_config->wo_path);
+    HDremove(vfd_config->log_file_path);
+
+done:
+    return ret_value;
+} /* end splitter_prepare_file_paths() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    splitter_crate_single_file_at
+ *
+ * Purpose:     Create a file, optionally w/ dataset.
+ *
+ * Return:      Success:        0
+ *              Failure:        -1
+ *
+ * Description:
+ *              Create a file at the given location with the given FAPL,
+ *              and write data as defined in `data` in a pre-determined location in the file.
+ *
+ *              If the dataset definition pointer is NULL, no data is written
+ *              to the file.
+ *
+ *              Will always overwrite an existing file with the given name/path.
+ *
+ *-------------------------------------------------------------------------
+ */
+static int
+splitter_create_single_file_at(
+        const char     *filename,
+        hid_t           fapl_id,
+        const struct splitter_dataset_def *data)
+{
+    hid_t       file_id = H5I_INVALID_HID;
+    hid_t       space_id = H5I_INVALID_HID;
+    hid_t       dset_id = H5I_INVALID_HID;
+    int         ret_value = 0;
+
+    if (filename == NULL || *filename == '\0') {
+        SPLITTER_TEST_FAULT("filename is invalid\n");
+    }
+    /* TODO: sanity-check fapl id? */
+
+    file_id = H5Fcreate(filename, H5F_ACC_TRUNC, H5P_DEFAULT, fapl_id);
+    if (file_id < 0) {
+        SPLITTER_TEST_FAULT("can't create file\n");
+    }
+
+    if (data) {
+        /* TODO: sanity-check data, if it exists? */
+        space_id = H5Screate_simple(data->n_dims, data->dims, NULL);
+        if (space_id < 0) {
+            SPLITTER_TEST_FAULT("can't create dataspace\n");
+        }
+
+        dset_id = H5Dcreate2(
+                file_id,
+                data->dset_name,
+                data->mem_type_id,
+                space_id,
+                H5P_DEFAULT,
+                H5P_DEFAULT,
+                H5P_DEFAULT);
+        if (dset_id < 0) {
+            SPLITTER_TEST_FAULT("can't create dataset\n");
+        }
+
+        if (H5Dwrite(dset_id, data->mem_type_id, H5S_ALL, H5S_ALL, H5P_DEFAULT, data->buf) < 0) {
+            SPLITTER_TEST_FAULT("can't write data to dataset\n");
+        }
+
+        if (H5Dclose(dset_id) < 0) {
+            SPLITTER_TEST_FAULT("can't close dset\n");
+        }
+        if (H5Sclose(space_id) < 0) {
+            SPLITTER_TEST_FAULT("can't close space\n");
+        }
+    } /* end if data definition is provided */
+
+    if (H5Fclose(file_id) < 0) {
+        SPLITTER_TEST_FAULT("can't close file\n");
+    }
+
+done:
+    if (ret_value < 0) {
+        H5E_BEGIN_TRY {
+            (void)H5Dclose(dset_id);
+            (void)H5Sclose(space_id);
+            (void)H5Fclose(file_id);
+        } H5E_END_TRY;
+    } /* end if error */
+    return ret_value;
+} /* end splitter_create_single_file_at() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    splitter_compare_expected_data
+ *
+ * Purpose:     Compare data within a predermined dataset.
+ *
+ * Return:      Success:        0
+ *              Failure:        -1
+ *
+ * Description: Read data from the file at a predetermined location, and
+ *              compare its contents byte-for-byte with that expected in
+ *              the `data` definition structure.
+ *
+ *-------------------------------------------------------------------------
+ */
+static int
+splitter_compare_expected_data(hid_t file_id,
+                               const struct splitter_dataset_def *data)
+{
+    hid_t dset_id = H5I_INVALID_HID;
+    int buf[SPLITTER_SIZE][SPLITTER_SIZE];
+    int expected[SPLITTER_SIZE][SPLITTER_SIZE];
+    size_t i = 0;
+    size_t j = 0;
+    int ret_value = 0;
+
+    if (sizeof((void *)buf) != sizeof(data->buf)) {
+        SPLITTER_TEST_FAULT("invariant size of expected data does not match that received!\n");
+    }
+    HDmemcpy(expected, data->buf, sizeof(expected));
+
+    dset_id = H5Dopen2(file_id, data->dset_name, H5P_DEFAULT);
+    if (dset_id < 0) {
+        SPLITTER_TEST_FAULT("can't open dataset\n");
+    }
+
+    if (H5Dread(dset_id, data->mem_type_id, H5S_ALL, H5S_ALL, H5P_DEFAULT, (void *)buf) < 0) {
+        SPLITTER_TEST_FAULT("can't read dataset\n");
+    }
+
+    for (i=0; i < SPLITTER_SIZE; i++) {
+        for (j=0; j < SPLITTER_SIZE; j++) {
+            if (buf[i][j] != expected[i][j]) {
+                SPLITTER_TEST_FAULT("mismatch in expected data\n");
+            }
+        }
+    }
+
+    if (H5Dclose(dset_id) < 0) {
+        SPLITTER_TEST_FAULT("can't close dataset\n");
+    }
+
+done:
+    if (ret_value < 0) {
+        H5E_BEGIN_TRY {
+            (void)H5Dclose(dset_id);
+        } H5E_END_TRY;
+    }
+    return ret_value;
+} /* end splitter_compare_expected_data() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    splitter_tentative_open_test()
+ *
+ * Purpose:     Verifies Splitter behavior with "tentative" H5F_open.
+ *
+ * Return:      Success:        0
+ *              Failure:        -1
+ *
+ * Description:
+ *              H5F_open() has a two-stage opening process when given a
+ *              Read/Write access flag -- first it performs a "tentative
+ *              open", where it checks to see whether files already exist
+ *              on the system, done in such a way as to not "alter its state"
+ *              (i.e., truncate).
+ *              This can cause problems with the Splitter VFD, as the
+ *              file on the R/W channel might exist already, but that on the
+ *              W/O channel will not, and vice-versa.
+ *
+ *              This test exists to verify that in any event, files will be
+ *              created as required.
+ *
+ *-------------------------------------------------------------------------
+ */
+static int
+splitter_tentative_open_test(hid_t child_fapl_id)
+{
+    char        filename_rw[H5FD_SPLITTER_PATH_MAX + 1];
+    H5FD_splitter_vfd_config_t  vfd_config;
+    hid_t       fapl_id = H5I_INVALID_HID;
+    hid_t       file_id = H5I_INVALID_HID;
+    int         buf[SPLITTER_SIZE][SPLITTER_SIZE]; /* for comparison */
+    hsize_t     dims[2]        = { SPLITTER_SIZE, SPLITTER_SIZE }; /* for comparison */
+    int         i              = 0; /* for comparison */
+    int         j              = 0; /* for comparison */
+    struct splitter_dataset_def data; /* for comparison */
+    int         ret_value = 0;
+
+    /* pre-fill data buffer to write */
+    for (i=0; i < SPLITTER_SIZE; i++) {
+        for (j=0; j < SPLITTER_SIZE; j++) {
+            buf[i][j] = i*100+j;
+        }
+    }
+
+    /* Dataset info */
+    data.buf = (void *)buf;
+    data.mem_type_id = H5T_NATIVE_INT;
+    data.dims = dims;
+    data.n_dims = 2;
+    data.dset_name = SPLITTER_DATASET_NAME;
+
+    vfd_config.magic = H5FD_SPLITTER_MAGIC;
+    vfd_config.version = H5FD_CURR_SPLITTER_VFD_CONFIG_VERSION;
+    vfd_config.ignore_wo_errs = FALSE;
+    vfd_config.rw_fapl_id = child_fapl_id;
+    vfd_config.wo_fapl_id = child_fapl_id;
+
+    if (splitter_prepare_file_paths(&vfd_config, filename_rw) < 0) {
+        SPLITTER_TEST_FAULT("can't prepare splitter file paths\n");
+    }
+
+    /* Create a new fapl to use the SPLITTER file driver */
+    if ((fapl_id = H5Pcreate(H5P_FILE_ACCESS)) == H5I_INVALID_HID) {
+        SPLITTER_TEST_FAULT("can't create FAPL ID\n");
+    }
+    if (H5Pset_fapl_splitter(fapl_id, &vfd_config) < 0) {
+        SPLITTER_TEST_FAULT("can't set splitter FAPL\n");
+    }
+    if (H5Pget_driver(fapl_id) != H5FD_SPLITTER) {
+        SPLITTER_TEST_FAULT("set FAPL not SPLITTER\n");
+    }
+
+    /* H5Fopen() with RDWR access.
+     * Neither file exist already
+     * Should fail.
+     */
+
+    H5E_BEGIN_TRY {
+        file_id = H5Fopen(filename_rw, H5F_ACC_RDWR, fapl_id);
+    } H5E_END_TRY;
+    if (file_id != H5I_INVALID_HID) {
+        SPLITTER_TEST_FAULT("open with both nonexistent files unexpectedly succeeded\n");
+    }
+    if (file_exists(filename_rw, child_fapl_id)) {
+        SPLITTER_TEST_FAULT("R/W file unexpectedly created\n");
+    }
+    if (file_exists(vfd_config.wo_path, child_fapl_id)) {
+        SPLITTER_TEST_FAULT("W/O file unexpectedly created\n");
+    }
+
+    /* H5Fopen() with RDWR access.
+     * W/O file exists already.
+     * Should fail.
+     */
+
+    if (splitter_create_single_file_at(vfd_config.wo_path, child_fapl_id, &data) < 0) {
+        SPLITTER_TEST_FAULT("can't write W/O file\n");
+    }
+    H5E_BEGIN_TRY {
+        file_id = H5Fopen(filename_rw, H5F_ACC_RDWR, fapl_id);
+    } H5E_END_TRY;
+    if (file_id != H5I_INVALID_HID) {
+        SPLITTER_TEST_FAULT("open with nonexistent R/W file unexpectedly succeeded\n");
+    }
+    if (file_exists(filename_rw, child_fapl_id)) {
+        SPLITTER_TEST_FAULT("R/W file unexpectedly created\n");
+    }
+    if (!file_exists(vfd_config.wo_path, child_fapl_id)) {
+        SPLITTER_TEST_FAULT("W/O file mysteriously disappeared\n");
+    }
+    HDremove(vfd_config.wo_path);
+    if (file_exists(vfd_config.wo_path, child_fapl_id)) {
+        SPLITTER_TEST_FAULT("failed to remove W/O file\n");
+    }
+
+    /* H5Fopen() with RDWR access.
+     * R/W file exists already.
+     * Should fail.
+     */
+
+    if (splitter_create_single_file_at(filename_rw, child_fapl_id, &data) < 0) {
+        SPLITTER_TEST_FAULT("can't write R/W file\n");
+    }
+    H5E_BEGIN_TRY {
+        file_id = H5Fopen(filename_rw, H5F_ACC_RDWR, fapl_id);
+    } H5E_END_TRY;
+    if (file_id != H5I_INVALID_HID) {
+        SPLITTER_TEST_FAULT("open with nonexistent W/O unexpectedly succeeded\n");
+    }
+    if (!file_exists(filename_rw, child_fapl_id)) {
+        SPLITTER_TEST_FAULT("R/W file mysteriously disappeared\n");
+    }
+    if (file_exists(vfd_config.wo_path, child_fapl_id)) {
+        SPLITTER_TEST_FAULT("W/O file unexpectedly created\n");
+    }
+
+    /* H5Fopen() with RDWR access.
+     * Both files already exist.
+     */
+
+    if (splitter_create_single_file_at(vfd_config.wo_path, child_fapl_id, &data) < 0) {
+        SPLITTER_TEST_FAULT("can't write W/O file\n");
+    }
+    file_id = H5Fopen(filename_rw, H5F_ACC_RDWR, fapl_id);
+    if (file_id == H5I_INVALID_HID) {
+        SPLITTER_TEST_FAULT("file-open failed with both present\n");
+    }
+    /* Open successful; close file then inspect presence again */
+    if (H5Fclose(file_id) < 0) {
+        SPLITTER_TEST_FAULT("can't close file ID\n");
+    }
+    if (!file_exists(filename_rw, child_fapl_id)) {
+        SPLITTER_TEST_FAULT("R/W file mysteriously disappared\n");
+    }
+    if (!file_exists(vfd_config.wo_path, child_fapl_id)) {
+        SPLITTER_TEST_FAULT("W/O file mysteriously disappeared\n");
+    }
+    if (h5_compare_file_bytes(filename_rw, vfd_config.wo_path) < 0) {
+        SPLITTER_TEST_FAULT("files are not byte-for-byte equivalent\n");
+    }
+
+    /* H5Fcreate() with TRUNC access.
+     * Both files already exist.
+     */
+
+    file_id = H5Fcreate(filename_rw, H5F_ACC_TRUNC, H5P_DEFAULT, fapl_id);
+    if (file_id == H5I_INVALID_HID) {
+        SPLITTER_TEST_FAULT("file-open failed with both present\n");
+    }
+    /* Open successful; close file then inspect presence again */
+    if (H5Fclose(file_id) < 0) {
+        SPLITTER_TEST_FAULT("can't close file ID\n");
+    }
+    if (!file_exists(filename_rw, child_fapl_id)) {
+        SPLITTER_TEST_FAULT("R/W file mysteriously disappared\n");
+    }
+    if (!file_exists(vfd_config.wo_path, child_fapl_id)) {
+        SPLITTER_TEST_FAULT("W/O file mysteriously disappeared\n");
+    }
+    if (h5_compare_file_bytes(filename_rw, vfd_config.wo_path) < 0) {
+        SPLITTER_TEST_FAULT("files are not byte-for-byte equivalent\n");
+    }
+
+    /* H5Fcreate() with TRUNC access.
+     * R/W already exists.
+     */
+
+    HDremove(filename_rw);
+    HDremove(vfd_config.wo_path);
+    if (splitter_create_single_file_at(filename_rw, child_fapl_id, &data) < 0) {
+        SPLITTER_TEST_FAULT("can't write R/W file\n");
+    }
+
+    if (file_exists(vfd_config.wo_path, child_fapl_id)) {
+        SPLITTER_TEST_FAULT("failed to remove W/O file\n");
+    }
+    file_id = H5Fcreate(filename_rw, H5F_ACC_TRUNC, H5P_DEFAULT, fapl_id);
+    if (file_id == H5I_INVALID_HID) {
+        SPLITTER_TEST_FAULT("file-open failed with both present\n");
+    }
+    /* Open successful; close file then inspect presence again */
+    if (H5Fclose(file_id) < 0) {
+        SPLITTER_TEST_FAULT("can't close file ID\n");
+    }
+    if (!file_exists(filename_rw, child_fapl_id)) {
+        SPLITTER_TEST_FAULT("R/W file mysteriously disappared\n");
+    }
+    if (!file_exists(vfd_config.wo_path, child_fapl_id)) {
+        SPLITTER_TEST_FAULT("W/O file mysteriously disappeared\n");
+    }
+    if (h5_compare_file_bytes(filename_rw, vfd_config.wo_path) < 0) {
+        SPLITTER_TEST_FAULT("files are not byte-for-byte equivalent\n");
+    }
+
+    /* H5Fcreate() with TRUNC access.
+     * W/O already exists.
+     */
+
+    HDremove(filename_rw);
+    HDremove(vfd_config.wo_path);
+    if (splitter_create_single_file_at(vfd_config.wo_path, child_fapl_id, &data) < 0) {
+        SPLITTER_TEST_FAULT("can't write R/W file\n");
+    }
+
+    if (file_exists(filename_rw, child_fapl_id)) {
+        SPLITTER_TEST_FAULT("failed to remove R/W file\n");
+    }
+    file_id = H5Fcreate(filename_rw, H5F_ACC_TRUNC, H5P_DEFAULT, fapl_id);
+    if (file_id == H5I_INVALID_HID) {
+        SPLITTER_TEST_FAULT("file-open failed with both present\n");
+    }
+    /* Open successful; close file then inspect presence again */
+    if (H5Fclose(file_id) < 0) {
+        SPLITTER_TEST_FAULT("can't close file ID\n");
+    }
+    if (!file_exists(filename_rw, child_fapl_id)) {
+        SPLITTER_TEST_FAULT("R/W file mysteriously disappared\n");
+    }
+    if (!file_exists(vfd_config.wo_path, child_fapl_id)) {
+        SPLITTER_TEST_FAULT("W/O file mysteriously disappeared\n");
+    }
+    if (h5_compare_file_bytes(filename_rw, vfd_config.wo_path) < 0) {
+        SPLITTER_TEST_FAULT("files are not byte-for-byte equivalent\n");
+    }
+
+    /* H5Fcreate with both files absent is tested elsewhere */
+
+    /* Cleanup
+     */
+
+    if (H5Pclose(fapl_id) < 0) {
+        SPLITTER_TEST_FAULT("can't close splitter FAPL ID\n");
+    }
+
+done:
+    if (ret_value < 0) {
+        H5E_BEGIN_TRY {
+            (void)H5Pclose(fapl_id);
+            (void)H5Fclose(file_id);
+        } H5E_END_TRY;
+    } /* end if error */
+    return ret_value;
+} /* end splitter_tentative_open_test() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    file_exists()
+ *
+ * Purpose:     Determine whether a file exists on-system
+ *
+ * Return:      Non-zero (1) if it exists (H5Fopen successful),
+ *              zero (0) if absent (cannot be opened).
+ *
+ * Description: Attempt H5Fopen with the given FAPL ID and RDONLY access flag.
+ *
+ *-------------------------------------------------------------------------
+ */
+static int
+file_exists(const char *filename, hid_t fapl_id)
+{
+    hid_t file_id = H5I_INVALID_HID;
+    int ret_value = 0;
+
+    H5E_BEGIN_TRY {
+        file_id = H5Fopen(filename, H5F_ACC_RDONLY, fapl_id);
+    } H5E_END_TRY;
+    if (file_id != H5I_INVALID_HID) {
+        ret_value = 1;
+        if (H5Fclose(file_id) < 0) {
+            FAIL_PUTS_ERROR("can't close file ID\n");
+        }
+    }
+
+    return ret_value;
+
+error:
+    H5E_BEGIN_TRY {
+        (void)H5Fclose(file_id);
+    } H5E_END_TRY;
+    return ret_value;
+} /* end file_exists() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    test_splitter
+ *
+ * Purpose:     Tests the Splitter VFD
+ *
+ * Return:      Success:        0
+ *              Failure:        -1
+ *
+ * Description:
+ *              This test function uses the Splitter VFD to produce a r/w
+ *              file and a w/o file. It will verify that the two files
+ *              are identical.
+ *
+ *-------------------------------------------------------------------------
+ */
+static herr_t
+test_splitter(void)
+{
+    int         buf[SPLITTER_SIZE][SPLITTER_SIZE];
+    hsize_t     dims[2]        = { SPLITTER_SIZE, SPLITTER_SIZE };
+    hid_t       child_fapl_id  = H5I_INVALID_HID;
+    int         i              = 0;
+    int         j              = 0;
+    struct splitter_dataset_def data;
+
+    TESTING("SPLITTER file driver");
+
+    /* pre-fill data buffer to write */
+    for (i=0; i < SPLITTER_SIZE; i++) {
+        for (j=0; j < SPLITTER_SIZE; j++) {
+            buf[i][j] = i*100+j;
+        }
+    }
+
+    /* Dataset info */
+    data.buf = (void *)buf;
+    data.mem_type_id = H5T_NATIVE_INT;
+    data.dims = dims;
+    data.n_dims = 2;
+    data.dset_name = SPLITTER_DATASET_NAME;
+
+    /* Stand-in for manual FAPL creation
+     * Enables verification with arbitrary VFDs via `make check-vfd`
+     */
+    child_fapl_id = h5_fileaccess();
+    if (child_fapl_id < 0) {
+        TEST_ERROR;
+    }
+
+    if (!driver_is_splitter_compatible(child_fapl_id)) {
+        SKIPPED();
+        HDprintf("  given driver is not Splitter W/O compatible.\n");
+        return 0;
+    }
+
+    /* Test Read-Only access, including when a file on the W/O channel
+     * does not exist.
+     */
+    if (splitter_RO_test(&data, child_fapl_id) < 0) {
+        TEST_ERROR;
+    }
+
+    /* Test opening of files when the W/O channel does not exist.
+     */
+    if (splitter_tentative_open_test(child_fapl_id) < 0) {
+        TEST_ERROR;
+    }
+
+
+    /* Test file creation, utilizing different child FAPLs (default vs.
+     * specified), logfile, and Write Channel error ignoring behavior.
+     */
+    for (i=0; i < 4; i++) {
+        hbool_t ignore_wo_errors = (i & 1) ? TRUE : FALSE;
+        hbool_t provide_logfile_path = (i & 2) ? TRUE : FALSE;
+        hid_t   child_fapl_ids[2] = { H5P_DEFAULT, H5P_DEFAULT };
+
+        /* Test child driver definition/default combination */
+        for (j=0; j < 4; j++) {
+
+            child_fapl_ids[0] = (j & 1) ? child_fapl_id : H5P_DEFAULT;
+            child_fapl_ids[1] = (j & 2) ? child_fapl_id : H5P_DEFAULT;
+
+            if (run_splitter_test(&data, ignore_wo_errors, provide_logfile_path, child_fapl_ids) < 0) {
+                TEST_ERROR;
+            }
+
+        } /* end for child fapl definition/pairing */
+
+    } /* end for behavior-flag loops */
+
+/* TODO: SWMR open? */
+/* Concurrent opens with both drivers using the Splitter */
+
+
+    if (H5Pclose(child_fapl_id) == FAIL) {
+        TEST_ERROR;
+    }
+
+    PASSED();
+    return 0;
+
+error:
+    if (child_fapl_id != H5I_INVALID_HID) {
+        (void)H5Pclose(child_fapl_id);
+    }
+    return -1;
+} /* end test_splitter() */
+
+#undef SPLITTER_TEST_FAULT
+
+
 /*-------------------------------------------------------------------------
  * Function:    main
  *
@@ -2187,9 +3276,6 @@ error:
  * Return:      Success:        0
  *              Failure:        1
  *
- * Programmer:  Raymond Lu
- *              Tuesday, Sept 24, 2002
- *
  *-------------------------------------------------------------------------
  */
 int
@@ -2213,6 +3299,7 @@ main(void)
     nerrors += test_stdio() < 0          ? 1 : 0;
     nerrors += test_windows() < 0        ? 1 : 0;
     nerrors += test_ros3() < 0           ? 1 : 0;
+    nerrors += test_splitter() < 0       ? 1 : 0;
 
     if(nerrors) {
         HDprintf("***** %d Virtual File Driver TEST%s FAILED! *****\n",
diff --git a/utils/CMakeLists.txt b/utils/CMakeLists.txt
new file mode 100644
index 0000000..2d5626e
--- /dev/null
+++ b/utils/CMakeLists.txt
@@ -0,0 +1,4 @@
+cmake_minimum_required (VERSION 3.10)
+project (HDF5_UTILS C)
+
+add_subdirectory (mirror_vfd)
diff --git a/utils/COPYING b/utils/COPYING
new file mode 100644
index 0000000..4a23112
--- /dev/null
+++ b/utils/COPYING
@@ -0,0 +1,12 @@
+
+  Copyright by The HDF Group.
+  All rights reserved.
+
+  The files and subdirectories in this directory are part of HDF5.
+  The full HDF5 copyright notice, including terms governing use,
+  modification, and redistribution, is contained in the COPYING file
+  which can be found at the root of the source code distribution tree
+  or in https://support.hdfgroup.org/ftp/HDF5/releases.  If you do
+  not have access to either file, you may request a copy from
+  help@hdfgroup.org.
+
diff --git a/utils/Makefile.am b/utils/Makefile.am
new file mode 100644
index 0000000..b4409ac
--- /dev/null
+++ b/utils/Makefile.am
@@ -0,0 +1,26 @@
+#
+# Copyright by The HDF Group.
+# All rights reserved.
+#
+# This file is part of HDF5.  The full HDF5 copyright notice, including
+# terms governing use, modification, and redistribution, is contained in
+# the COPYING file, which can be found at the root of the source code
+# distribution tree, or in https://support.hdfgroup.org/ftp/HDF5/releases.
+# If you do not have access to either file, you may request a copy from
+# help@hdfgroup.org.
+##
+## Makefile.am
+## Run automake to generate a Makefile.in from this file.
+##
+#
+# Utils HDF5 Makefile(.in)
+#
+
+include $(top_srcdir)/config/commence.am
+
+CONFIG=ordered
+
+# All subdirectories
+SUBDIRS=mirror_vfd
+
+include $(top_srcdir)/config/conclude.am
diff --git a/utils/mirror_vfd/CMakeLists.txt b/utils/mirror_vfd/CMakeLists.txt
new file mode 100644
index 0000000..1926352
--- /dev/null
+++ b/utils/mirror_vfd/CMakeLists.txt
@@ -0,0 +1,64 @@
+cmake_minimum_required (VERSION 3.10)
+project (HDF5_UTILS_MIRRORVFD C)
+
+#-----------------------------------------------------------------------------
+# Add the mirror_server executable
+#-----------------------------------------------------------------------------
+
+set (mirror_server_SOURCES
+        ${HDF5_UTILS_MIRRORVFD_SOURCE_DIR}/mirror_remote.c
+        ${HDF5_UTILS_MIRRORVFD_SOURCE_DIR}/mirror_server.c
+        ${HDF5_UTILS_MIRRORVFD_SOURCE_DIR}/mirror_writer.c)
+add_executable (mirror_server ${mirror_server_SOURCES})
+target_include_directories (mirror_server PRIVATE "${HDF5_UITLS_DIR};${HDF5_SRC_DIR};${HDF5_BINARY_DIR};$<$<BOOL:${HDF5_ENABLE_PARALLEL}>:${MPI_C_INCLUDE_DIRS}>")
+if (NOT BUILD_SHARED_LIBS)
+  TARGET_C_PROPERTIES (mirror_server STATIC)
+  target_link_libraries (mirror_server PRIVATE ${HDF5_TOOLS_LIB_TARGET} ${HDF5_LIB_TARGET})
+else ()
+  TARGET_C_PROPERTIES (mirror_server SHARED)
+  target_link_libraries (mirror_server PRIVATE ${HDF5_TOOLS_LIBSH_TARGET} ${HDF5_LIBSH_TARGET})
+endif ()
+set_target_properties (mirror_server PROPERTIES FOLDER utils)
+set_global_variable (HDF5_UTILS_TO_EXPORT "${HDF5_UTILS_TO_EXPORT};mirror_server")
+set (H5_DEP_EXECUTABLES ${H5_DEP_EXECUTABLES} mirror_server)
+
+#-----------------------------------------------------------------------------
+# Add the mirror_server_stop executable
+#-----------------------------------------------------------------------------
+
+set (mirror_server_stop_SOURCES ${HDF5_UTILS_MIRRORVFD_SOURCE_DIR}/mirror_server_stop.c)
+add_executable (mirror_server_stop ${mirror_server_stop_SOURCES})
+target_include_directories (mirror_server_stop PRIVATE "${HDF5_UITLS_DIR};${HDF5_SRC_DIR};${HDF5_BINARY_DIR};$<$<BOOL:${HDF5_ENABLE_PARALLEL}>:${MPI_C_INCLUDE_DIRS}>")
+if (NOT BUILD_SHARED_LIBS)
+  TARGET_C_PROPERTIES (mirror_server_stop STATIC)
+  target_link_libraries (mirror_server_stop PRIVATE ${HDF5_TOOLS_LIB_TARGET} ${HDF5_LIB_TARGET})
+else ()
+  TARGET_C_PROPERTIES (mirror_server_stop SHARED)
+  target_link_libraries (mirror_server_stop PRIVATE ${HDF5_TOOLS_LIBSH_TARGET} ${HDF5_LIBSH_TARGET})
+endif ()
+set_target_properties (mirror_server_stop PROPERTIES FOLDER utils)
+set_global_variable (HDF5_UTILS_TO_EXPORT "${HDF5_UTILS_TO_EXPORT};mirror_server_stop")
+set (H5_DEP_EXECUTABLES ${H5_DEP_EXECUTABLES} mirror_server_stop)
+
+##############################################################################
+##############################################################################
+###           I N S T A L L A T I O N                                      ###
+##############################################################################
+##############################################################################
+
+#-----------------------------------------------------------------------------
+# Rules for Installation of tools using make Install target
+#-----------------------------------------------------------------------------
+if (HDF5_EXPORTED_TARGETS)
+  foreach (exec ${H5_DEP_EXECUTABLES})
+    INSTALL_PROGRAM_PDB (${exec} ${HDF5_INSTALL_BIN_DIR} utilsapplications)
+  endforeach ()
+
+  install (
+      TARGETS
+          ${H5_DEP_EXECUTABLES}
+      EXPORT
+          ${HDF5_EXPORTED_TARGETS}
+      RUNTIME DESTINATION ${HDF5_INSTALL_BIN_DIR} COMPONENT utilsapplications
+  )
+endif ()
diff --git a/utils/mirror_vfd/Makefile.am b/utils/mirror_vfd/Makefile.am
new file mode 100644
index 0000000..96d3104
--- /dev/null
+++ b/utils/mirror_vfd/Makefile.am
@@ -0,0 +1,30 @@
+#
+# Copyright by The HDF Group.
+# All rights reserved.
+#
+# This file is part of HDF5.  The full HDF5 copyright notice, including
+# terms governing use, modification, and redistribution, is contained in
+# the COPYING file, which can be found at the root of the source code
+# distribution tree, or in https://support.hdfgroup.org/ftp/HDF5/releases.
+# If you do not have access to either file, you may request a copy from
+# help@hdfgroup.org.
+##
+## Makefile.am
+## Run automake to generate a Makefile.in from this file.
+#
+# HDF5 Library Makefile(.in)
+#
+
+include $(top_srcdir)/config/commence.am
+
+AM_CPPFLAGS+=-I$(top_srcdir)/src
+
+bin_PROGRAMS = mirror_server mirror_server_stop
+
+mirror_server_SOURCES = mirror_server.c mirror_writer.c mirror_remote.c
+#mirror_writer_SOURCES = mirror_writer.c mirror_remote.c
+
+# All programs depend on the hdf5 library
+LDADD=$(LIBHDF5)
+
+include $(top_srcdir)/config/conclude.am
diff --git a/utils/mirror_vfd/mirror_remote.c b/utils/mirror_vfd/mirror_remote.c
new file mode 100644
index 0000000..81a3625
--- /dev/null
+++ b/utils/mirror_vfd/mirror_remote.c
@@ -0,0 +1,225 @@
+/* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
+ * Copyright by The HDF Group.                                               *
+ * All rights reserved.                                                      *
+ *                                                                           *
+ * This file is part of HDF5.  The full HDF5 copyright notice, including     *
+ * terms governing use, modification, and redistribution, is contained in    *
+ * the COPYING file, which can be found at the root of the source code       *
+ * distribution tree, or in https://support.hdfgroup.org/ftp/HDF5/releases.  *
+ * If you do not have access to either file, you may request a copy from     *
+ * help@hdfgroup.org.                                                        *
+ * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */
+
+/* Common operations for "remote" processes for the Mirror VFD.
+ *
+ * Jacob Smith, 2020-03-06
+ */
+
+#include "mirror_remote.h"
+
+#ifdef H5_HAVE_MIRROR_VFD
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    mirror_log
+ *
+ * Purpose:     Write message to the logging stream/file.
+ *              If logging info pointer is NULL, uses logging defaults.
+ * ----------------------------------------------------------------------------
+ */
+void
+mirror_log(struct mirror_log_info *info,
+           unsigned int            level,
+           const char             *format,
+           ...)
+{
+    FILE *stream = MIRROR_LOG_DEFAULT_STREAM;
+    unsigned int verbosity = MIRROR_LOG_DEFAULT_VERBOSITY;
+    hbool_t      custom = FALSE;
+
+    if (info != NULL && info->magic == MIRROR_LOG_INFO_MAGIC) {
+        stream = info->stream;
+        verbosity = info->verbosity;
+        custom = TRUE;
+    }
+
+    if (level == V_NONE) {
+        return;
+    }
+    else
+    if (level <= verbosity) {
+        if (custom == TRUE && info->prefix[0] != '\0') {
+            HDfprintf(stream, "%s", info->prefix);
+        }
+
+        switch (level) {
+        case (V_ERR) :
+            HDfprintf(stream, "ERROR ");
+            break;
+        case (V_WARN) :
+            HDfprintf(stream, "WARNING ");
+            break;
+        default:
+            break;
+        }
+
+        if (format != NULL) {
+            va_list args;
+            HDva_start(args, format);
+            HDvfprintf(stream, format, args);
+            HDva_end(args);
+        }
+
+        HDfprintf(stream, "\n");
+        HDfflush(stream);
+    } /* end if sufficiently verbose to print */
+} /* end mirror_log() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    session_log_bytes
+ *
+ * Purpose:     "Pretty-print" raw binary data to logging stream/file.
+ *              If info pointer is NULL, uses logging defaults.
+ * ----------------------------------------------------------------------------
+ */
+void
+mirror_log_bytes(struct mirror_log_info *info,
+                 unsigned int            level,
+                 size_t                  n_bytes,
+                 const unsigned char    *buf)
+{
+    FILE *stream = MIRROR_LOG_DEFAULT_STREAM;
+    unsigned int verbosity = MIRROR_LOG_DEFAULT_VERBOSITY;
+
+    if (buf == NULL) {
+        return;
+    }
+
+    if (info != NULL && info->magic == MIRROR_LOG_INFO_MAGIC) {
+        stream = info->stream;
+        verbosity = info->verbosity;
+    }
+
+    if (level <= verbosity) {
+        size_t bytes_written = 0;
+        const unsigned char *b = NULL;
+
+        /* print whole lines */
+        while ((n_bytes - bytes_written) >= 32) {
+            b = buf + bytes_written; /* point to region in buffer */
+            HDfprintf(stream,
+                    "%04zX  %02X%02X%02X%02X %02X%02X%02X%02X" \
+                          " %02X%02X%02X%02X %02X%02X%02X%02X" \
+                          " %02X%02X%02X%02X %02X%02X%02X%02X" \
+                          " %02X%02X%02X%02X %02X%02X%02X%02X\n",
+                    bytes_written,
+                    b[0], b[1], b[2], b[3],   b[4], b[5], b[6], b[7],
+                    b[8], b[9], b[10],b[11],  b[12],b[13],b[14],b[15],
+                    b[16],b[17],b[18],b[19],  b[20],b[21],b[22],b[23],
+                    b[24],b[25],b[26],b[27],  b[28],b[29],b[30],b[31]);
+            bytes_written += 32;
+        }
+
+        /* start partial line */
+        if (n_bytes > bytes_written) {
+            HDfprintf(stream, "%04zX ", bytes_written);
+        }
+
+        /* partial line blocks */
+        while ((n_bytes - bytes_written) >= 4) {
+            HDfprintf(stream, " %02X%02X%02X%02X",
+                    buf[bytes_written],   buf[bytes_written+1],
+                    buf[bytes_written+2], buf[bytes_written+3]);
+            bytes_written += 4;
+        }
+
+        /* block separator before partial block */
+        if (n_bytes > bytes_written) {
+            HDfprintf(stream, " ");
+        }
+
+        /* partial block individual bytes */
+        while (n_bytes > bytes_written) {
+            HDfprintf(stream, "%02X", buf[bytes_written++]);
+        }
+
+        /* end partial line */
+        HDfprintf(stream, "\n");
+    } /* end if suitably verbose to log */
+} /* end mirror_log_bytes() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    mirror_log_init
+ *
+ * Purpose:     Prepare a loginfo_t structure for use.
+ *
+ * Return:      Success: Pointer to newly-ceated info.
+ *              Failure: NULL. Either unable to allocate or cannot open file.
+ * ----------------------------------------------------------------------------
+ */
+loginfo_t *
+mirror_log_init(char *path, char *prefix, unsigned int verbosity)
+{
+    loginfo_t *info = NULL;
+
+    info = (loginfo_t *)HDmalloc(sizeof(loginfo_t));
+    if (info != NULL) {
+        info->magic = MIRROR_LOG_INFO_MAGIC;
+        info->verbosity = verbosity;
+        info->stream = MIRROR_LOG_DEFAULT_STREAM;
+        info->prefix[0] = '\0';
+
+        if (prefix && *prefix) {
+            HDstrncpy(info->prefix, prefix, MIRROR_LOG_PREFIX_MAX);
+        }
+
+        if (path && *path) {
+            FILE *f = NULL;
+            f = HDfopen(path, "w");
+            if (NULL == f) {
+                HDfprintf(MIRROR_LOG_DEFAULT_STREAM,
+                        "WARN custom logging path could not be opened: %s\n",
+                        path);
+                info->magic += 1;
+                HDfree(info);
+            }
+            else {
+                info->stream = f;
+            }
+        }
+
+    } /* end if able to allocate */
+
+    return info;
+} /* end mirror_log_init() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    mirror_log_term
+ *
+ * Purpose:     Shut down and clean up a loginfo_t structure.
+ *
+ * Return:      Success: SUCCEED. Resources released.
+ *              Failure: FAIL.    Indeterminite state.
+ * ----------------------------------------------------------------------------
+ */
+herr_t
+mirror_log_term(loginfo_t *info)
+{
+    if (info == NULL || info->magic != MIRROR_LOG_INFO_MAGIC) {
+        return FAIL;
+    }
+    if (info->stream != stderr || info->stream != stdout) {
+        if (HDfclose(info->stream) < 0) {
+            return FAIL;
+        }
+    }
+    info->magic += 1;
+    HDfree(info);
+    return SUCCEED;
+} /* end mirror_log_term() */
+
+#endif /* H5_HAVE_MIRROR_VFD */
+
diff --git a/utils/mirror_vfd/mirror_remote.h b/utils/mirror_vfd/mirror_remote.h
new file mode 100644
index 0000000..9132d51
--- /dev/null
+++ b/utils/mirror_vfd/mirror_remote.h
@@ -0,0 +1,51 @@
+/* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
+ * Copyright by The HDF Group.                                               *
+ * All rights reserved.                                                      *
+ *                                                                           *
+ * This file is part of HDF5.  The full HDF5 copyright notice, including     *
+ * terms governing use, modification, and redistribution, is contained in    *
+ * the COPYING file, which can be found at the root of the source code       *
+ * distribution tree, or in https://support.hdfgroup.org/ftp/HDF5/releases.  *
+ * If you do not have access to either file, you may request a copy from     *
+ * help@hdfgroup.org.                                                        *
+ * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */
+
+/* Common definitions for "remote" processes for the Mirror VFD.
+ *
+ * Jacob Smith, 2020-03-06
+ */
+
+#include "hdf5.h"
+#include "H5private.h"
+
+#ifdef H5_HAVE_MIRROR_VFD
+
+#define V_NONE 0
+#define V_ERR  1
+#define V_WARN 2
+#define V_INFO 3
+#define V_ALL  4
+
+#define MIRROR_LOG_DEFAULT_STREAM stdout
+#define MIRROR_LOG_DEFAULT_VERBOSITY V_WARN
+#define MIRROR_LOG_PREFIX_MAX 79
+#define MIRROR_LOG_INFO_MAGIC 0x569D589A
+
+typedef struct mirror_log_info {
+    uint32_t magic;
+    FILE     *stream;
+    unsigned int verbosity;
+    char     prefix[MIRROR_LOG_PREFIX_MAX+1];
+} loginfo_t;
+
+void mirror_log(loginfo_t *info, unsigned int level,
+        const char *format, ...);
+void mirror_log_bytes(loginfo_t *info, unsigned int level,
+        size_t n_bytes, const unsigned char *buf);
+loginfo_t *mirror_log_init(char *path, char *prefix, unsigned int verbosity);
+int mirror_log_term(loginfo_t *loginfo);
+
+herr_t run_writer(int socketfd, H5FD_mirror_xmit_open_t *xmit_open);
+
+#endif /* H5_HAVE_MIRROR_VFD */
+
diff --git a/utils/mirror_vfd/mirror_server.c b/utils/mirror_vfd/mirror_server.c
new file mode 100644
index 0000000..2583e60
--- /dev/null
+++ b/utils/mirror_vfd/mirror_server.c
@@ -0,0 +1,666 @@
+/* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
+ * Copyright by The HDF Group.                                               *
+ * All rights reserved.                                                      *
+ *                                                                           *
+ * This file is part of HDF5.  The full HDF5 copyright notice, including     *
+ * terms governing use, modification, and redistribution, is contained in    *
+ * the COPYING file, which can be found at the root of the source code       *
+ * distribution tree, or in https://support.hdfgroup.org/ftp/HDF5/releases.  *
+ * If you do not have access to either file, you may request a copy from     *
+ * help@hdfgroup.org.                                                        *
+ * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */
+
+/*
+ * "Server" application to associate a Mirror VFD Driver with a Writer.
+ *
+ * Server waits on a dedicated port for a Driver to attempt to connect.
+ * When connection is made, reads a message from the Driver.
+ * If message is "SHUTDOWN", Server closes connection and terminates.
+ * Else, if it receives an encoded OPEN xmit (from Driver), the Server forks
+ * itself; the child becomes a dedicated Writer and maintains connection with
+ * the Driver instance, and the parent process remains a Server and returns
+ * to listening for incoming requests.
+ * Else, the message is not recognized and is ignored.
+ *
+ *
+ *
+ * mirror_server [args]
+ *
+ * Primary server for coordinating mirror VFD connections with the remote
+ * process.
+ *
+ * args:
+ * --help, -h     Print help message and exit.
+ * --port=N       Positive integer for primary listening port.
+ * --verbosity=N  Debugging verbosity
+ *                0: none
+ *                1: errors
+ *                2: details
+ *                3: all
+ * --logpath=S    File path to direct debugging output, if any.
+ *                Default of none prints output to stdout.
+ *
+ */
+
+#include "mirror_remote.h"
+
+#ifdef H5_HAVE_MIRROR_VFD
+
+#define MAXBUF 2048             /* max buffer length.                        */
+#define LISTENQ 80              /* max pending mirrorS requests              */
+#define DEFAULT_PORT 3000       /* default listening port                    */
+#define MAX_PORT_LOOPS 20       /* max iteratations through port range       */
+#define PORT_LOOP_RETRY_DELAY 1 /* seconds to wait between port scans        */
+
+/* semi-unique "magic" numbers to sanity-check structure pointers */
+#define OP_ARGS_MAGIC    0xCF074379u
+#define SERVER_RUN_MAGIC 0x741B459Au
+
+
+/* ---------------------------------------------------------------------------
+ * Structure:   struct op_args
+ *
+ * Purpose:     Convenience structure for holding arguments from command-line.
+ *
+ * `magic` (uint32_t)
+ *      Semi-unique number to help validate a pointer to this struct type.
+ *      Must be OP_ARGS_MAGIC to be considered valid.
+ *
+ * `help` (int)
+ *      Flag that the help argument was present in the command line.
+ *
+ * `main_port` (int)
+ *      Flag that the help argument was present in the command line.
+ *
+ * `verbosity` (int)
+ *      Number between 0 (none) and 4 (all) that controls how much detail
+ *      the program prints as a course of logging.
+ *
+ * `log_prepend_serv` (int)
+ *      Flag that the logging messages should have 'S- ' at the start of each
+ *      line.
+ *
+ * `log_prepend_type` (int)
+ *      Flag that the logging messages should have the assocaited verbosity
+ *      level present in the line (e.g., "WARN", "ERROR", or "INFO").
+ *
+ * `log_path` (char *)
+ *      Path string from the command line, giving the absolute path
+ *      for the file for logging output. Can be empty.
+ *
+ * `writer_log_path` (char *)
+ *      Path string from the command line, giving the absolute path
+ *      for the file for writer's logging output. Can be empty.
+ *
+ * ---------------------------------------------------------------------------
+ */
+struct op_args {
+    uint32_t    magic;
+    int         help;
+    int         main_port;
+    int         verbosity;
+    int         log_prepend_serv;
+    int         log_prepend_type;
+    char        log_path[PATH_MAX+1];
+    char        writer_log_path[PATH_MAX+1];
+};
+
+
+/* ---------------------------------------------------------------------------
+ * Structure:   struct server_run
+ *
+ * Purpose:     Convenience structure for holding information about a server
+ *              in operation.
+ *
+ * `magic` (uint32_t)
+ *      Semi-unique number to help validate a pointer to this struct type.
+ *      Must be SERVER_RUN_MAGIC to be considered valid.
+ *
+ * `log_stream` (FILE *)
+ *      File handle where logging output is directed.
+ *      By default, is stdout.
+ *
+ * `opts` (struct opt_args)
+ *      Contained structure, holds the server's configuration.
+ *
+ * `listenfd` (int)
+ *      File descriptor of the listening socket.
+ *
+ * ---------------------------------------------------------------------------
+ */
+struct server_run {
+    uint32_t            magic;
+    struct op_args      opts;
+    struct mirror_log_info *loginfo;
+    int                 listenfd;
+};
+
+
+/* ---------------------------------------------------------------------------
+ * Function:   mybzero
+ *
+ * Purpose:    Introduce bzero without neededing it on the system.
+ *
+ * Programmer: Jacob Smith
+ *             2020-03-30
+ * ---------------------------------------------------------------------------
+ */
+static void mybzero(void *dest, size_t size)
+{
+    size_t i = 0;
+    char  *s = NULL;
+    HDassert(dest);
+    s = (char *)dest;
+    for (i = 0; i < size; i++) {
+        *(s+i) = 0;
+    }
+} /* end mybzero() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    usage
+ *
+ * Purpose:     Print the usage message to stdout.
+ * ---------------------------------------------------------------------------
+ */
+static void
+usage(void)
+{
+    HDfprintf(stdout,
+              "mirror_server [options]\n"                                     \
+              "\n"                                                            \
+              "Application for providing Mirror Writer process to "           \
+              " Mirror VFD on file-open.\n"                                   \
+              "Listens on a dedicated socket; forks as a Writer upon receipt" \
+              " of a valid OPEN xmit.\n"                                      \
+              "\n"                                                            \
+              "Options:\n"                                                    \
+              "--help [-h]         : Print this help message and quit.\n"     \
+              "--logpath=PATH      : File path for logging output "           \
+                  "(default none, to stdout).\n"                              \
+              "--port=PORT         : Primary port (default %d).\n"            \
+              "--verbosity=NUM     : Debug printing level "                   \
+                  "0..4, (default %d).\n",
+              DEFAULT_PORT,
+              MIRROR_LOG_DEFAULT_VERBOSITY);
+} /* end usage() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    parse_args
+ *
+ * Purpose:     Read command line options and store results in args_out
+ *              structure. Fails in event of unrecognized option.
+ *
+ * Return:      0 on success, -1 on failure.
+ * ---------------------------------------------------------------------------
+ */
+static int
+parse_args(int argc, char **argv, struct op_args *args_out)
+{
+    int i;
+
+    /* preset default values
+     */
+    args_out->main_port = DEFAULT_PORT;
+    args_out->help = 0;
+    args_out->log_prepend_serv = 1;
+    args_out->log_prepend_type = 1;
+    args_out->verbosity = MIRROR_LOG_DEFAULT_VERBOSITY;
+    /* preset empty strings */
+    mybzero(args_out->log_path,        PATH_MAX+1);
+    mybzero(args_out->writer_log_path, PATH_MAX+1);
+
+    if (argv == NULL || *argv == NULL) {
+        mirror_log(NULL, V_ERR, "invalid argv pointer");
+        return -1;
+    }
+
+    /* Loop over arguments after program name and writer_path */
+    for (i=2; i < argc; i++) {
+        if (!HDstrncmp(argv[i], "-h", 3) || !HDstrncmp(argv[i], "--help", 7)) {
+            mirror_log(NULL, V_INFO, "found help argument");
+            args_out->help = 1;
+            return 0;
+        } /* end if help */
+        else
+        if (!HDstrncmp(argv[i], "--port=", 7)) {
+            mirror_log(NULL, V_INFO, "parsing 'main_port' (%s)", argv[i]+7);
+            args_out->main_port = HDatoi(argv[i]+7);
+        } /* end if port */
+        else
+        if (!HDstrncmp(argv[i], "--verbosity=", 12)) {
+            mirror_log(NULL, V_INFO, "parsing 'verbosity' (%s)", argv[i]+12);
+            args_out->verbosity = HDatoi(argv[i]+12);
+        } /* end if verbosity */
+        else
+        if (!HDstrncmp(argv[i], "--logpath=", 10)) {
+            mirror_log(NULL, V_INFO, "parsing 'logpath' (%s)", argv[i]+10);
+            HDstrncpy(args_out->log_path, argv[i]+10, PATH_MAX);
+        } /* end if logpath */
+        else {
+            mirror_log(NULL, V_ERR, "unrecognized argument: %s", argv[i]);
+            return -1;
+        } /* end if unrecognized argument */
+    } /* end for each arg after the path to writer "receiver process" */
+
+    mirror_log(NULL, V_INFO, "all args parsed");
+
+    return 0;
+} /* end parse_args() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    prepare_listening_socket
+ *
+ * Purpose:     Configure and open a socket.
+ *              In event of error, attempts to undo its processes.
+ *
+ * Return:      Success: non-negative (the file descriptor of the socket)
+ *              Failure: -1
+ * ---------------------------------------------------------------------------
+ */
+static int
+prepare_listening_socket(struct server_run *run)
+{
+    struct sockaddr_in server_addr;
+    int _true = 1; /* needed for setsockopt() */
+    int ret_value = -1;
+    int ret = 0; /* for checking return value of function calls */
+
+    if (run == NULL || run->magic != SERVER_RUN_MAGIC) {
+        mirror_log(NULL, V_ERR, "invalid server_run pointer");
+        return -1;
+    }
+
+    mirror_log(run->loginfo, V_INFO, "preparing socket");
+
+    server_addr.sin_family      = AF_INET;
+    server_addr.sin_addr.s_addr = HDhtonl(INADDR_ANY);
+    server_addr.sin_port        = HDhtons((uint16_t)run->opts.main_port);
+
+    mirror_log(run->loginfo, V_INFO, "socket()");
+    ret_value = HDsocket(AF_INET, SOCK_STREAM, 0);
+    if (ret_value < 0) {
+        mirror_log(run->loginfo, V_ERR, "listening socket:%d", ret_value);
+        goto error;
+    }
+
+    mirror_log(run->loginfo, V_ALL, "setsockopt()");
+    HDsetsockopt(ret_value, SOL_SOCKET, SO_REUSEADDR, &_true, sizeof(int));
+
+    mirror_log(run->loginfo, V_INFO, "bind()");
+    ret = HDbind(ret_value, (struct sockaddr *)&server_addr,
+            sizeof(server_addr));
+    if (ret < 0) {
+        mirror_log(run->loginfo, V_ERR, "bind() %s", HDstrerror(errno));
+        goto error;
+    }
+
+    mirror_log(run->loginfo, V_INFO, "listen()");
+    ret = HDlisten(ret_value, LISTENQ);
+    if (ret < 0) {
+        mirror_log(run->loginfo, V_ERR, "H5FD server listen:%d", ret);
+        goto error;
+    }
+
+    return ret_value;
+
+error:
+    if (ret_value >= 0) {
+        HDshutdown(ret_value, SHUT_RDWR);
+        HDclose(ret_value);
+    }
+    return -1;
+} /* end prepare_listening_socket() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    init_server_run
+ *
+ * Purpose:     Set up server_run struct with default and specified values.
+ *
+ * Return:      Zero (0) if successful, -1 if an error occurred.
+ * ---------------------------------------------------------------------------
+ */
+static struct server_run *
+init_server_run(int argc, char **argv)
+{
+    struct server_run *run;
+
+    run = (struct server_run *)HDmalloc(sizeof(struct server_run));
+    if (run == NULL) {
+        mirror_log(NULL, V_ERR, "can't allocate server_run struct");
+        return NULL;
+    }
+
+    run->magic = (uint32_t)SERVER_RUN_MAGIC;
+    run->opts.magic = (uint32_t)OP_ARGS_MAGIC;
+    run->listenfd = -1;
+
+    if (parse_args(argc, argv, &(run->opts)) < 0) {
+        mirror_log(NULL, V_ERR, "can't parse arguments");
+        usage();
+        goto error;
+    }
+
+    if (run->opts.help) {
+        usage();
+        return run; /* early exit */
+    }
+
+    run->loginfo = mirror_log_init(run->opts.log_path, "s- ",
+            run->opts.verbosity);
+
+    run->listenfd = prepare_listening_socket(run);
+    if (run->listenfd < 0) {
+        mirror_log(NULL, V_ERR, "can't prepare listening socket");
+        goto error;
+    }
+
+    return run;
+
+error:
+    if (run != NULL) {
+        HDfree(run);
+    }
+    return NULL;
+
+} /* end init_server_run() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    term_server_run
+ *
+ * Purpose:     Close opened items in a sever_run and release the pointer.
+ *
+ * Return:      Zero (0) if successful, -1 if an error occurred.
+ * ---------------------------------------------------------------------------
+ */
+static int
+term_server_run(struct server_run *run)
+{
+    if (run == NULL || run->magic != SERVER_RUN_MAGIC) {
+        mirror_log(NULL, V_ERR, "invalid server_run pointer");
+        return -1;
+    }
+
+    mirror_log(run->loginfo, V_INFO, "shutting down");
+
+    if (run->listenfd >= 0) {
+        HDshutdown(run->listenfd, SHUT_RDWR); /* TODO: error-checking? */
+        HDclose(run->listenfd); /* TODO: error-checking? */
+        run->listenfd = -1;
+    }
+
+    if (mirror_log_term(run->loginfo) < 0) {
+        mirror_log(NULL, V_ERR, "can't close logging stream");
+        return -1; /* doesn't solve the problem, but informs of error */
+    }
+    run->loginfo = NULL;
+
+    (run->magic)++;
+    (run->opts.magic)++;
+    HDfree(run);
+    return 0;
+} /* end term_server_run() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    accept_connection
+ *
+ * Purpose:     Main working loop; process requests as they are received.
+ *              Does nothing if the run option help is set.
+ *
+ * Return:      -1 on error, else a non-negative file descriptor of the socket.
+ * ---------------------------------------------------------------------------
+ */
+static int
+accept_connection(struct server_run *run)
+{
+    struct sockaddr_in client_addr;        /**/
+    socklen_t          clilen;             /**/
+    struct hostent    *host_port = NULL;   /**/
+    char              *hostaddrp;          /**/
+    int                connfd = -1; /* connection file descriptor */
+
+    if (run == NULL || run->magic != SERVER_RUN_MAGIC) {
+        mirror_log(NULL, V_ERR, "invalid server_run pointer");
+        return -1;
+    }
+
+    /*------------------------------*/
+    /* accept a connection on a socket */
+    clilen = sizeof(client_addr);
+    connfd = HDaccept(run->listenfd, (struct sockaddr *)&client_addr, &clilen);
+    if (connfd < 0) {
+        mirror_log(run->loginfo, V_ERR, "accept:%d", connfd);
+        goto error;
+    }
+    mirror_log(run->loginfo, V_INFO, "connection achieved");
+
+    /*------------------------------*/
+    /* get client address information */
+    host_port = HDgethostbyaddr(
+            (const char *)&client_addr.sin_addr.s_addr,
+            sizeof(client_addr.sin_addr.s_addr),
+            AF_INET);
+    if (host_port == NULL) {
+        mirror_log(run->loginfo, V_ERR, "gethostbyaddr()");
+        goto error;
+    }
+
+    /* function has the string space statically scoped -- OK until next call */
+    hostaddrp = HDinet_ntoa(client_addr.sin_addr);
+    /* TODO? proper error-checking */
+
+    mirror_log(run->loginfo, V_INFO,
+            "server connected with %s (%s)",
+            host_port->h_name,
+            hostaddrp);
+
+    return connfd;
+
+error:
+    if (connfd >= 0) {
+        close(connfd);
+    }
+    return -1;
+} /* end accept_connection() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    wait_for_child
+ *
+ * Purpose:     Signal handler to reap zombie processes.
+ * ---------------------------------------------------------------------------
+ */
+static void
+wait_for_child(int sig)
+{
+    while (HDwaitpid(-1, NULL, WNOHANG) > 0);
+} /* end wait_for_child() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    handle_requests
+ *
+ * Purpose:     Main working loop; process requests as they are received.
+ *              Does nothing if the run option `help` is set.
+ *
+ * Return:      -1 on error, else 0 for successful operation.
+ * ---------------------------------------------------------------------------
+ */
+static int
+handle_requests(struct server_run *run)
+{
+    int         connfd = -1;        /**/
+    char        mybuf[H5FD_MIRROR_XMIT_OPEN_SIZE];      /**/
+    int         ret;                /* general-purpose error-checking */
+    int         pid;                /* process ID of fork */
+    struct sigaction sa;
+    int         ret_value = 0;
+
+    if (run == NULL || run->magic != SERVER_RUN_MAGIC) {
+        mirror_log(NULL, V_ERR, "invalid server_run pointer");
+        return -1;
+    }
+
+    if (run->opts.help) {
+        return 0;
+    }
+
+    if (run->listenfd < 0) {
+        mirror_log(NULL, V_ERR, "invalid listening socket");
+        return -1;
+    }
+
+    /* Set up the signal handler */
+    sa.sa_handler = wait_for_child;
+    HDsigemptyset(&sa.sa_mask);
+    sa.sa_flags = SA_RESTART;
+    if (HDsigaction(SIGCHLD, &sa, NULL) == -1) {
+        perror("sigaction");
+        return 1;
+    }
+
+    /* Keep listening for attempts to connect.
+     */
+
+    while (1) { /* infinite loop, exited via break or goto */
+        mirror_log(run->loginfo, V_INFO, "server waiting for connections...");
+
+        connfd = -1;
+
+        connfd = accept_connection(run);
+        if (connfd < 0) {
+            mirror_log(run->loginfo, V_ERR, "unable to receive connection");
+            goto error;
+        }
+
+        /* Read handshake from port connection.
+         */
+
+        ret = (int)HDread(connfd, &mybuf, H5FD_MIRROR_XMIT_OPEN_SIZE);
+        if (-1 == ret) {
+            mirror_log(run->loginfo, V_ERR, "read:%d", ret);
+            goto error;
+        }
+        mirror_log(run->loginfo, V_INFO, "received %d bytes", ret);
+        mirror_log(run->loginfo, V_ALL, "```");
+        mirror_log_bytes(run->loginfo, V_ALL, ret,
+                (const unsigned char *)mybuf);
+        mirror_log(run->loginfo, V_ALL, "```");
+
+        /* Respond to handshake message.
+         */
+
+        if (!HDstrncmp("SHUTDOWN", mybuf, 8)) {
+            /* Stop operation if told to stop */
+            mirror_log(run->loginfo, V_INFO, "received SHUTDOWN!", ret);
+            HDclose(connfd);
+            connfd = -1;
+            goto done;
+        } /* end if explicit "SHUTDOWN" directive */
+        else
+        if (H5FD_MIRROR_XMIT_OPEN_SIZE == ret) {
+            H5FD_mirror_xmit_open_t xopen;
+
+            mirror_log(run->loginfo, V_INFO,
+                     "probable OPEN xmit received");
+
+            H5FD_mirror_xmit_decode_open(&xopen, (const unsigned char *)mybuf);
+            if (FALSE == H5FD_mirror_xmit_is_open(&xopen)) {
+                mirror_log(run->loginfo, V_WARN,
+                         "expected OPEN xmit was malformed");
+                HDclose(connfd);
+                continue;
+            }
+
+            mirror_log(run->loginfo, V_INFO,
+                     "probable OPEN xmit confirmed");
+
+            pid = HDfork();
+            if (pid < 0) { /* fork error */
+                mirror_log(run->loginfo, V_ERR, "cannot fork");
+                goto error;
+            } /* end if fork error */
+            else
+            if (pid == 0) { /* child process (writer side of fork) */
+                mirror_log(run->loginfo, V_INFO,
+                         "executing writer");
+                if (run_writer(connfd, &xopen) < 0) {
+                    HDprintf("can't run writer\n");
+                }
+                else {
+                    HDprintf("writer OK\n");
+                }
+                HDclose(connfd);
+
+                HDexit(EXIT_SUCCESS);
+            } /* end if writer side of fork */
+            else { /* parent process (server side of fork) */
+                mirror_log(run->loginfo, V_INFO, "tidying up from handshake");
+                HDclose(connfd);
+            } /* end if server side of fork */
+
+        } /* end else-if valid request for service */
+        else {
+            /* Ignore unrecognized messages */
+            HDclose(connfd);
+            continue;
+        } /* end else (not a valid message, to be ignored) */
+
+    } /* end while listening for new connections */
+
+done:
+    if (connfd >= 0) {
+        mirror_log(run->loginfo, V_WARN, "connfd still open upon cleanup");
+        HDclose(connfd);
+    }
+
+    return ret_value;
+
+error:
+    if (connfd >= 0) {
+        HDclose(connfd);
+    }
+    return -1;
+} /* end handle_requests() */
+
+
+/* ------------------------------------------------------------------------- */
+int
+main(int argc, char **argv)
+{
+    struct server_run  *run;
+
+    run = init_server_run(argc, argv);
+    if (NULL == run) {
+        mirror_log(NULL, V_ERR, "can't initialize run");
+        HDexit(EXIT_FAILURE);
+    }
+
+    if (handle_requests(run) < 0) {
+        mirror_log(run->loginfo, V_ERR, "problem handling requests");
+    }
+
+    if (term_server_run(run) < 0) {
+        mirror_log(NULL, V_ERR, "problem closing server run");
+        HDexit(EXIT_FAILURE);
+    }
+
+    HDexit(EXIT_SUCCESS);
+} /* end main() */
+
+#else /* H5_HAVE_MIRROR_VFD */
+
+int
+main(int argc, char **argv)
+{
+    HDprintf("Mirror VFD was not built -- cannot launch server.\n");
+    HDexit(EXIT_FAILURE);
+}
+
+#endif /* H5_HAVE_MIRROR_VFD */
+
diff --git a/utils/mirror_vfd/mirror_server_stop.c b/utils/mirror_vfd/mirror_server_stop.c
new file mode 100644
index 0000000..ccd5824
--- /dev/null
+++ b/utils/mirror_vfd/mirror_server_stop.c
@@ -0,0 +1,214 @@
+/* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
+ * Copyright by The HDF Group.                                               *
+ * All rights reserved.                                                      *
+ *                                                                           *
+ * This file is part of HDF5.  The full HDF5 copyright notice, including     *
+ * terms governing use, modification, and redistribution, is contained in    *
+ * the COPYING file, which can be found at the root of the source code       *
+ * distribution tree, or in https://support.hdfgroup.org/ftp/HDF5/releases.  *
+ * If you do not have access to either file, you may request a copy from     *
+ * help@hdfgroup.org.                                                        *
+ * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */
+
+/*
+ * Purpose:     Stop the mirror server
+ *              Exists for cross-platform, optionally remote shutdown.
+ */
+
+#include "H5private.h" /* System compatability call-wrapper macros */
+
+#ifdef H5_HAVE_MIRROR_VFD
+
+#define MSHS_OPTS_MAGIC 0x613B1C15u /* sanity-checking constant */
+#define MSHS_IP_STR_SIZE 20
+#define MSHS_DEFAULT_IP "127.0.0.1"
+#define MSHS_DEFAULT_PORTNO 3000
+
+
+/* ----------------------------------------------------------------------------
+ * Structure:   struct mshs_opts
+ *
+ * Purpose:     Convenience structure to hold options as parsed from the
+ *              command line.
+ *
+ * `magic` (uint32_t)
+ *      Semi-unique constant to help verify pointer integrity.
+ *
+ * `help` (int)
+ *      Flag that the help argument was present.
+ *
+ * `portno` (int)
+ *      Port number, as received from arguments.
+ *
+ * `ip` (char *)
+ *      IP address string as received from arguments.
+ *
+ * ----------------------------------------------------------------------------
+ */
+struct mshs_opts {
+    uint32_t    magic;
+    int         help;
+    int         portno;
+    char        ip[MSHS_IP_STR_SIZE + 1];
+};
+
+
+/* ----------------------------------------------------------------------------
+ * Function:    usage
+ *
+ * Purpose:     Print usage message to stdout.
+ * ----------------------------------------------------------------------------
+ */
+static void
+usage(void)
+{
+    HDprintf("mirror_server_halten_sie [options]\n"                           \
+             "System-independent Mirror Server shutdown program.\n"           \
+             "Sends shutdown message to Mirror Server at given IP:port\n"     \
+             "\n"                                                             \
+             "Options:\n"                                                     \
+             "    -h | --help Print this usage message and exit.\n"           \
+             "    --ip=ADDR   IP Address of remote server (defaut %s)\n"      \
+             "    --port=PORT Handshake port of remote server (default %d)\n",
+             MSHS_DEFAULT_IP,
+             MSHS_DEFAULT_PORTNO);
+} /* end usage() */
+
+
+/* ----------------------------------------------------------------------------
+ * Function:    parse_args
+ *
+ * Purpose:     Parse command-line arguments, populating the options struct
+ *              pointer as appropriate.
+ *              Default values will be set for unspecified options.
+ *
+ * Return:      0 on success, negative (-1) if error.
+ * ----------------------------------------------------------------------------
+ */
+static int
+parse_args(int argc, char **argv, struct mshs_opts *opts)
+{
+    int i = 0;
+
+    opts->magic = MSHS_OPTS_MAGIC;
+    opts->help = 0;
+    opts->portno = MSHS_DEFAULT_PORTNO;
+    HDstrncpy(opts->ip, MSHS_DEFAULT_IP, MSHS_IP_STR_SIZE);
+
+    for (i=1; i < argc; i++) { /* start with first possible option argument */
+        if (!HDstrncmp(argv[i], "-h", 3) || !HDstrncmp(argv[i], "--help", 7)) {
+            opts->help = 1;
+        }
+        else
+        if (!HDstrncmp(argv[i], "--ip=", 5)) {
+            HDstrncpy(opts->ip, argv[i]+5, MSHS_IP_STR_SIZE);
+        }
+        else
+        if (!HDstrncmp(argv[i], "--port=", 7)) {
+            opts->portno = HDatoi(argv[i]+7);
+        }
+        else {
+            HDprintf("Unrecognized option: '%s'\n", argv[i]);
+            usage();
+            opts->magic++; /* invalidate for sanity */
+            return -1;
+        }
+    } /* end for each argument from command line */
+
+    /* auto-replace 'localhost' with numeric IP */
+    if (!HDstrncmp(opts->ip, "localhost", 10)) { /* include null terminator */
+        HDstrncpy(opts->ip, "127.0.0.1", MSHS_IP_STR_SIZE);
+    }
+
+    return 0;
+} /* end parse_args() */
+
+
+/* ----------------------------------------------------------------------------
+ * Function:    send_shutdown
+ *
+ * Purpose:     Create socket and send shutdown signal to remote server.
+ *
+ * Return:      0 on success, negative (-1) if error.
+ * ----------------------------------------------------------------------------
+ */
+static int
+send_shutdown(struct mshs_opts *opts)
+{
+    int                live_socket;
+    struct sockaddr_in target_addr;
+
+    if (opts->magic != MSHS_OPTS_MAGIC) {
+       HDprintf("invalid options structure\n");
+       return -1;
+    }
+
+    live_socket = HDsocket(AF_INET, SOCK_STREAM, 0);
+    if (live_socket < 0) {
+        HDprintf("ERROR socket()\n");
+        return -1;
+    }
+
+    target_addr.sin_family = AF_INET;
+    target_addr.sin_port = HDhtons((uint16_t)opts->portno);
+    target_addr.sin_addr.s_addr = HDinet_addr(opts->ip);
+    HDmemset(target_addr.sin_zero, '\0', sizeof(target_addr.sin_zero));
+
+    if (HDconnect(live_socket, (struct sockaddr *)&target_addr,
+            (socklen_t)sizeof(target_addr))
+        < 0)
+    {
+        HDprintf("ERROR connect() (%d)\n%s\n", errno, HDstrerror(errno));
+        return -1;
+    }
+
+    if (HDwrite(live_socket, "SHUTDOWN", 9) == -1) {
+        HDprintf("ERROR write() (%d)\n%s\n", errno, HDstrerror(errno));
+        return -1;
+    }
+
+    if (HDclose(live_socket) < 0) {
+        HDprintf("ERROR close() can't close socket\n");
+        return -1;
+    }
+
+    return 0;
+} /* end send_shutdown() */
+
+
+/* ------------------------------------------------------------------------- */
+int
+main(int argc, char **argv)
+{
+    struct mshs_opts opts;
+
+    if (parse_args(argc, argv, &opts) < 0) {
+        HDprintf("Unable to parse arguments\n");
+        return EXIT_FAILURE;
+    }
+
+    if (opts.help) {
+        usage();
+        return EXIT_SUCCESS;
+    }
+
+    if (send_shutdown(&opts) < 0) {
+        HDprintf("Unable to send shutdown command\n");
+        return EXIT_FAILURE;
+    }
+
+    return EXIT_SUCCESS;
+} /* end main() */
+
+#else /* H5_HAVE_MIRROR_VFD */
+
+
+/* ------------------------------------------------------------------------- */
+int
+main(int argc, char **argv)
+{
+    HDprintf("Mirror VFD not built -- unable to perform shutdown.\n");
+    return EXIT_FAILURE;
+}
+
+#endif /* H5_HAVE_MIRROR_VFD */
diff --git a/utils/mirror_vfd/mirror_writer.c b/utils/mirror_vfd/mirror_writer.c
new file mode 100644
index 0000000..e1ab1b2
--- /dev/null
+++ b/utils/mirror_vfd/mirror_writer.c
@@ -0,0 +1,1103 @@
+/* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
+ * Copyright by The HDF Group.                                               *
+ * All rights reserved.                                                      *
+ *                                                                           *
+ * This file is part of HDF5.  The full HDF5 copyright notice, including     *
+ * terms governing use, modification, and redistribution, is contained in    *
+ * the COPYING file, which can be found at the root of the source code       *
+ * distribution tree, or in https://support.hdfgroup.org/ftp/HDF5/releases.  *
+ * If you do not have access to either file, you may request a copy from     *
+ * help@hdfgroup.org.                                                        *
+ * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */
+
+/*
+ * Remote writer process for the mirror (socket) VFD.
+ *
+ * Writer is started with arguments for slaved port.
+ * Awaits a connection on the socket.
+ * Handles instructions from the master 'Driver' process.
+ *
+ * Current implementation uses Sec2 as the underlying driver when opening a
+ * file. This is reflected in the source (H5FDmirror.c) of the Mirror driver.
+ */
+
+#include "mirror_remote.h"
+
+#ifdef H5_HAVE_MIRROR_VFD
+
+#define HEXDUMP_XMITS 1           /* Toggle whether to print xmit bytes-blob */
+                                  /* in detailed logging  */
+#define HEXDUMP_WRITEDATA 0       /* Toggle whether to print bytes to write */
+                                  /* in detailed logging  */
+#define LISTENQ 80                /* max pending Driver requests */
+
+#define MW_SESSION_MAGIC        0x88F36B32u
+#define MW_SOCK_COMM_MAGIC      0xDF10A157u
+#define MW_OPTS_MAGIC   0x3BA8B462u
+
+
+/* ---------------------------------------------------------------------------
+ * Structure: struct mirror_session
+ *
+ * Bundle of information used to manage the operation of this remote Writer
+ * in a "session" with the Driver process.
+ *
+ * magic (uint32_t)
+ *       Semi-unique "magic" number used to sanity-check a structure for
+ *       validity. MUST equal MW_SESSION_MAGIC to be valid.
+ *
+ * sockfd (int)
+ *       File descriptor to the socket.
+ *       Used for receiving bytes from and writing bytes to the Driver
+ *       across the network.
+ *       If not NULL, should be a valid descriptor.
+ *
+ * token (uint32t)
+ *      Number used to help sanity-check received transmission from the Writer.
+ *      Each Driver/Writer pairing should have a semi-unique "token" to help
+ *      guard against commands from the wrong entity.
+ *
+ * xmit_count (uint32_t)
+ *      Record of trasmissions received from the Driver. While the transmission
+ *      protocol should be trustworthy, this serves as an additional guard.
+ *      Starts a 0 and should be incremented for each one-way transmission.
+ *
+ * file (H5FD_t *)
+ *      Virtual File handle for the hdf5 file.
+ *      Set on file open if H5Fopen() is successful. If NULL, it is invalid.
+ *
+ * log_verbosity (unsigned int)
+ *      The verbosity level for logging. Should be set to one of the values
+ *      defined at the top of this file.
+ *
+ * log_stream (FILE *)
+ *      File pointer to which logging output is written. Starts (and ends)
+ *      with a default stream, such as stdout, but can be overridden at
+ *      runtime.
+ *
+ * reply (H5FD_mirror_xmit_reply_t)
+ *      Structure space for persistent reply data.
+ *      Should be initialized with basic header info (magic, version, op),
+ *      then with session info (token, xmit count), and finally with specific
+ *      reply info (update xmit_count, status code, and message) before
+ *      transmission.
+ *
+ * ----------------------------------------------------------------------------
+ */
+struct mirror_session {
+    uint32_t                  magic;
+    int                       sockfd;
+    uint32_t                  token;
+    uint32_t                  xmit_count;
+    H5FD_t                   *file;
+    loginfo_t                *loginfo;
+    H5FD_mirror_xmit_reply_t  reply;
+};
+
+
+/* ---------------------------------------------------------------------------
+ * Structure: struct sock_comm
+ *
+ * Structure for placing the data read and pre-processed from Driver in an
+ * organized fashion. Useful for pre-processing a received xmit.
+ *
+ * magic (uint32_t)
+ *      Semi-unique number to sanity-check structure pointer and validity.
+ *      Must equal MW_SOCK_COMM_MAGIC to be valid.
+ *
+ * recd_die (int)
+ *      "Boolean" flag indicating that an explicit shutdown/kill/die command
+ *      was received. Potentially useful for debugging and or "manual"
+ *      operation of the program.
+ *      0 indicates normal operation, non-0 (1) indicates to die.
+ *
+ * xmit_recd (H5FD_mirror_xmit_t *)
+ *      Structure pointer for the "xmit header" as decoded from the raw
+ *      binary stream read from the socket.
+ *
+ * raw (char *)
+ *      Pointer to a raw byte array, for storing data as read from the
+ *      socket. Bytes buffer is decoded into xmit_t header and derivative
+ *      structures.
+ *
+ * raw_size (size_t)
+ *      Give the size of the `raw` buffer.
+ *
+ * ---------------------------------------------------------------------------
+ */
+struct sock_comm {
+    uint32_t            magic;
+    int                 recd_die;
+    H5FD_mirror_xmit_t *xmit_recd;
+    char               *raw;
+    size_t              raw_size;
+};
+
+
+/* ---------------------------------------------------------------------------
+ * Structure: struct mirror_writer_opts
+ *
+ * Container for default values and options as parsed from the command line.
+ * Currently rather vestigal, but may be expanded and/or moved to be set by
+ * Server and passed around as an argument.
+ *
+ * magic (uint32_t)
+ *      Semi-unique number to sanity-check structure pointer and validity.
+ *      Must equal MW_OPTS_MAGIC to be valid.
+ *
+ * logpath (char *)
+ *      String pointer. Allocated at runtime.
+ *      Specifies file location for logging output.
+ *      May be NULL -- uses default output (e.g., stdout).
+ *
+ * ----------------------------------------------------------------------------
+ */
+struct mirror_writer_opts {
+    uint32_t    magic;
+    char       *logpath;
+};
+
+static void mybzero(void *dest, size_t size);
+
+static int do_open(struct mirror_session *session,
+        const H5FD_mirror_xmit_open_t *xmit_open);
+
+
+/* ---------------------------------------------------------------------------
+ * Function:   mybzero
+ *
+ * Purpose:    Introduce bzero without neededing it on the system.
+ *
+ * Programmer: Jacob Smith
+ *             2020-03-30
+ * ---------------------------------------------------------------------------
+ */
+static void mybzero(void *dest, size_t size)
+{
+    size_t i = 0;
+    char *s = NULL;
+    HDassert(dest);
+    s = (char *)dest;
+    for (i = 0; i < size; i++) {
+        *(s+i) = 0;
+    }
+} /* end mybzero() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    session_init
+ *
+ * Purpose:     Populate mirror_session structure with default and
+ *              options-drived values.
+ *
+ * Return:      An allocated mirror_session structure pointer on success,
+ *              else NULL.
+ * ----------------------------------------------------------------------------
+ */
+static struct mirror_session *
+session_init(struct mirror_writer_opts *opts)
+{
+    struct mirror_session *session = NULL;
+
+    mirror_log(NULL, V_INFO, "session_init()");
+
+    if (NULL == opts || opts->magic != MW_OPTS_MAGIC) {
+        mirror_log(NULL, V_ERR, "invalid opts pointer");
+        goto error;
+    }
+
+    session = (struct mirror_session *)HDmalloc(sizeof(struct mirror_session));
+    if (session == NULL) {
+        mirror_log(NULL, V_ERR, "can't allocate session structure");
+        goto error;
+    }
+
+    session->magic         = MW_SESSION_MAGIC;
+    session->sockfd        = -1;
+    session->xmit_count    = 0;
+    session->token         = 0;
+    session->file          = NULL;
+
+    session->reply.pub.magic         = H5FD_MIRROR_XMIT_MAGIC;
+    session->reply.pub.version       = H5FD_MIRROR_XMIT_CURR_VERSION;
+    session->reply.pub.op            = H5FD_MIRROR_OP_REPLY;
+    session->reply.pub.session_token = 0;
+    mybzero(session->reply.message, H5FD_MIRROR_STATUS_MESSAGE_MAX);
+
+    /* Options-derived population
+     */
+
+    session->loginfo = mirror_log_init(opts->logpath, "W- ",
+            MIRROR_LOG_DEFAULT_VERBOSITY);
+
+    return session;
+
+error:
+    if (session) {
+        HDfree(session);
+    }
+    return NULL;
+} /* end session_init() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    session_stop
+ *
+ * Purpose:     Stop and clean up a session.
+ *              Only do this as part of program termination or aborting startup.
+ *
+ * Return:      0 on success, or negative sum of errors encountered.
+ * ----------------------------------------------------------------------------
+ */
+static int
+session_stop(struct mirror_session *session)
+{
+    int ret_value = 0;
+
+    HDassert(session && (session->magic == MW_SESSION_MAGIC));
+
+    mirror_log(session->loginfo, V_INFO, "session_stop()");
+
+    /* Close HDF5 file if it is still open (probably in error) */
+    if (session->file) {
+        mirror_log(session->loginfo, V_WARN, "HDF5 file still open at cleanup");
+        if (H5FDclose(session->file) < 0) {
+            mirror_log(session->loginfo, V_ERR, "H5FDclose() during cleanup!");
+            ret_value--;
+        }
+    }
+
+    /* Socket will be closed by parent side of server fork after exit */
+
+    /* Close custom logging stream */
+    if (mirror_log_term(session->loginfo) < 0) {
+        mirror_log(NULL, V_ERR, "Problem closing logging stream");
+        ret_value--;
+    }
+    session->loginfo = NULL;
+
+    /* Invalidate and release structure */
+    session->magic++;
+    HDfree(session);
+
+    return ret_value;
+} /* end session_stop() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    session_start
+ *
+ * Purpose:     Initiate session, open files.
+ *
+ * Return:      Success: A valid mirror_session pointer which must later be
+ *                       cleaned up with session_stop().
+ *              Failure: NULL, after cleaning up after itself.
+ * ---------------------------------------------------------------------------
+ */
+static struct mirror_session *
+session_start(int socketfd, const H5FD_mirror_xmit_open_t *xmit_open)
+{
+    struct mirror_session *session = NULL;
+    struct mirror_writer_opts opts;
+#if 0 /* TODO: behaviro option */
+    char logpath[H5FD_MIRROR_XMIT_FILEPATH_MAX] = "";
+#endif
+
+    mirror_log(NULL, V_INFO, "session_start()");
+
+    if (FALSE == H5FD_mirror_xmit_is_open(xmit_open)) {
+        mirror_log(NULL, V_ERR, "invalid OPEN xmit");
+        return NULL;
+    }
+
+    opts.magic = MW_OPTS_MAGIC;
+#if 0 /* TODO: behavior option */
+    HDsnprintf(logpath, H5FD_MIRROR_XMIT_FILEPATH_MAX, "%s.log",
+            xmit_open->filename);
+    opts.logpath = logpath;
+#else
+    opts.logpath = NULL;
+#endif
+
+    session = session_init(&opts);
+    if (NULL == session) {
+        mirror_log(NULL, V_ERR, "can't instantiate session");
+        goto error;
+    }
+
+    session->sockfd = socketfd;
+
+    if (do_open(session, xmit_open) < 0) {
+        mirror_log(NULL, V_ERR, "unable to open file");
+        goto error;
+    }
+
+    return session;
+
+error:
+    if (session != NULL) {
+        if (session_stop(session) < 0) {
+            mirror_log(NULL, V_WARN, "Can't abort session init");
+        }
+        session = NULL;
+    }
+    return NULL;
+}
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    _xmit_reply
+ *
+ * Purpose:     Common operations to send a reply xmit through the session.
+ *
+ * Return:      0 on success, -1 if error.
+ * ----------------------------------------------------------------------------
+ */
+static int
+_xmit_reply(struct mirror_session *session)
+{
+    unsigned char             xmit_buf[H5FD_MIRROR_XMIT_REPLY_SIZE];
+    H5FD_mirror_xmit_reply_t *reply = &(session->reply);
+
+    HDassert(session && (session->magic == MW_SESSION_MAGIC));
+
+    mirror_log(session->loginfo, V_ALL, "_xmit_reply()");
+
+    reply->pub.xmit_count = session->xmit_count++;
+    if (H5FD_mirror_xmit_encode_reply(xmit_buf,
+            (const H5FD_mirror_xmit_reply_t *)reply)
+        != H5FD_MIRROR_XMIT_REPLY_SIZE)
+    {
+        mirror_log(session->loginfo, V_ERR, "can't encode reply");
+        return -1;
+    }
+
+    mirror_log(session->loginfo, V_ALL, "reply xmit data\n```");
+    mirror_log_bytes(session->loginfo, V_ALL, H5FD_MIRROR_XMIT_REPLY_SIZE,
+            (const unsigned char *)xmit_buf);
+    mirror_log(session->loginfo, V_ALL, "```");
+
+    if (HDwrite(session->sockfd, xmit_buf, H5FD_MIRROR_XMIT_REPLY_SIZE) < 0) {
+        mirror_log(session->loginfo, V_ERR, "can't write reply to Driver");
+        return -1;
+    }
+
+    return 0;
+} /* end _xmit_reply() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    reply_ok
+ *
+ * Purpose:     Send an OK reply through the session.
+ *
+ * Return:      0 on success, -1 if error.
+ * ---------------------------------------------------------------------------
+ */
+static int
+reply_ok(struct mirror_session *session)
+{
+    H5FD_mirror_xmit_reply_t *reply = &(session->reply);
+
+    HDassert(session && (session->magic == MW_SESSION_MAGIC));
+
+    mirror_log(session->loginfo, V_ALL, "reply_ok()");
+
+    reply->status = H5FD_MIRROR_STATUS_OK;
+    mybzero(reply->message, H5FD_MIRROR_STATUS_MESSAGE_MAX);
+    return _xmit_reply(session);
+} /* end reply_ok() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    reply_error
+ *
+ * Purpose:     Send an ERROR reply with message through the session.
+ *              Message may be cut short if it would overflow the available
+ *              buffer in the xmit.
+ *
+ * Return:      0 on success, -1 if error.
+ * ---------------------------------------------------------------------------
+ */
+static int
+reply_error(struct mirror_session  *session,
+            const char             *msg)
+{
+    H5FD_mirror_xmit_reply_t *reply = &(session->reply);
+
+    HDassert(session && (session->magic == MW_SESSION_MAGIC));
+
+    mirror_log(session->loginfo, V_ALL, "reply_error(%s)", msg);
+
+    reply->status = H5FD_MIRROR_STATUS_ERROR;
+    HDsnprintf(reply->message, H5FD_MIRROR_STATUS_MESSAGE_MAX-1, "%s", msg);
+    return _xmit_reply(session);
+} /* end reply_error() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    do_close
+ *
+ * Purpose:     Handle an CLOSE operation.
+ *
+ * Return:      0 on success, -1 if error.
+ * ---------------------------------------------------------------------------
+ */
+static int
+do_close(struct mirror_session *session)
+{
+
+    HDassert(session && (session->magic == MW_SESSION_MAGIC));
+
+    mirror_log(session->loginfo, V_INFO, "do_close()");
+
+    if (NULL == session->file) {
+        mirror_log(session->loginfo, V_ERR, "no file to close!");
+        reply_error(session, "no file to close");
+        return -1;
+    }
+
+    if (H5FDclose(session->file) < 0) {
+        mirror_log(session->loginfo, V_ERR, "H5FDclose()");
+        reply_error(session, "H5FDclose()");
+        return -1;
+    }
+    session->file = NULL;
+
+    if (reply_ok(session) < 0) {
+        mirror_log(session->loginfo, V_ERR, "can't reply");
+        reply_error(session, "ok reply failed; session contaminated");
+        return -1;
+    }
+
+    return 0;
+} /* end do_close() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    do_lock
+ *
+ * Purpose:     Handle a LOCK operation.
+ *
+ * Return:      0 on success, -1 if error.
+ * ---------------------------------------------------------------------------
+ */
+static int
+do_lock(struct mirror_session  *session,
+        const unsigned char    *xmit_buf)
+{
+    size_t                  decode_ret = 0;
+    H5FD_mirror_xmit_lock_t xmit_lock;
+
+    HDassert(session && (session->magic == MW_SESSION_MAGIC) && xmit_buf);
+
+    mirror_log(session->loginfo, V_INFO, "do_lock()");
+
+    decode_ret = H5FD_mirror_xmit_decode_lock(&xmit_lock, xmit_buf);
+    if (H5FD_MIRROR_XMIT_LOCK_SIZE != decode_ret) {
+        mirror_log(session->loginfo, V_ERR, "can't decode set-eoa xmit");
+        reply_error(session, "remote xmit_eoa_t decoding size failure");
+        return -1;
+    }
+
+    if (!H5FD_mirror_xmit_is_lock(&xmit_lock)) {
+        mirror_log(session->loginfo, V_ERR, "not a set-eoa xmit");
+        reply_error(session, "remote xmit_eoa_t decode failure");
+        return -1;
+    }
+    mirror_log(session->loginfo, V_INFO, "lock rw: (%d)", xmit_lock.rw);
+
+    if (H5FDlock(session->file, (hbool_t)xmit_lock.rw) < 0) {
+        mirror_log(session->loginfo, V_ERR, "H5FDlock()");
+        reply_error(session, "remote H5FDlock() failure");
+        return -1;
+    }
+
+    if (reply_ok(session) < 0) {
+        mirror_log(session->loginfo, V_ERR, "can't reply");
+        reply_error(session, "ok reply failed; session contaminated");
+        return -1;
+    }
+
+    return 0;
+} /* end do_lock() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    do_open
+ *
+ * Purpose:     Handle an OPEN operation.
+ *
+ * Return:      0 on success, -1 if error.
+ * ---------------------------------------------------------------------------
+ */
+static int
+do_open(struct mirror_session  *session,
+        const H5FD_mirror_xmit_open_t *xmit_open)
+{
+    hid_t fapl_id = H5I_INVALID_HID;
+    unsigned _flags = 0;
+    haddr_t _maxaddr = HADDR_UNDEF;
+
+    HDassert(session && (session->magic == MW_SESSION_MAGIC) && xmit_open &&
+             TRUE == H5FD_mirror_xmit_is_open(xmit_open));
+
+    mirror_log(session->loginfo, V_INFO, "do_open()");
+
+    if (0 != xmit_open->pub.xmit_count) {
+        mirror_log(session->loginfo, V_ERR, "open with xmit count not zero!");
+        reply_error(session, "initial transmission count not zero");
+        goto error;
+    }
+    if (0 != session->token) {
+        mirror_log(session->loginfo, V_ERR, "open with token already set!");
+        reply_error(session, "initial session token not zero");
+        goto error;
+    }
+
+    session->xmit_count = 1;
+    session->token = xmit_open->pub.session_token;
+    session->reply.pub.session_token = session->token;
+
+    _flags = (unsigned)xmit_open->flags;
+    _maxaddr = (haddr_t)xmit_open->maxaddr;
+
+    /* Check whether the native size_t on the remote machine (Driver) is larger
+     * than that on the local machine; if so, issue a warning.
+     * The blob is always an 8-byte bitfield -- check its contents.
+     */
+    if (xmit_open->size_t_blob > (uint64_t)((size_t)(-1))) {
+        mirror_log(session->loginfo, V_WARN,
+            "Driver size_t is larger than our own");
+    }
+
+    mirror_log(session->loginfo, V_INFO,
+            "to open file %s (flags %d) (maxaddr %d)",
+            xmit_open->filename, _flags, _maxaddr);
+
+    /* Explicitly use Sec2 as the underlying driver for now.
+     */
+    fapl_id = H5Pcreate(H5P_FILE_ACCESS);
+    if (fapl_id < 0) {
+        mirror_log(session->loginfo, V_ERR, "can't create FAPL");
+        reply_error(session, "H5Pcreate() failure");
+        goto error;
+    }
+    if (H5Pset_fapl_sec2(fapl_id) < 0) {
+        mirror_log(session->loginfo, V_ERR, "can't set FAPL as Sec2");
+        reply_error(session, "H5Pset_fapl_sec2() failure");
+        goto error;
+    }
+
+    session->file = H5FDopen(xmit_open->filename, _flags, fapl_id, _maxaddr);
+    if (NULL == session->file) {
+        mirror_log(session->loginfo, V_ERR, "H5FDopen()");
+        reply_error(session, "remote H5FDopen() failure");
+        goto error;
+    }
+
+    /* FAPL is set and in use; clean up */
+    if (H5Pclose(fapl_id) < 0) {
+        mirror_log(session->loginfo, V_ERR, "can't set FAPL as Sec2");
+        reply_error(session, "H5Pset_fapl_sec2() failure");
+        goto error;
+    }
+
+    if (reply_ok(session) < 0) {
+        mirror_log(session->loginfo, V_ERR, "can't reply");
+        reply_error(session, "ok reply failed; session contaminated");
+        return -1;
+    }
+
+    return 0;
+
+error:
+    if (fapl_id > 0) {
+        H5E_BEGIN_TRY {
+            (void)H5Pclose(fapl_id);
+        } H5E_END_TRY;
+    }
+    return -1;
+} /* end do_open() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    do_set_eoa
+ *
+ * Purpose:     Handle a SET_EOA operation.
+ *
+ * Return:      0 on success, -1 if error.
+ * ---------------------------------------------------------------------------
+ */
+static int
+do_set_eoa(struct mirror_session  *session,
+           const unsigned char    *xmit_buf)
+{
+    size_t                 decode_ret = 0;
+    H5FD_mirror_xmit_eoa_t xmit_seoa;
+
+    HDassert(session && (session->magic == MW_SESSION_MAGIC) && xmit_buf);
+
+    mirror_log(session->loginfo, V_INFO, "do_set_eoa()");
+
+    decode_ret = H5FD_mirror_xmit_decode_set_eoa(&xmit_seoa, xmit_buf);
+    if (H5FD_MIRROR_XMIT_EOA_SIZE != decode_ret) {
+        mirror_log(session->loginfo, V_ERR, "can't decode set-eoa xmit");
+        reply_error(session, "remote xmit_eoa_t decoding size failure");
+        return -1;
+    }
+
+    if (!H5FD_mirror_xmit_is_set_eoa(&xmit_seoa)) {
+        mirror_log(session->loginfo, V_ERR, "not a set-eoa xmit");
+        reply_error(session, "remote xmit_eoa_t decode failure");
+        return -1;
+    }
+
+    mirror_log(session->loginfo, V_INFO, "set EOA addr %d",
+            xmit_seoa.eoa_addr);
+
+    if (H5FDset_eoa(session->file, (H5FD_mem_t)xmit_seoa.type,
+            (haddr_t)xmit_seoa.eoa_addr)
+        < 0)
+    {
+        mirror_log(session->loginfo, V_ERR, "H5FDset_eoa()");
+        reply_error(session, "remote H5FDset_eoa() failure");
+        return -1;
+    }
+
+    if (reply_ok(session) < 0) {
+        mirror_log(session->loginfo, V_ERR, "can't reply");
+        reply_error(session, "ok reply failed; session contaminated");
+        return -1;
+    }
+
+    return 0;
+} /* end do_set_eoa() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    do_truncate
+ *
+ * Purpose:     Handle a TRUNCATE operation.
+ *
+ * Return:      0 on success, -1 if error.
+ * ---------------------------------------------------------------------------
+ */
+static int
+do_truncate(struct mirror_session  *session)
+{
+
+    HDassert(session && (session->magic == MW_SESSION_MAGIC));
+
+    mirror_log(session->loginfo, V_INFO, "do_truncate()");
+
+    /* default DXPL ID (0), 0 for "FALSE" closing -- both probably unused */
+    if (H5FDtruncate(session->file, 0, 0) < 0) {
+        mirror_log(session->loginfo, V_ERR, "H5FDtruncate()");
+        reply_error(session, "remote H5FDtruncate() failure");
+        return -1;
+    }
+
+    if (reply_ok(session) < 0) {
+        mirror_log(session->loginfo, V_ERR, "can't reply");
+        reply_error(session, "ok reply failed; session contaminated");
+        return -1;
+    }
+
+    return 0;
+} /* end do_truncate() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    do_unlock
+ *
+ * Purpose:     Handle an UNLOCK operation.
+ *
+ * Return:      0 on success, -1 if error.
+ * ---------------------------------------------------------------------------
+ */
+static int
+do_unlock(struct mirror_session  *session)
+{
+    HDassert(session && (session->magic == MW_SESSION_MAGIC));
+
+    mirror_log(session->loginfo, V_INFO, "do_unlock()");
+
+    if (H5FDunlock(session->file) < 0) {
+        mirror_log(session->loginfo, V_ERR, "H5FDunlock()");
+        reply_error(session, "remote H5FDunlock() failure");
+        return -1;
+    }
+
+    if (reply_ok(session) < 0) {
+        mirror_log(session->loginfo, V_ERR, "can't reply");
+        reply_error(session, "ok reply failed; session contaminated");
+        return -1;
+    }
+
+    return 0;
+} /* end do_unlock() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    do_write
+ *
+ * Purpose:     Handle a WRITE operation.
+ *              Receives command, replies; receives & writes data, replies.
+ *
+ *              It is known that this results in suboptimal performance,
+ *              but handling both small and very, very large write buffers
+ *              with a single "over the wire" exchange
+ *              poses design challenges not worth tackling as of March 2020.
+ *
+ * Return:      0 on success, -1 if error.
+ * ---------------------------------------------------------------------------
+ */
+static int
+do_write(struct mirror_session  *session,
+         const unsigned char    *xmit_buf)
+{
+    size_t      decode_ret        = 0;
+    haddr_t     addr              = 0;
+    haddr_t     sum_bytes_written = 0;
+    H5FD_mem_t  type              = 0;
+    char       *buf               = NULL;
+    ssize_t     nbytes_in_packet  = 0;
+    H5FD_mirror_xmit_write_t xmit_write;
+
+    HDassert(session && (session->magic == MW_SESSION_MAGIC) && xmit_buf);
+
+    mirror_log(session->loginfo, V_INFO, "do_write()");
+
+    if (NULL == session->file) {
+        mirror_log(session->loginfo, V_ERR, "no open file!");
+        reply_error(session, "no file open on remote");
+        return -1;
+    }
+
+    decode_ret = H5FD_mirror_xmit_decode_write(&xmit_write, xmit_buf);
+    if (H5FD_MIRROR_XMIT_WRITE_SIZE != decode_ret) {
+        mirror_log(session->loginfo, V_ERR, "can't decode write xmit");
+        reply_error(session, "remote xmit_write_t decoding size failure");
+        return -1;
+    }
+
+    if (!H5FD_mirror_xmit_is_write(&xmit_write)) {
+        mirror_log(session->loginfo, V_ERR, "not a write xmit");
+        reply_error(session, "remote xmit_write_t decode failure");
+        return -1;
+    }
+
+    addr = (haddr_t)xmit_write.offset;
+    type = (H5FD_mem_t)xmit_write.type;
+
+    /* Allocate the buffer once -- re-use between loops.
+     */
+    buf = (char *)HDmalloc(sizeof(char) * H5FD_MIRROR_DATA_BUFFER_MAX);
+    if (NULL == buf) {
+        mirror_log(session->loginfo, V_ERR, "can't allocate databuffer");
+        reply_error(session, "can't allocate buffer for receiving data");
+        return -1;
+    }
+
+    /* got write signal; ready for data */
+    if (reply_ok(session) < 0) {
+        mirror_log(session->loginfo, V_ERR, "can't reply");
+        reply_error(session, "ok reply failed; session contaminated");
+        return -1;
+    }
+
+    mirror_log(session->loginfo, V_INFO, "to write %zu bytes at %zu",
+            xmit_write.size,
+            addr);
+
+    /* The given write may be:
+     * 1. larger than the allowed single buffer size
+     * 2. larger than the native size_t of this system
+     *
+     * Handle all cases by looping, ingesting as much of the stream as possible
+     * and writing that part to the file.
+     */
+    sum_bytes_written = 0;
+    do {
+        nbytes_in_packet = HDread(session->sockfd, buf,
+                H5FD_MIRROR_DATA_BUFFER_MAX);
+        if (-1 == nbytes_in_packet) {
+            mirror_log(session->loginfo, V_ERR, "can't read into databuffer");
+            reply_error(session, "can't read data buffer");
+            return -1;
+        }
+
+        mirror_log(session->loginfo, V_INFO, "received %zd bytes",
+            nbytes_in_packet);
+        if (HEXDUMP_WRITEDATA) {
+            mirror_log(session->loginfo, V_ALL, "DATA:\n```");
+            mirror_log_bytes(session->loginfo, V_ALL, nbytes_in_packet,
+                    (const unsigned char *)buf);
+            mirror_log(session->loginfo, V_ALL, "```");
+        }
+
+        mirror_log(session->loginfo, V_INFO, "writing %zd bytes at %zu",
+                 nbytes_in_packet,
+                 (addr + sum_bytes_written));
+
+        if (H5FDwrite(session->file, type, H5P_DEFAULT,
+                (addr + sum_bytes_written), (size_t)nbytes_in_packet, buf)
+            < 0)
+        {
+            mirror_log(session->loginfo, V_ERR, "H5FDwrite()");
+            reply_error(session, "remote H5FDwrite() failure");
+            return -1;
+        }
+
+        sum_bytes_written += (haddr_t)nbytes_in_packet;
+
+    } while (sum_bytes_written < xmit_write.size); /* end while ingesting */
+
+    HDfree(buf);
+
+    /* signal that we're done here and a-ok */
+    if (reply_ok(session) < 0) {
+        mirror_log(session->loginfo, V_ERR, "can't reply");
+        reply_error(session, "ok reply failed; session contaminated");
+        return -1;
+    }
+
+    return 0;
+} /* end do_write() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    receive_communique
+ *
+ * Purpose:     Accept bytes from the socket, check for emergency shutdown, and
+ *              sanity-check received bytes.
+ *              The raw bytes read are stored in the sock_comm structure at
+ *              comm->raw.
+ *              The raw bytes are decoded and a xmit_t (header) struct pointer
+ *              in comm is populated at comm->xmit_recd.
+ *
+ * Return:      0 on success, -1 if error.
+ * ---------------------------------------------------------------------------
+ */
+static int
+receive_communique(
+        struct mirror_session  *session,
+        struct sock_comm       *comm)
+{
+    ssize_t     read_ret = 0;
+    size_t      decode_ret;
+    H5FD_mirror_xmit_t *X = comm->xmit_recd;
+
+    HDassert((session != NULL)                    && \
+             (session->magic == MW_SESSION_MAGIC) && \
+             (comm != NULL)                       && \
+             (comm->magic == MW_SOCK_COMM_MAGIC)  && \
+             (comm->xmit_recd != NULL)            && \
+             (comm->raw != NULL)                  && \
+             (comm->raw_size >= H5FD_MIRROR_XMIT_BUFFER_MAX) );
+
+    mirror_log(session->loginfo, V_INFO, "receive_communique()");
+
+    mybzero(comm->raw, comm->raw_size);
+    comm->recd_die = 0;
+
+    mirror_log(session->loginfo, V_INFO, "ready to receive"); /* TODO */
+
+    read_ret = HDread(session->sockfd, comm->raw, H5FD_MIRROR_XMIT_BUFFER_MAX);
+    if (-1 == read_ret) {
+        mirror_log(session->loginfo, V_ERR, "read:%zd", read_ret);
+        goto error;
+    }
+
+    mirror_log(session->loginfo, V_INFO, "received %zd bytes", read_ret);
+    if (HEXDUMP_XMITS) {
+        mirror_log(session->loginfo, V_ALL, "```", read_ret);
+        mirror_log_bytes(session->loginfo, V_ALL, (size_t)read_ret,
+                (const unsigned char *)comm->raw);
+        mirror_log(session->loginfo, V_ALL, "```");
+    } /* end if hexdump transmissions received */
+
+    /* old-fashioned manual kill (for debugging) */
+    if (!HDstrncmp("GOODBYE", comm->raw, 7)) {
+        mirror_log(session->loginfo, V_INFO, "received GOODBYE");
+        comm->recd_die = 1;
+        goto done;
+    }
+
+    decode_ret = H5FD_mirror_xmit_decode_header(X,
+            (const unsigned char *)comm->raw);
+    if (H5FD_MIRROR_XMIT_HEADER_SIZE != decode_ret) {
+        mirror_log(session->loginfo, V_ERR,
+                "header decode size mismatch: expected (%z), got (%z)",
+                H5FD_MIRROR_XMIT_HEADER_SIZE, decode_ret);
+        /* Try to tell Driver that it should stop */
+        reply_error(session, "xmit size mismatch");
+        goto error;
+    }
+
+    if (!H5FD_mirror_xmit_is_xmit(X)) {
+        mirror_log(session->loginfo, V_ERR, "bad magic: 0x%X", X->magic);
+        /* Try to tell Driver that it should stop */
+        reply_error(session, "bad magic");
+        goto error;
+    }
+
+    if (session->xmit_count != X->xmit_count) {
+        mirror_log(session->loginfo, V_ERR,
+                "xmit_count mismatch exp:%d recd:%d",
+                session->xmit_count, X->xmit_count);
+        /* Try to tell Driver that it should stop */
+        reply_error(session, "xmit_count mismatch");
+        goto error;
+    }
+
+    if ( (session->token > 0) && (session->token != X->session_token) ) {
+        mirror_log(session->loginfo, V_ERR, "wrong session");
+        /* Try to tell Driver that it should stop */
+        reply_error(session, "wrong session");
+        goto error;
+    }
+
+    session->xmit_count++;
+
+done:
+    return 0;
+
+error:
+    return -1;
+} /* end receive_communique() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    process_instructions
+ *
+ * Purpose:     Receive and handle all instructions from Driver.
+ *
+ * Return:      0 on success, -1 if error.
+ * ---------------------------------------------------------------------------
+ */
+static int
+process_instructions(struct mirror_session *session)
+{
+    struct sock_comm    comm;
+    char                xmit_buf[H5FD_MIRROR_XMIT_BUFFER_MAX]; /* raw bytes */
+    H5FD_mirror_xmit_t  xmit_recd; /* for decoded xmit header */
+
+    HDassert(session && (session->magic == MW_SESSION_MAGIC));
+
+    mirror_log(session->loginfo, V_INFO, "process_instructions()");
+
+    comm.magic = MW_SOCK_COMM_MAGIC;
+    comm.recd_die = 0; /* Flag for program to terminate */
+    comm.xmit_recd = &xmit_recd;
+    comm.raw = xmit_buf;
+    comm.raw_size = sizeof(xmit_buf);
+
+    while (1) { /* sill-listening infinite loop */
+
+        /* Use convenience structure for raw/decoded info in/out */
+        if (receive_communique(session, &comm) < 0) {
+            mirror_log(session->loginfo, V_ERR, "problem reading socket");
+            return -1;
+        }
+
+        if (comm.recd_die) {
+            goto done;
+        }
+
+        switch(xmit_recd.op) {
+        case H5FD_MIRROR_OP_CLOSE:
+            if (do_close(session) < 0) {
+                return -1;
+            }
+            goto done;
+        case H5FD_MIRROR_OP_LOCK:
+            if (do_lock(session, (const unsigned char *)xmit_buf) < 0) {
+                return -1;
+            }
+            break;
+        case H5FD_MIRROR_OP_OPEN:
+            mirror_log(session->loginfo, V_ERR, "OPEN xmit during session");
+            reply_error(session, "illegal OPEN xmit during session");
+            return -1;
+        case H5FD_MIRROR_OP_SET_EOA:
+            if (do_set_eoa(session, (const unsigned char *)xmit_buf) < 0) {
+                return -1;
+            }
+            break;
+        case H5FD_MIRROR_OP_TRUNCATE:
+            if (do_truncate(session) < 0) {
+                return -1;
+            }
+            break;
+        case H5FD_MIRROR_OP_UNLOCK:
+            if (do_unlock(session) < 0) {
+                return -1;
+            }
+            break;
+        case H5FD_MIRROR_OP_WRITE:
+            if (do_write(session, (const unsigned char *)xmit_buf) < 0) {
+                return -1;
+            }
+            break;
+        default:
+            mirror_log(session->loginfo, V_ERR, "unrecognized transmission");
+            reply_error(session, "unrecognized transmission");
+            return -1;
+        } /* end switch (xmit_recd.op) */
+
+    } /* end while still listening */
+
+done:
+    comm.magic = 0; /* invalidate structure, on principle */
+    xmit_recd.magic = 0; /* invalidate structure, on principle */
+    return 0;
+} /* end process_instructions() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    run_writer
+ *
+ * Purpose:     Initiate Writer operations.
+ *
+ *              Receives as parameters a socket which has accepted the
+ *              connection to the Driver and the OPEN xmit (which must be
+ *              decoded into the structure and verified prior to being passed
+ *              to this function).
+ *
+ *              Is not responsible for closing or cleaning up any of the
+ *              received parameters.
+ *
+ * Return:      Success: SUCCEED
+ *              Failure: FAIL
+ * ---------------------------------------------------------------------------
+ */
+herr_t
+run_writer(int socketfd, H5FD_mirror_xmit_open_t *xmit_open)
+{
+    struct mirror_session *session = NULL;
+    int ret_value = SUCCEED;
+
+    session = session_start(socketfd, xmit_open);
+    if (NULL == session) {
+        mirror_log(NULL, V_ERR, "Can't start session -- aborting");
+        ret_value = FAIL;
+    }
+    else {
+        if (process_instructions(session) < 0) {
+            mirror_log(session->loginfo, V_ERR,
+                    "problem processing instructions");
+            ret_value = FAIL;
+        }
+        if (session_stop(session) < 0) {
+            mirror_log(NULL, V_ERR, "Can't stop session -- going down hard");
+            ret_value = FAIL;
+        }
+    }
+
+    return ret_value;
+} /* end run_writer */
+
+#endif /* H5_HAVE_MIRROR_VFD */
+