Brings Mirror VFD to 1.10 from develop

33 files changed, 8333 insertions, 16 deletions

diff --git a/CMakeLists.txt b/CMakeLists.txt
index ef4caca..5d1295f 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)
@@ -886,6 +891,16 @@ 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 9e89e57..60bb838 100644
--- a/MANIFEST
+++ b/MANIFEST
@@ -682,6 +682,9 @@
 ./src/H5FDint.c
 ./src/H5FDlog.c
 ./src/H5FDlog.h
+./src/H5FDmirror.c
+./src/H5FDmirror.h
+./src/H5FDmirror_priv.h
 ./src/H5FDmodule.h
 ./src/H5FDmpi.c
 ./src/H5FDmpi.h
@@ -1003,6 +1006,7 @@
 ./test/cache_image.c
 ./test/cache_logging.c
 ./test/cache_tagging.c
+./test/chunk_info.c
 ./test/cmpd_dset.c
 ./test/cork.c
 ./test/corrupt_stab_msg.h5
@@ -1068,6 +1072,7 @@
 ./test/gen_deflate.c
 ./test/gen_file_image.c
 ./test/gen_filespace.c
+./test/gen_filters.c
 ./test/gen_mergemsg.c
 ./test/gen_new_array.c
 ./test/gen_new_fill.c
@@ -1106,6 +1111,7 @@
 ./test/corrupted_name_len.h5
 ./test/mergemsg.h5
 ./test/mf.c
+./test/mirror_vfd.c
 ./test/mount.c
 ./test/mtime.c
 ./test/multi_file_v16-r.h5
@@ -1163,6 +1169,7 @@
 ./test/testhdf5.c
 ./test/testhdf5.h
 ./test/testlibinfo.sh.in
+./test/test_mirror.sh.in
 ./test/test_usecases.sh.in
 ./test/testmeta.c
 ./test/testswmr.sh.in
@@ -1204,19 +1211,18 @@
 ./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
 ./test/use.h
-./test/vfd.c
-./test/gen_filters.c
-./test/chunk_info.c
 ./test/vds.c
 ./test/vds_env.c
 ./test/vds_swmr.h
 ./test/vds_swmr_gen.c
 ./test/vds_swmr_reader.c
 ./test/vds_swmr_writer.c
+./test/vfd.c
 
 ./test/testfiles/err_compat_1
 ./test/testfiles/err_compat_2
@@ -2730,6 +2736,18 @@
 ./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
@@ -3458,6 +3476,8 @@
 ./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 19522f7..d31f7c7 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
@@ -190,10 +190,10 @@ 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                                 \
-	  if test $$d != .; then                                       \
-	    (cd $$d && $(MAKE) $(AM_MAKEFLAGS) $@) || exit 1;          \
-	  fi;                                                          \
+	for d in src utils test; do                                     \
+	  if test $$d != .; then                                        \
+	    (cd $$d && $(MAKE) $(AM_MAKEFLAGS) $@) || exit 1;           \
+	  fi;                                                           \
 	done
 
 # Automake wants to make config.status depend on configure.  This
diff --git a/bin/trace b/bin/trace
index 6b5dd17..42b89b5 100755
--- a/bin/trace
+++ b/bin/trace
@@ -127,6 +127,7 @@ $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 4188e45..ab121e9 100644
--- a/config/cmake/ConfigureChecks.cmake
+++ b/config/cmake/ConfigureChecks.cmake
@@ -196,6 +196,23 @@ 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 05903d4..7d4b28e 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@
 
@@ -270,6 +273,9 @@
 /* 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@
 
@@ -282,6 +288,12 @@
 /* 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 7dd72b4..ebcbd61 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/configure.ac b/configure.ac
index 5e5db4a..93cb2e3 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1147,6 +1147,7 @@ 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_SUBST([H5_IS_DARWIN])
@@ -2928,6 +2929,50 @@ 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])
@@ -3796,9 +3841,12 @@ 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
                  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 909401e..ef85584 100644
--- a/release_docs/RELEASE.txt
+++ b/release_docs/RELEASE.txt
@@ -303,6 +303,12 @@ 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
 
diff --git a/src/H5FDmirror.c b/src/H5FDmirror.c
new file mode 100644
index 0000000..6bee4a7
--- /dev/null
+++ b/src/H5FDmirror.c
@@ -0,0 +1,2017 @@
+/* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
+ * 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 "H5FDmirror_priv.h"    /* Private header for the mirror VFD */
+#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)); \
+    HDfflush(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 transmission buffers */
+H5FL_BLK_DEFINE_STATIC(xmit);
+
+/* Declare a free list to manage the H5FD_mirror_t struct */
+H5FL_DEFINE_STATIC(H5FD_mirror_t);
+
+/* Declare a free list to manage the H5FD_mirror_xmit_open_t struct */
+H5FL_DEFINE_STATIC(H5FD_mirror_xmit_open_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(FUNC);
+
+    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(FUNC);
+
+    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_STATIC_NOERR
+
+    /* Reset VFL ID */
+    H5FD_MIRROR_g = 0;
+
+    LOG_OP_CALL(FUNC);
+
+    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(__func__);
+
+    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(__func__);
+
+    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(__func__);
+
+    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(__func__);
+
+    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(__func__);
+
+    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(__func__);
+
+    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(__func__);
+
+    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(__func__);
+
+    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(__func__);
+
+    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(__func__);
+
+    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(__func__);
+
+    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(__func__);
+
+    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(__func__);
+
+    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(__func__);
+
+    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(__func__);
+
+    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(__func__);
+
+    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(__func__);
+
+    HDassert(dest && x);
+
+    /* clear entire structure, but especially its filepath string area */
+    HDmemset(dest, 0, H5FD_MIRROR_XMIT_OPEN_SIZE);
+
+    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(__func__);
+
+    HDassert(dest && x);
+
+    /* clear entire structure, but especially its message string area */
+    HDmemset(dest, 0, H5FD_MIRROR_XMIT_REPLY_SIZE);
+
+    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(__func__);
+
+    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(__func__);
+
+    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.
+ * ---------------------------------------------------------------------------
+ */
+H5_ATTR_PURE hbool_t
+H5FD_mirror_xmit_is_close(const H5FD_mirror_xmit_t *xmit)
+{
+    LOG_OP_CALL(__func__);
+
+    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.
+ * ---------------------------------------------------------------------------
+ */
+H5_ATTR_PURE hbool_t
+H5FD_mirror_xmit_is_lock(const H5FD_mirror_xmit_lock_t *xmit)
+{
+    LOG_OP_CALL(__func__);
+
+    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.
+ * ---------------------------------------------------------------------------
+ */
+H5_ATTR_PURE hbool_t
+H5FD_mirror_xmit_is_open(const H5FD_mirror_xmit_open_t *xmit)
+{
+    LOG_OP_CALL(__func__);
+
+    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.
+ * ---------------------------------------------------------------------------
+ */
+H5_ATTR_PURE hbool_t
+H5FD_mirror_xmit_is_set_eoa(const H5FD_mirror_xmit_eoa_t *xmit)
+{
+    LOG_OP_CALL(__func__);
+
+    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.
+ * ---------------------------------------------------------------------------
+ */
+H5_ATTR_PURE hbool_t
+H5FD_mirror_xmit_is_reply(const H5FD_mirror_xmit_reply_t *xmit)
+{
+    LOG_OP_CALL(__func__);
+
+    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.
+ * ---------------------------------------------------------------------------
+ */
+H5_ATTR_PURE hbool_t
+H5FD_mirror_xmit_is_write(const H5FD_mirror_xmit_write_t *xmit)
+{
+    LOG_OP_CALL(__func__);
+
+    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.
+ * ---------------------------------------------------------------------------
+ */
+H5_ATTR_PURE hbool_t
+H5FD_mirror_xmit_is_xmit(const H5FD_mirror_xmit_t *xmit)
+{
+    LOG_OP_CALL(__func__);
+
+    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.
+ * ----------------------------------------------------------------------------
+ */
+static herr_t
+H5FD__mirror_verify_reply(H5FD_mirror_t *file)
+{
+    unsigned char                      *xmit_buf = NULL;
+    struct H5FD_mirror_xmit_reply_t     reply;
+    ssize_t                             read_ret = 0;
+    herr_t                              ret_value = SUCCEED;
+
+    FUNC_ENTER_STATIC
+
+    LOG_OP_CALL(FUNC);
+
+    HDassert(file && file->sock_fd);
+
+    xmit_buf = H5FL_BLK_MALLOC(xmit, H5FD_MIRROR_XMIT_BUFFER_MAX);
+    if(NULL == xmit_buf)
+        HGOTO_ERROR(H5E_VFL, H5E_CANTALLOC, FAIL, "unable to allocate xmit buffer");
+
+    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, 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:
+    if(xmit_buf)
+        xmit_buf = H5FL_BLK_FREE(xmit, xmit_buf);
+
+    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_STATIC
+
+    LOG_OP_CALL(FUNC);
+
+    fa = (H5FD_mirror_fapl_t *)H5MM_calloc(sizeof(H5FD_mirror_fapl_t));
+    if(NULL == fa)
+        HGOTO_ERROR(H5E_VFL, H5E_CANTALLOC, 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_STATIC
+
+    LOG_OP_CALL(FUNC);
+
+    new_fa = (H5FD_mirror_fapl_t *)H5MM_malloc(sizeof(H5FD_mirror_fapl_t));
+    if(new_fa == NULL)
+        HGOTO_ERROR(H5E_VFL, H5E_CANTALLOC, 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_STATIC_NOERR
+
+    LOG_OP_CALL(FUNC);
+
+    /* 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(FUNC);
+
+    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(FUNC);
+
+    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)
+{
+    int                      live_socket = -1;
+    struct sockaddr_in       target_addr;
+    socklen_t                addr_size;
+    unsigned char           *xmit_buf = NULL;
+    H5FD_mirror_fapl_t       fa;
+    H5FD_mirror_t           *file      = NULL;
+    H5FD_mirror_xmit_open_t *open_xmit = NULL;
+    H5FD_t                  *ret_value = NULL;
+
+    FUNC_ENTER_STATIC
+
+    LOG_OP_CALL(FUNC);
+
+    /* --------------- */
+    /* 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_VFL, H5E_CANTALLOC, 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? */
+
+    open_xmit = (H5FD_mirror_xmit_open_t *)H5FL_CALLOC(H5FD_mirror_xmit_open_t);
+    if(NULL == open_xmit)
+        HGOTO_ERROR(H5E_VFL, H5E_CANTALLOC, NULL, "unable to allocate open_xmit struct");
+
+    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);
+
+    xmit_buf = H5FL_BLK_MALLOC(xmit, H5FD_MIRROR_XMIT_BUFFER_MAX);
+    if(NULL == xmit_buf)
+        HGOTO_ERROR(H5E_VFL, H5E_CANTALLOC, NULL, "unable to allocate xmit buffer");
+
+    if(H5FD_mirror_xmit_encode_open(xmit_buf, 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");
+    }
+
+    if(open_xmit)
+        open_xmit = H5FL_FREE(H5FD_mirror_xmit_open_t, open_xmit);
+    if(xmit_buf)
+        xmit_buf = H5FL_BLK_FREE(xmit, xmit_buf);
+
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* 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 = NULL;
+    int xmit_encoded = 0; /* monitor point of failure */
+    herr_t         ret_value = SUCCEED;
+
+    FUNC_ENTER_STATIC
+
+    LOG_OP_CALL(FUNC);
+
+    /* Sanity check */
+    HDassert(file);
+    HDassert(file->sock_fd >= 0);
+
+    file->xmit.xmit_count = (file->xmit_i)++;
+    file->xmit.op = H5FD_MIRROR_OP_CLOSE;
+
+    xmit_buf = H5FL_BLK_MALLOC(xmit, H5FD_MIRROR_XMIT_BUFFER_MAX);
+    if(NULL == xmit_buf)
+        HGOTO_ERROR(H5E_VFL, H5E_CANTALLOC, FAIL, "unable to allocate xmit buffer");
+
+    if(H5FD_mirror_xmit_encode_header(xmit_buf, &(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 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 error */
+
+    file = H5FL_FREE(H5FD_mirror_t, file); /* always release resources */
+
+    if(xmit_buf)
+        xmit_buf = H5FL_BLK_FREE(xmit, xmit_buf);
+
+    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_STATIC_NOERR;
+
+    LOG_OP_CALL(FUNC);
+
+    /* 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 = 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_STATIC_NOERR
+
+    LOG_OP_CALL(FUNC);
+
+    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  = NULL;
+    H5FD_mirror_t      *file      = (H5FD_mirror_t *)_file;
+    herr_t              ret_value = SUCCEED;
+
+    FUNC_ENTER_STATIC
+
+    LOG_OP_CALL(FUNC);
+
+    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;
+
+    xmit_buf = H5FL_BLK_MALLOC(xmit, H5FD_MIRROR_XMIT_BUFFER_MAX);
+    if(NULL == xmit_buf)
+        HGOTO_ERROR(H5E_VFL, H5E_CANTALLOC, FAIL, "unable to allocate xmit buffer");
+
+    if(H5FD_mirror_xmit_encode_set_eoa(xmit_buf, &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:
+    if(xmit_buf)
+        xmit_buf = H5FL_BLK_FREE(xmit, xmit_buf);
+
+    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_STATIC_NOERR
+
+    LOG_OP_CALL(FUNC);
+
+    HDassert(file);
+
+    FUNC_LEAVE_NOAPI(file->eof)
+} /* end H5FD__mirror_get_eof() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD__mirror_read
+ *
+ * Purpose:     Required to register the driver, but 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)
+{
+    FUNC_ENTER_STATIC_NOERR
+
+    LOG_OP_CALL(FUNC);
+
+    FUNC_LEAVE_NOAPI(FAIL)
+} /* 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 = NULL;
+    H5FD_mirror_t              *file        = (H5FD_mirror_t *)_file;
+    herr_t                      ret_value   = SUCCEED;
+
+    FUNC_ENTER_STATIC
+
+    LOG_OP_CALL(FUNC);
+
+    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;
+
+    xmit_buf = H5FL_BLK_MALLOC(xmit, H5FD_MIRROR_XMIT_BUFFER_MAX);
+    if(NULL == xmit_buf)
+        HGOTO_ERROR(H5E_VFL, H5E_CANTALLOC, FAIL, "unable to allocate xmit buffer");
+
+    /* Notify Writer of incoming data to write. */
+    if(H5FD_mirror_xmit_encode_write(xmit_buf, &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:
+    if(xmit_buf)
+        xmit_buf = H5FL_BLK_FREE(xmit, xmit_buf);
+
+    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 = NULL;
+    H5FD_mirror_t      *file = (H5FD_mirror_t *)_file;
+    herr_t      ret_value = SUCCEED;
+
+    FUNC_ENTER_STATIC
+
+    LOG_OP_CALL(FUNC);
+
+    file->xmit.xmit_count = (file->xmit_i)++;
+    file->xmit.op = H5FD_MIRROR_OP_TRUNCATE;
+
+    xmit_buf = H5FL_BLK_MALLOC(xmit, H5FD_MIRROR_XMIT_BUFFER_MAX);
+    if(NULL == xmit_buf)
+        HGOTO_ERROR(H5E_VFL, H5E_CANTALLOC, FAIL, "unable to allocate xmit buffer");
+
+    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:
+    if(xmit_buf)
+        xmit_buf = H5FL_BLK_FREE(xmit, xmit_buf);
+
+    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 = NULL;
+    H5FD_mirror_t      *file = (H5FD_mirror_t *)_file;
+    herr_t ret_value = SUCCEED;
+
+    FUNC_ENTER_STATIC
+
+    LOG_OP_CALL(FUNC);
+
+    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;
+
+    xmit_buf = H5FL_BLK_MALLOC(xmit, H5FD_MIRROR_XMIT_BUFFER_MAX);
+    if(NULL == xmit_buf)
+        HGOTO_ERROR(H5E_VFL, H5E_CANTALLOC, FAIL, "unable to allocate xmit buffer");
+
+    if(H5FD_mirror_xmit_encode_lock(xmit_buf, &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:
+    if(xmit_buf)
+        xmit_buf = H5FL_BLK_FREE(xmit, xmit_buf);
+
+    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 = NULL;
+    H5FD_mirror_t      *file = (H5FD_mirror_t *)_file;
+    herr_t ret_value = SUCCEED;
+
+    FUNC_ENTER_STATIC
+
+    LOG_OP_CALL(FUNC);
+
+    file->xmit.xmit_count = (file->xmit_i)++;
+    file->xmit.op = H5FD_MIRROR_OP_UNLOCK;
+
+    xmit_buf = H5FL_BLK_MALLOC(xmit, H5FD_MIRROR_XMIT_BUFFER_MAX);
+    if(NULL == xmit_buf)
+        HGOTO_ERROR(H5E_VFL, H5E_CANTALLOC, FAIL, "unable to allocate xmit buffer");
+
+    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:
+    if(xmit_buf)
+        xmit_buf = H5FL_BLK_FREE(xmit, xmit_buf);
+
+    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..7d15c1b
--- /dev/null
+++ b/src/H5FDmirror.h
@@ -0,0 +1,79 @@
+/* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
+ * 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);
+
+#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/H5FDmirror_priv.h b/src/H5FDmirror_priv.h
new file mode 100644
index 0000000..dc15441
--- /dev/null
+++ b/src/H5FDmirror_priv.h
@@ -0,0 +1,323 @@
+/* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
+ * 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_priv_H
+#define H5FDmirror_priv_H
+
+#ifdef H5_HAVE_MIRROR_VFD
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/* = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = =
+ * 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
+
+#endif /* H5_HAVE_MIRROR_VFD */
+
+#endif /* H5FDmirror_priv_H */
+
diff --git a/src/H5FDsec2.c b/src/H5FDsec2.c
index cbb707a..ce11c05 100644
--- a/src/H5FDsec2.c
+++ b/src/H5FDsec2.c
@@ -545,6 +545,12 @@ 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/H5private.h b/src/H5private.h
index 78746c3..6c481c9 100644
--- a/src/H5private.h
+++ b/src/H5private.h
@@ -362,6 +362,22 @@
 #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
@@ -665,6 +681,9 @@ 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 */
@@ -711,6 +730,9 @@ 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 */
@@ -753,6 +775,9 @@ 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 */
@@ -997,6 +1022,9 @@ 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 /* HDgethostname */
@@ -1036,6 +1064,18 @@ 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 */
@@ -1090,6 +1130,9 @@ 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 */
@@ -1171,6 +1214,12 @@ 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 */
@@ -1302,12 +1351,21 @@ 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 */
@@ -1353,6 +1411,9 @@ 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 a077a19..05a5856 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -127,6 +127,11 @@ if DIRECT_VFD_CONDITIONAL
     libhdf5_la_SOURCES += H5FDdirect.c
 endif
 
+# Only compile the mirror VFD if necessary
+if MIRROR_VFD_CONDITIONAL
+    libhdf5_la_SOURCES += H5FDmirror.c
+endif
+
 # Only compile the read-only S3 VFD if necessary
 if ROS3_VFD_CONDITIONAL
     libhdf5_la_SOURCES += H5FDros3.c H5FDs3comms.c
@@ -138,7 +143,7 @@ include_HEADERS = hdf5.h H5api_adpt.h H5overflow.h H5pubconf.h H5public.h H5vers
         H5Cpublic.h H5Dpublic.h \
         H5Epubgen.h H5Epublic.h H5Fpublic.h \
         H5FDpublic.h H5FDcore.h H5FDdirect.h H5FDfamily.h H5FDhdfs.h \
-        H5FDlog.h H5FDmpi.h H5FDmpio.h H5FDmulti.h H5FDros3.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 \
         H5MMpublic.h H5Opublic.h H5Ppublic.h \
diff --git a/src/hdf5.h b/src/hdf5.h
index 9a0ea40..b717c6d 100644
--- a/src/hdf5.h
+++ b/src/hdf5.h
@@ -44,6 +44,7 @@
 #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                        */
diff --git a/src/libhdf5.settings.in b/src/libhdf5.settings.in
index b1c1b2b..128e107 100644
--- a/src/libhdf5.settings.in
+++ b/src/libhdf5.settings.in
@@ -79,6 +79,7 @@ Parallel Filtered Dataset Writes: @PARALLEL_FILTERED_WRITES@
           I/O filters (external): @EXTERNAL_FILTERS@
                              MPE: @MPE@
                       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 8e553d0..d4c33b9 100644
--- a/test/CMakeLists.txt
+++ b/test/CMakeLists.txt
@@ -173,6 +173,11 @@ 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
@@ -206,7 +211,7 @@ set (H5_TESTS
     page_buffer
     dtypes
     dsets
-    chunk_info
+    chunk_info # compression lib link
     cmpd_dset
     filter_fail
     extend
@@ -235,6 +240,7 @@ set (H5_TESTS
     ros3
     s3comms
     hdfs
+    mirror_vfd
     ntypes
     dangle
     dtransform
@@ -272,6 +278,7 @@ set (H5_TESTS_MULTIPLE
     cache_image
     ttsafe
     thread_id # special link
+    mirror_vfd
 )
 # Only build single source tests here
 foreach (h5_test ${H5_TESTS})
@@ -284,6 +291,19 @@ endforeach ()
 ###           M U L T I P L E  S O U R C E   T E S T S                     ###
 ##############################################################################
 ######### Also special handling of link libs #############
+#-- Adding test for chunk_info
+add_executable (chunk_info ${HDF5_TEST_SOURCE_DIR}/chunk_info.c)
+target_compile_options(chunk_info PRIVATE "${HDF5_CMAKE_C_FLAGS}")
+target_include_directories (chunk_info PRIVATE "${HDF5_SRC_DIR};${HDF5_BINARY_DIR};${HDF5_TEST_BINARY_DIR};$<$<BOOL:${HDF5_ENABLE_PARALLEL}>:${MPI_C_INCLUDE_DIRS}>")
+if (NOT BUILD_SHARED_LIBS)
+  TARGET_C_PROPERTIES (chunk_info STATIC)
+  target_link_libraries (chunk_info PRIVATE ${HDF5_TEST_LIB_TARGET} ${LINK_COMP_LIBS})
+else ()
+  TARGET_C_PROPERTIES (chunk_info SHARED)
+  target_link_libraries (chunk_info PRIVATE ${HDF5_TEST_LIBSH_TARGET} ${LINK_COMP_LIBS})
+endif ()
+set_target_properties (chunk_info PROPERTIES FOLDER test)
+
 #-- Adding test for direct_chunk
 add_executable (direct_chunk ${HDF5_TEST_SOURCE_DIR}/direct_chunk.c)
 target_compile_options(direct_chunk PRIVATE "${HDF5_CMAKE_C_FLAGS}")
@@ -362,6 +382,18 @@ 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 949b926..4e14067 100644
--- a/test/CMakeTests.cmake
+++ b/test/CMakeTests.cmake
@@ -448,6 +448,8 @@ set (test_CLEANFILES
     direct_chunk.h5
     splitter*.h5
     splitter.log
+    mirror_rw/*
+    mirror_wo/*
 )
 
 # Remove any output file left over from previous test run
diff --git a/test/Makefile.am b/test/Makefile.am
index 1c759dc..874e35d 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
 endif
@@ -137,6 +141,7 @@ LDADD=libh5test.la $(LIBHDF5)
 ttsafe_SOURCES=ttsafe.c ttsafe_dcreate.c ttsafe_error.c ttsafe_cancel.c       \
                ttsafe_acreate.c ttsafe_attr_vlen.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
@@ -202,7 +207,7 @@ CHECK_CLEANFILES+=accum.h5 cmpd_dset.h5 compact_dataset.h5 dataset.h5 dset_offse
     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 \
-    splitter*.h5 splitter.log
+    splitter*.h5 splitter.log mirror_rw mirror_ro
 
 # Sources for testhdf5 executable
 testhdf5_SOURCES=testhdf5.c tarray.c tattr.c tchecksum.c tconfig.c tfile.c \
@@ -212,11 +217,13 @@ 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
+    testexternal_env.sh testswmr.sh testvds_env.sh testvdsswmr.sh test_usecases.sh testflushrefresh.sh \
+    testabort_fail.sh test_mirror.sh
 
 include $(top_srcdir)/config/conclude.am
diff --git a/test/mirror_vfd.c b/test/mirror_vfd.c
new file mode 100644
index 0000000..35c3f90
--- /dev/null
+++ b/test/mirror_vfd.c
@@ -0,0 +1,2763 @@
+/* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
+ * 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
+
+#include "H5FDmirror_priv.h"    /* Private header for the 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 $RUN_DIR
+    fi
+    exit 0
+else
+    echo "Mirror VFD tests FAILED."
+    exit 1
+fi
+
diff --git a/test/use_append_chunk_mirror.c b/test/use_append_chunk_mirror.c
new file mode 100644
index 0000000..6ee01c0
--- /dev/null
+++ b/test/use_append_chunk_mirror.c
@@ -0,0 +1,403 @@
+/* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
+ * Copyright by The HDF Group.                                               *
+ * All rights reserved.                                                      *
+ *                                                                           *
+ * This file is part of HDF5.  The full HDF5 copyright notice, including     *
+ * terms governing use, modification, and redistribution, is contained in    *
+ * the COPYING file, which can be found at the root of the source code       *
+ * distribution tree, or in https://support.hdfgroup.org/ftp/HDF5/releases.  *
+ * If you do not have access to either file, you may request a copy from     *
+ * help@hdfgroup.org.                                                        *
+ * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */
+
+/* HACKED VERSION
+ * Demonstrate SWMR with a mirrored file.
+ *
+ * Must be built with SERVER_IP as the IP address of the target system
+ * with a running mirror server, and SERVER_PORT as the primary server port.
+ *
+ * In addition to the local file, 'shinano.h5' will be created on the remote
+ * system, mirroring the local file. The file location will be local to
+ * Server's/Writer's invocation directory.
+ *
+ * Template for demonstration purposes:
+ *
+ * # Launch mirror server on remote machine (in foreground to easily stop)
+ * REMOTE(1)$ ./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/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..67e95a5
--- /dev/null
+++ b/utils/mirror_vfd/mirror_remote.h
@@ -0,0 +1,53 @@
+/* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
+ * 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
+
+#include "H5FDmirror_priv.h"    /* Private header for the 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..db7cf04
--- /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(void)
+{
+    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..f2414e0
--- /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");
+        HDexit(EXIT_FAILURE);
+    }
+
+    if (opts.help) {
+        usage();
+        HDexit(EXIT_FAILURE);
+    }
+
+    if (send_shutdown(&opts) < 0) {
+        HDprintf("Unable to send shutdown command\n");
+        HDexit(EXIT_FAILURE);
+    }
+
+    HDexit(EXIT_SUCCESS);
+} /* end main() */
+
+#else /* H5_HAVE_MIRROR_VFD */
+
+
+/* ------------------------------------------------------------------------- */
+int
+main(void)
+{
+    HDprintf("Mirror VFD not built -- unable to perform shutdown.\n");
+    HDexit(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 */
+