Add Splitter VFD to library.

* "Simultaneous and equivalent" Read-Write and Write-Only channels for
  file I/O.
* Only supports drivers with the H5FD_FEAT_DEFAULT_VFD_COMPATIBLE flag for
  now, preventing issues with multi-file drivers.

Add Mirror VFD to library.

* Write-only operations over a network.
* Uses TCP/IP sockets.
* Server and auxiliary server-shutdown programs provided in a new directory,
  `utils/mirror_vfd`.
* Automated testing via loopback ("remote" of localhost).

11 files changed, 4013 insertions, 5 deletions

diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index f582056..6678aaf 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -231,6 +231,7 @@ set (H5FD_SOURCES
     ${HDF5_SRC_DIR}/H5FDhdfs.c
     ${HDF5_SRC_DIR}/H5FDint.c
     ${HDF5_SRC_DIR}/H5FDlog.c
+    ${HDF5_SRC_DIR}/H5FDmirror.c
     ${HDF5_SRC_DIR}/H5FDmpi.c
     ${HDF5_SRC_DIR}/H5FDmpio.c
     ${HDF5_SRC_DIR}/H5FDmulti.c
@@ -238,6 +239,7 @@ set (H5FD_SOURCES
     ${HDF5_SRC_DIR}/H5FDs3comms.c
     ${HDF5_SRC_DIR}/H5FDsec2.c
     ${HDF5_SRC_DIR}/H5FDspace.c
+    ${HDF5_SRC_DIR}/H5FDsplitter.c
     ${HDF5_SRC_DIR}/H5FDstdio.c
     ${HDF5_SRC_DIR}/H5FDtest.c
     ${HDF5_SRC_DIR}/H5FDwindows.c
@@ -249,6 +251,7 @@ set (H5FD_HDRS
     ${HDF5_SRC_DIR}/H5FDfamily.h
     ${HDF5_SRC_DIR}/H5FDhdfs.h
     ${HDF5_SRC_DIR}/H5FDlog.h
+    ${HDF5_SRC_DIR}/H5FDmirror.h
     ${HDF5_SRC_DIR}/H5FDmpi.h
     ${HDF5_SRC_DIR}/H5FDmpio.h
     ${HDF5_SRC_DIR}/H5FDmulti.h
@@ -256,6 +259,7 @@ set (H5FD_HDRS
     ${HDF5_SRC_DIR}/H5FDros3.h
     ${HDF5_SRC_DIR}/H5FDs3comms.h
     ${HDF5_SRC_DIR}/H5FDsec2.h
+    ${HDF5_SRC_DIR}/H5FDsplitter.h
     ${HDF5_SRC_DIR}/H5FDstdio.h
     ${HDF5_SRC_DIR}/H5FDwindows.h
 )
diff --git a/src/H5FDmirror.c b/src/H5FDmirror.c
new file mode 100644
index 0000000..ea1f516
--- /dev/null
+++ b/src/H5FDmirror.c
@@ -0,0 +1,1991 @@
+/* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
+ * Copyright by The HDF Group.                                               *
+ * All rights reserved.                                                      *
+ *                                                                           *
+ * This file is part of HDF5.  The full HDF5 copyright notice, including     *
+ * terms governing use, modification, and redistribution, is contained in    *
+ * the COPYING file, which can be found at the root of the source code       *
+ * distribution tree, or in https://support.hdfgroup.org/ftp/HDF5/releases.  *
+ * If you do not have access to either file, you may request a copy from     *
+ * help@hdfgroup.org.                                                        *
+ * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */
+
+/*
+ * Purpose: Transmit write-only operations to a receiver/writer process on
+ *          a remote host.
+ */
+
+#include "H5FDdrvr_module.h" /* This source code file is part of the H5FD driver module */
+
+#include "H5private.h"      /* Generic Functions        */
+
+#ifdef H5_HAVE_MIRROR_VFD
+
+#include "H5Eprivate.h"     /* Error handling           */
+#include "H5Fprivate.h"     /* File access              */
+#include "H5FDprivate.h"    /* File drivers             */
+#include "H5FDmirror.h"     /* "Mirror" definitions     */
+#include "H5FLprivate.h"    /* Free Lists               */
+#include "H5Iprivate.h"     /* IDs                      */
+#include "H5MMprivate.h"    /* Memory management        */
+#include "H5Pprivate.h"     /* Property lists           */
+
+/* The driver identification number, initialized at runtime */
+static hid_t H5FD_MIRROR_g = 0;
+
+/* Virtual file structure for a Mirror Driver */
+typedef struct H5FD_mirror_t {
+    H5FD_t             pub;     /* Public stuff, must be first            */
+    H5FD_mirror_fapl_t fa;      /* Configuration structure                */
+    haddr_t            eoa;     /* End of allocated region                */
+    haddr_t            eof;     /* End of file; current file size         */
+    int                sock_fd; /* Handle of socket to remote operator    */
+    H5FD_mirror_xmit_t xmit;    /* Primary communication header           */
+    uint32_t           xmit_i;  /* Counter of transmission sent and rec'd */
+} H5FD_mirror_t;
+
+/*
+ * These macros check for overflow of various quantities.  These macros
+ * assume that HDoff_t is signed and haddr_t and size_t are unsigned.
+ *
+ * ADDR_OVERFLOW:   Checks whether a file address of type `haddr_t'
+ *                  is too large to be represented by the second argument
+ *                  of the file seek function.
+ *
+ * SIZE_OVERFLOW:   Checks whether a buffer size of type `hsize_t' is too
+ *                  large to be represented by the `size_t' type.
+ *
+ * REGION_OVERFLOW: Checks whether an address and size pair describe data
+ *                  which can be addressed entirely by the second
+ *                  argument of the file seek function.
+ */
+#define MAXADDR (((haddr_t)1<<(8*sizeof(HDoff_t)-1))-1)
+#define ADDR_OVERFLOW(A)    (HADDR_UNDEF==(A) || ((A) & ~(haddr_t)MAXADDR))
+
+#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))
+
+/* Debugging flabs for verbose tracing -- nonzero to enable */
+#define MIRROR_DEBUG_OP_CALLS 0
+#define MIRROR_DEBUG_XMIT_BYTES 0
+
+#if MIRROR_DEBUG_XMIT_BYTES
+#define LOG_XMIT_BYTES(label, buf, len) do {                                  \
+    ssize_t bytes_written = 0;                                                \
+    const unsigned char *b = NULL;                                            \
+                                                                              \
+    HDfprintf(stdout, "%s bytes:\n```\n", (label));                           \
+                                                                              \
+    /* print whole lines */                                                   \
+    while ((len - bytes_written) >= 32) {                                     \
+        b = (const unsigned char *)(buf) + bytes_written;                     \
+        HDfprintf(stdout,                                                     \
+                "%04zX  %02X%02X%02X%02X %02X%02X%02X%02X"                    \
+                      " %02X%02X%02X%02X %02X%02X%02X%02X"                    \
+                      " %02X%02X%02X%02X %02X%02X%02X%02X"                    \
+                      " %02X%02X%02X%02X %02X%02X%02X%02X\n",                 \
+                bytes_written,                                                \
+                b[0], b[1], b[2], b[3],   b[4], b[5], b[6], b[7],             \
+                b[8], b[9], b[10],b[11],  b[12],b[13],b[14],b[15],            \
+                b[16],b[17],b[18],b[19],  b[20],b[21],b[22],b[23],            \
+                b[24],b[25],b[26],b[27],  b[28],b[29],b[30],b[31]);           \
+        bytes_written += 32;                                                  \
+    }                                                                         \
+                                                                              \
+    /* start partial line */                                                  \
+    if (len > bytes_written) {                                                \
+        HDfprintf(stdout, "%04zX ", bytes_written);                           \
+    }                                                                         \
+                                                                              \
+    /* partial line blocks */                                                 \
+    while ((len - bytes_written) >= 4) {                                      \
+        HDfprintf(stdout, " %02X%02X%02X%02X",                                \
+                (buf)[bytes_written],   (buf)[bytes_written+1],               \
+                (buf)[bytes_written+2], (buf)[bytes_written+3]);              \
+        bytes_written += 4;                                                   \
+    }                                                                         \
+                                                                              \
+    /* block separator before partial block */                                \
+    if (len > bytes_written) {                                                \
+        HDfprintf(stdout, " ");                                               \
+    }                                                                         \
+                                                                              \
+    /* partial block individual bytes */                                      \
+    while (len > bytes_written) {                                             \
+        HDfprintf(stdout, "%02X", (buf)[bytes_written++]);                    \
+    }                                                                         \
+                                                                              \
+    /* end partial line */                                                    \
+    HDfprintf(stdout, "\n");                                                  \
+    HDfprintf(stdout, "```\n");                                               \
+    HDfflush(stdout);                                                         \
+} while (0)
+#else
+#define LOG_XMIT_BYTES(label, buf, len) /* no-op */
+#endif /* MIRROR_DEBUG_XMIT_BYTE */
+
+#if MIRROR_DEBUG_OP_CALLS
+#define LOG_OP_CALL(name) do {         \
+    HDprintf("called %s()\n", (name)); \
+    fflush(stdout);                    \
+} while (0)
+#else
+#define LOG_OP_CALL(name) /* no-op */
+#endif /* MIRROR_DEBUG_OP_CALLS */
+
+/* Prototypes */
+static herr_t H5FD_mirror_term(void);
+static void *H5FD_mirror_fapl_get(H5FD_t *_file);
+static void *H5FD_mirror_fapl_copy(const void *_old_fa);
+static herr_t H5FD_mirror_fapl_free(void *_fa);
+static haddr_t H5FD_mirror_get_eoa(const H5FD_t *_file, H5FD_mem_t type);
+static herr_t H5FD_mirror_set_eoa(H5FD_t *_file, H5FD_mem_t type, haddr_t addr);
+static haddr_t H5FD_mirror_get_eof(const H5FD_t *_file, H5FD_mem_t type);
+static H5FD_t *H5FD_mirror_open(const char *name, unsigned flags, \
+        hid_t fapl_id, haddr_t maxaddr);
+static herr_t H5FD_mirror_close(H5FD_t *_file);
+static herr_t H5FD_mirror_query(const H5FD_t *_file, unsigned long *flags);
+static herr_t H5FD_mirror_write(H5FD_t *_file, H5FD_mem_t type, hid_t fapl_id, \
+        haddr_t addr, size_t size, const void *buf);
+static herr_t H5FD_mirror_read(H5FD_t *_file, H5FD_mem_t type, hid_t fapl_id,
+        haddr_t addr, size_t size, void *buf);
+static herr_t H5FD_mirror_truncate(H5FD_t *_file, hid_t dxpl_id, \
+        hbool_t closing);
+static herr_t H5FD_mirror_lock(H5FD_t *_file, hbool_t rw);
+static herr_t H5FD_mirror_unlock(H5FD_t *_file);
+
+static herr_t H5FD__mirror_verify_reply(H5FD_mirror_t *file);
+
+static const H5FD_class_t H5FD_mirror_g = {
+    "mirror",                   /* name                 */
+    MAXADDR,                    /* maxaddr              */
+    H5F_CLOSE_WEAK,             /* fc_degree            */
+    H5FD_mirror_term,           /* terminate            */
+    NULL,                       /* sb_size              */
+    NULL,                       /* sb_encode            */
+    NULL,                       /* sb_decode            */
+    0,                          /* fapl_size            */
+    H5FD_mirror_fapl_get,       /* fapl_get             */
+    H5FD_mirror_fapl_copy,      /* fapl_copy            */
+    H5FD_mirror_fapl_free,      /* fapl_free            */
+    0,                          /* dxpl_size            */
+    NULL,                       /* dxpl_copy            */
+    NULL,                       /* dxpl_free            */
+    H5FD_mirror_open,           /* open                 */
+    H5FD_mirror_close,          /* close                */
+    NULL,                       /* cmp                  */
+    H5FD_mirror_query,          /* query                */
+    NULL,                       /* get_type_map         */
+    NULL,                       /* alloc                */
+    NULL,                       /* free                 */
+    H5FD_mirror_get_eoa,        /* get_eoa              */
+    H5FD_mirror_set_eoa,        /* set_eoa              */
+    H5FD_mirror_get_eof,        /* get_eof              */
+    NULL,                       /* get_handle           */
+    H5FD_mirror_read,           /* read                 */
+    H5FD_mirror_write,          /* write                */
+    NULL,                       /* flush                */
+    H5FD_mirror_truncate,       /* truncate             */
+    H5FD_mirror_lock,           /* lock                 */
+    H5FD_mirror_unlock,         /* unlock               */
+    H5FD_FLMAP_DICHOTOMY        /* fl_map               */
+};
+
+/* Declare a free list to manage the H5FD_mirror_t struct */
+H5FL_DEFINE_STATIC(H5FD_mirror_t);
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD__init_package
+ *
+ * Purpose:     Initializes any interface-specific data or routines.
+ *
+ * Return:      Non-negative on success/Negative on failure
+ *-------------------------------------------------------------------------
+ */
+static herr_t
+H5FD__init_package(void)
+{
+    herr_t ret_value = SUCCEED;
+
+    FUNC_ENTER_STATIC
+
+    LOG_OP_CALL("H5FD__init_package");
+
+    if (H5FD_mirror_init() < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_CANTINIT, FAIL,
+                    "unable to initialize mirror VFD");
+    }
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* H5FD__init_package() */
+
+
+/* -------------------------------------------------------------------------
+ * Function:    H5FD_mirror_init
+ *
+ * Purpose:     Initialize this driver by registering the driver with the
+ *              library.
+ *
+ * Return:      Success:    The driver ID for the mirror driver.
+ *              Failure:    Negative
+ * -------------------------------------------------------------------------
+ */
+hid_t
+H5FD_mirror_init(void)
+{
+    hid_t ret_value = H5I_INVALID_HID;
+
+    FUNC_ENTER_NOAPI(FAIL)
+
+    LOG_OP_CALL("H5FD_mirror_init");
+
+    if (H5I_VFL != H5I_get_type(H5FD_MIRROR_g)) {
+        H5FD_MIRROR_g = H5FD_register(&H5FD_mirror_g, sizeof(H5FD_class_t),
+                FALSE);
+    }
+
+    ret_value = H5FD_MIRROR_g;
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD_mirror_init() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    H5FD_mirror_term
+ *
+ * Purpose:     Shut down the VFD
+ *
+ * Returns:     SUCCEED (Can't fail)
+ * ---------------------------------------------------------------------------
+ */
+static herr_t
+H5FD_mirror_term(void)
+{
+    FUNC_ENTER_NOAPI_NOINIT_NOERR
+
+    /* Reset VFL ID */
+    H5FD_MIRROR_g = 0;
+
+    LOG_OP_CALL("H5FD_mirror_term");
+
+    FUNC_LEAVE_NOAPI(SUCCEED)
+} /* end H5FD_mirror_term() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    H5FD__mirror_xmit_decode_uint16
+ *
+ * Purpose:     Extract a 16-bit integer in "network" (Big-Endian) word order
+ *              from the byte-buffer and return it with the local word order at
+ *              the destination pointer.
+ *
+ *              The programmer must ensure that the received buffer holds
+ *              at least the expected size of data.
+ *
+ * Return:      The number of bytes read from the buffer (2).
+ * ---------------------------------------------------------------------------
+ */
+size_t
+H5FD__mirror_xmit_decode_uint16(uint16_t *out, const unsigned char *_buf)
+{
+    uint16_t n = 0;
+    LOG_OP_CALL("H5FD__mirror_xmit_decode_uint16");
+    HDassert(_buf && out);
+    HDmemcpy(&n, _buf, sizeof(n));
+    *out = (uint16_t)HDntohs(n);
+    return 2; /* number of bytes eaten */
+} /* end H5FD__mirror_xmit_decode_uint16() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    H5FD__mirror_xmit_decode_uint32
+ *
+ * Purpose:     Extract a 32-bit integer in "network" (Big-Endian) word order
+ *              from the byte-buffer and return it with the local word order at
+ *              the destination pointer.
+ *
+ *              The programmer must ensure that the received buffer holds
+ *              at least the expected size of data.
+ *
+ * Return:      The number of bytes read from the buffer (4).
+ * ---------------------------------------------------------------------------
+ */
+size_t
+H5FD__mirror_xmit_decode_uint32(uint32_t *out, const unsigned char *_buf)
+{
+    uint32_t n = 0;
+    LOG_OP_CALL("H5FD__mirror_xmit_decode_uint32");
+    HDassert(_buf && out);
+    HDmemcpy(&n, _buf, sizeof(n));
+    *out = (uint32_t)HDntohl(n);
+    return 4; /* number of bytes eaten */
+} /* end H5FD__mirror_xmit_decode_uint32() */
+
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    is_host_little_endian
+ *
+ * Purpose:     Determine whether the host machine is is little-endian.
+ *
+ *              Store an intger with a known value, re-map the memory to a
+ *              character array, and inspect the array's contents.
+ *
+ * Return:      The number of bytes written to the buffer (8).
+ *
+ * Programmer:  Jacob Smith
+ *              2020-03-05
+ * ---------------------------------------------------------------------------
+ */
+static hbool_t
+is_host_little_endian(void)
+{
+    union {
+        uint32_t u32;
+        uint8_t  u8[4];
+    } echeck;
+    echeck.u32 = 0xA1B2C3D4;
+    if (echeck.u8[0] == 0xD4) {
+        return TRUE;
+    }
+    else {
+        return FALSE;
+    }
+} /* end is_host_little_endian() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    H5FD__mirror_xmit_decode_uint64
+ *
+ * Purpose:     Extract a 64-bit integer in "network" (Big-Endian) word order
+ *              from the byte-buffer and return it with the local word order.
+ *
+ *              The programmer must ensure that the received buffer holds
+ *              at least the expected size of data.
+ *
+ *              WARNING: Does not accommodate other forms of endianness,
+ *              e.g. "middle-endian".
+ *
+ * Return:      The number of bytes written to the buffer (8).
+ * ---------------------------------------------------------------------------
+ */
+size_t
+H5FD__mirror_xmit_decode_uint64(uint64_t *out, const unsigned char *_buf)
+{
+    uint64_t n = 0;
+    LOG_OP_CALL("H5FD__mirror_xmit_decode_uint64");
+    HDassert(_buf && out);
+    HDmemcpy(&n, _buf, sizeof(n));
+    if (TRUE == is_host_little_endian()) {
+        *out = _BSWAP_64(n);
+    }
+    else {
+        *out = n;
+    }
+    return 8;
+} /* end H5FD__mirror_xmit_decode_uint64() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    H5FD__mirror_xmit_decode_uint8
+ *
+ * Purpose:     Extract a 8-bit integer in "network" (Big-Endian) word order
+ *              from the byte-buffer and return it with the local word order at
+ *              the destination pointer.
+ *              (yes, it's one byte).
+ *
+ * Return:      The number of bytes read from the buffer (1).
+ * ---------------------------------------------------------------------------
+ */
+size_t
+H5FD__mirror_xmit_decode_uint8(uint8_t *out, const unsigned char *_buf)
+{
+    LOG_OP_CALL("H5FD__mirror_xmit_decode_uint8");
+    HDassert(_buf && out);
+    HDmemcpy(out, _buf, sizeof(uint8_t));
+    return 1; /* number of bytes eaten */
+} /* end H5FD__mirror_xmit_decode_uint8() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    H5FD__mirror_xmit_encode_uint16
+ *
+ * Purpose:     Encode a 16-bit integer in "network" (Big-Endian) word order
+ *              in place in the destination bytes-buffer.
+ *
+ *              The programmer must ensure that the destination buffer is
+ *              large enough to hold the expected data.
+ *
+ * Return:      The number of bytes written to the buffer (2).
+ * ---------------------------------------------------------------------------
+ */
+size_t
+H5FD__mirror_xmit_encode_uint16(unsigned char *_dest, uint16_t v)
+{
+    uint16_t n = 0;
+    LOG_OP_CALL("H5FD__mirror_xmit_encode_uint16");
+    HDassert(_dest);
+    n = (uint16_t)HDhtons(v);
+    HDmemcpy(_dest, &n, sizeof(n));
+    return 2;
+} /* end H5FD__mirror_xmit_encode_uint16() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    H5FD__mirror_xmit_encode_uint32
+ *
+ * Purpose:     Encode a 32-bit integer in "network" (Big-Endian) word order
+ *              in place in the destination bytes-buffer.
+ *
+ *              The programmer must ensure that the destination buffer is
+ *              large enough to hold the expected data.
+ *
+ * Return:      The number of bytes written to the buffer (4).
+ * ---------------------------------------------------------------------------
+ */
+size_t
+H5FD__mirror_xmit_encode_uint32(unsigned char *_dest, uint32_t v)
+{
+    uint32_t n = 0;
+    LOG_OP_CALL("H5FD__mirror_xmit_encode_uint32");
+    HDassert(_dest);
+    n = (uint32_t)HDhtonl(v);
+    HDmemcpy(_dest, &n, sizeof(n));
+    return 4;
+} /* end H5FD__mirror_xmit_encode_uint32() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    H5FD__mirror_xmit_encode_uint64
+ *
+ * Purpose:     Encode a 64-bit integer in "network" (Big-Endian) word order
+ *              in place in the destination bytes-buffer.
+ *
+ *              The programmer must ensure that the destination buffer is
+ *              large enough to hold the expected data.
+ *
+ * Return:      The number of bytes written to the buffer (8).
+ * ---------------------------------------------------------------------------
+ */
+size_t
+H5FD__mirror_xmit_encode_uint64(unsigned char *_dest, uint64_t v)
+{
+    uint64_t n = v;
+    LOG_OP_CALL("H5FD__mirror_xmit_decode_uint64");
+    HDassert(_dest);
+    if (TRUE == is_host_little_endian()) {
+        n = _BSWAP_64(v);
+    }
+    HDmemcpy(_dest, &n, sizeof(n));
+    return 8;
+} /* H5FD__mirror_xmit_encode_uint64() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    H5FD__mirror_xmit_encode_uint8
+ *
+ * Purpose:     Encode a 8-bit integer in "network" (Big-Endian) word order
+ *              in place in the destination bytes-buffer.
+ *              (yes, it's one byte).
+ *
+ *              The programmer must ensure that the destination buffer is
+ *              large enough to hold the expected data.
+ *
+ * Return:      The number of bytes read from the buffer (1).
+ * ---------------------------------------------------------------------------
+ */
+size_t
+H5FD__mirror_xmit_encode_uint8(unsigned char *dest, uint8_t v)
+{
+    LOG_OP_CALL("H5FD__mirror_xmit_encode_uint8");
+    HDassert(dest);
+    HDmemcpy(dest, &v, sizeof(v));
+    return 1;
+} /* end H5FD__mirror_xmit_encode_uint8() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    H5FD_mirror_xmit_decode_header
+ *
+ * Purpose:     Extract a mirror_xmit_t "header" from the bytes-buffer.
+ *
+ *              Fields will be lifted from the buffer and stored in the
+ *              target structure, using in the correct location (different
+ *              systems may insert different padding between components) and
+ *              word order (Big- vs Little-Endian).
+ *
+ *              The resulting structure should be sanity-checked with
+ *              H5FD_mirror_xmit_is_xmit() before use.
+ *
+ *              The programmer must ensure that the received buffer holds
+ *              at least the expected size of data.
+ *
+ * Return:      The number of bytes consumed from the buffer.
+ * ---------------------------------------------------------------------------
+ */
+size_t
+H5FD_mirror_xmit_decode_header(H5FD_mirror_xmit_t  *out,
+                               const unsigned char *buf)
+{
+    size_t n_eaten = 0;
+    LOG_OP_CALL("H5FD_mirror_xmit_decode_header");
+    HDassert(out && buf);
+    n_eaten += H5FD__mirror_xmit_decode_uint32(&(out->magic), &buf[n_eaten]);
+    n_eaten += H5FD__mirror_xmit_decode_uint8(&(out->version), &buf[n_eaten]);
+    n_eaten += H5FD__mirror_xmit_decode_uint32(&(out->session_token),
+            &buf[n_eaten]);
+    n_eaten += H5FD__mirror_xmit_decode_uint32(&(out->xmit_count),
+            &buf[n_eaten]);
+    n_eaten += H5FD__mirror_xmit_decode_uint8(&(out->op), &buf[n_eaten]);
+    HDassert(n_eaten == H5FD_MIRROR_XMIT_HEADER_SIZE);
+    return n_eaten;
+} /* end H5FD_mirror_xmit_decode_header() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    H5FD_mirror_xmit_decode_lock
+ *
+ * Purpose:     Extract a mirror_xmit_lock_t from the bytes-buffer.
+ *
+ *              Fields will be lifted from the buffer and stored in the
+ *              target structure, using in the correct location (different
+ *              systems may insert different padding between components) and
+ *              word order (Big- vs Little-Endian).
+ *
+ *              The programmer must ensure that the received buffer holds
+ *              at least the expected size of data.
+ *
+ *              The resulting structure should be sanity-checked with
+ *              H5FD_mirror_xmit_is_lock() before use.
+ *
+ * Return:      The number of bytes consumed from the buffer.
+ * ---------------------------------------------------------------------------
+ */
+size_t
+H5FD_mirror_xmit_decode_lock(H5FD_mirror_xmit_lock_t *out,
+                             const unsigned char     *buf)
+{
+    size_t n_eaten = 0;
+    LOG_OP_CALL("H5FD_mirror_xmit_decode_lock");
+    HDassert(out && buf);
+    n_eaten += H5FD_mirror_xmit_decode_header(&(out->pub), buf);
+    n_eaten += H5FD__mirror_xmit_decode_uint64(&(out->rw), &buf[n_eaten]);
+    HDassert(n_eaten == H5FD_MIRROR_XMIT_LOCK_SIZE);
+    return n_eaten;
+} /* end H5FD_mirror_xmit_decode_lock() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    H5FD_mirror_xmit_decode_open
+ *
+ * Purpose:     Extract a mirror_xmit_open_t from the bytes-buffer.
+ *
+ *              Fields will be lifted from the buffer and stored in the
+ *              target structure, using in the correct location (different
+ *              systems may insert different padding between components) and
+ *              word order (Big- vs Little-Endian).
+ *
+ *              The programmer must ensure that the received buffer holds
+ *              at least the expected size of data.
+ *
+ *              The resulting structure should be sanity-checked with
+ *              H5FD_mirror_xmit_is_open() before use.
+ *
+ * Return:      The maximum number of bytes that this decoding operation might
+ *              have consumed from the buffer.
+ * ---------------------------------------------------------------------------
+ */
+size_t
+H5FD_mirror_xmit_decode_open(H5FD_mirror_xmit_open_t *out,
+                             const unsigned char     *buf)
+{
+    size_t n_eaten = 0;
+    LOG_OP_CALL("H5FD_mirror_xmit_decode_open");
+    HDassert(out && buf);
+    n_eaten += H5FD_mirror_xmit_decode_header(&(out->pub), buf);
+    n_eaten += H5FD__mirror_xmit_decode_uint32(&(out->flags), &buf[n_eaten]);
+    n_eaten += H5FD__mirror_xmit_decode_uint64(&(out->maxaddr), &buf[n_eaten]);
+    n_eaten += H5FD__mirror_xmit_decode_uint64(&(out->size_t_blob),
+            &buf[n_eaten]);
+    HDassert((H5FD_MIRROR_XMIT_OPEN_SIZE - H5FD_MIRROR_XMIT_FILEPATH_MAX)
+             == n_eaten);
+    HDstrncpy(out->filename, (const char *)&buf[n_eaten],
+            H5FD_MIRROR_XMIT_FILEPATH_MAX-1);
+    out->filename[H5FD_MIRROR_XMIT_FILEPATH_MAX-1] = 0; /* force final NULL */
+    return H5FD_MIRROR_XMIT_OPEN_SIZE;
+} /* end H5FD_mirror_xmit_decode_open() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    H5FD_mirror_xmit_decode_reply
+ *
+ * Purpose:     Extract a mirror_xmit_reply_t from the bytes-buffer.
+ *
+ *              Fields will be lifted from the buffer and stored in the
+ *              target structure, using in the correct location (different
+ *              systems may insert different padding between components) and
+ *              word order (Big- vs Little-Endian).
+ *
+ *              The programmer must ensure that the received buffer holds
+ *              at least the expected size of data.
+ *
+ *              The resulting structure should be sanity-checked with
+ *              H5FD_mirror_xmit_is_reply() before use.
+ *
+ * Return:      The maximum number of bytes that this decoding operation might
+ *              have consumed from the buffer.
+ * ---------------------------------------------------------------------------
+ */
+size_t
+H5FD_mirror_xmit_decode_reply(H5FD_mirror_xmit_reply_t *out,
+                              const unsigned char      *buf)
+{
+    size_t n_eaten = 0;
+    LOG_OP_CALL("H5FD_mirror_xmit_decode_reply");
+    HDassert(out && buf);
+    n_eaten += H5FD_mirror_xmit_decode_header(&(out->pub), buf);
+    n_eaten += H5FD__mirror_xmit_decode_uint32(&(out->status), &buf[n_eaten]);
+    HDassert((H5FD_MIRROR_XMIT_REPLY_SIZE - H5FD_MIRROR_STATUS_MESSAGE_MAX)
+             == n_eaten);
+    HDstrncpy(out->message, (const char *)&buf[n_eaten],
+            H5FD_MIRROR_STATUS_MESSAGE_MAX-1);
+    out->message[H5FD_MIRROR_STATUS_MESSAGE_MAX-1] = 0; /* force NULL term */
+    return H5FD_MIRROR_XMIT_REPLY_SIZE;
+} /* end H5FD_mirror_xmit_decode_reply() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    H5FD_mirror_xmit_decode_set_eoa
+ *
+ * Purpose:     Extract a mirror_xmit_eoa_t from the bytes-buffer.
+ *
+ *              Fields will be lifted from the buffer and stored in the
+ *              target structure, using in the correct location (different
+ *              systems may insert different padding between components) and
+ *              word order (Big- vs Little-Endian).
+ *
+ *              The programmer must ensure that the received buffer holds
+ *              at least the expected size of data.
+ *
+ *              The resulting structure should be sanity-checked with
+ *              H5FD_mirror_xmit_is_set_eoa() before use.
+ *
+ * Return:      The number of bytes consumed from the buffer.
+ * ---------------------------------------------------------------------------
+ */
+size_t
+H5FD_mirror_xmit_decode_set_eoa(H5FD_mirror_xmit_eoa_t *out,
+                                const unsigned char    *buf)
+{
+    size_t n_eaten = 0;
+    LOG_OP_CALL("H5FD_mirror_xmit_decode_set_eoa");
+    HDassert(out && buf);
+    n_eaten += H5FD_mirror_xmit_decode_header(&(out->pub), buf);
+    n_eaten += H5FD__mirror_xmit_decode_uint8(&(out->type), &buf[n_eaten]);
+    n_eaten += H5FD__mirror_xmit_decode_uint64(&(out->eoa_addr), &buf[n_eaten]);
+    HDassert(n_eaten == H5FD_MIRROR_XMIT_EOA_SIZE);
+    return n_eaten;
+} /* end H5FD_mirror_xmit_decode_set_eoa() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    H5FD_mirror_xmit_decode_write
+ *
+ * Purpose:     Extract a mirror_xmit_write_t from the bytes-buffer.
+ *
+ *              Fields will be lifted from the buffer and stored in the
+ *              target structure, using in the correct location (different
+ *              systems may insert different padding between components) and
+ *              word order (Big- vs Little-Endian).
+ *
+ *              The programmer must ensure that the received buffer holds
+ *              at least the expected size of data.
+ *
+ *              The resulting structure should be sanity-checked with
+ *              H5FD_mirror_xmit_is_write() before use.
+ *
+ * Return:      The number of bytes consumed from the buffer.
+ * ---------------------------------------------------------------------------
+ */
+size_t
+H5FD_mirror_xmit_decode_write(H5FD_mirror_xmit_write_t *out,
+                              const unsigned char      *buf)
+{
+    size_t n_eaten = 0;
+    LOG_OP_CALL("H5FD_mirror_xmit_decode_write");
+    HDassert(out && buf);
+    n_eaten += H5FD_mirror_xmit_decode_header(&(out->pub), buf);
+    n_eaten += H5FD__mirror_xmit_decode_uint8(&(out->type), &buf[n_eaten]);
+    n_eaten += H5FD__mirror_xmit_decode_uint64(&(out->offset), &buf[n_eaten]);
+    n_eaten += H5FD__mirror_xmit_decode_uint64(&(out->size), &buf[n_eaten]);
+    HDassert(n_eaten == H5FD_MIRROR_XMIT_WRITE_SIZE);
+    return n_eaten;
+} /* end H5FD_mirror_xmit_decode_write() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    H5FD_mirror_xmit_encode_header
+ *
+ * Purpose:     Encode a mirror_xmit_t "header" to the bytes-buffer.
+ *
+ *              Fields will be packed into the buffer in a predictable manner,
+ *              any numbers stored in "network" (Big-Endian) word order.
+ *
+ *              The programmer must ensure that the destination buffer is
+ *              large enough to hold the expected data.
+ *
+ * Return:      The number of bytes written to the buffer.
+ * ---------------------------------------------------------------------------
+ */
+size_t
+H5FD_mirror_xmit_encode_header(unsigned char            *dest,
+                               const H5FD_mirror_xmit_t *x)
+{
+    size_t n_writ = 0;
+    LOG_OP_CALL("H5FD_mirror_xmit_encode_header");
+    HDassert(dest && x);
+    n_writ += H5FD__mirror_xmit_encode_uint32((dest+n_writ), x->magic);
+    n_writ += H5FD__mirror_xmit_encode_uint8((dest+n_writ), x->version);
+    n_writ += H5FD__mirror_xmit_encode_uint32((dest+n_writ), x->session_token);
+    n_writ += H5FD__mirror_xmit_encode_uint32((dest+n_writ), x->xmit_count);
+    n_writ += H5FD__mirror_xmit_encode_uint8((dest+n_writ), x->op);
+    HDassert(n_writ == H5FD_MIRROR_XMIT_HEADER_SIZE);
+    return n_writ;
+} /* end H5FD_mirror_xmit_encode_header() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    H5FD_mirror_xmit_encode_lock
+ *
+ * Purpose:     Encode a mirror_xmit_lock_t to the bytes-buffer.
+ *              Fields will be packed into the buffer in a predictable manner,
+ *              any numbers stored in "network" (Big-Endian) word order.
+ *
+ *              The programmer must ensure that the destination buffer is
+ *              large enough to hold the expected data.
+ *
+ * Return:      The number of bytes written to the buffer.
+ * ---------------------------------------------------------------------------
+ */
+size_t
+H5FD_mirror_xmit_encode_lock(unsigned char *dest,
+                             const H5FD_mirror_xmit_lock_t *x)
+{
+    size_t n_writ = 0;
+    LOG_OP_CALL("H5FD_mirror_xmit_encode_lock");
+    HDassert(dest && x);
+    n_writ += H5FD_mirror_xmit_encode_header(dest,
+            (const H5FD_mirror_xmit_t *)&(x->pub));
+    n_writ += H5FD__mirror_xmit_encode_uint64(&dest[n_writ], x->rw);
+    HDassert(n_writ == H5FD_MIRROR_XMIT_LOCK_SIZE);
+    return n_writ;
+} /* end H5FD_mirror_xmit_encode_lock() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    H5FD_mirror_xmit_encode_open
+ *
+ * Purpose:     Encode a mirror_xmit_open_t to the bytes-buffer.
+ *              Fields will be packed into the buffer in a predictable manner,
+ *              any numbers stored in "network" (Big-Endian) word order.
+ *
+ *              The programmer must ensure that the destination buffer is
+ *              large enough to hold the expected data.
+ *
+ * Return:      The maximum number of bytes that this decoding operation might
+ *              have written into the buffer.
+ * ---------------------------------------------------------------------------
+ */
+size_t
+H5FD_mirror_xmit_encode_open(unsigned char                 *dest,
+                             const H5FD_mirror_xmit_open_t *x)
+{
+    size_t n_writ = 0;
+    LOG_OP_CALL("H5FD_mirror_xmit_encode_open");
+    HDassert(dest && x);
+    HDbzero(dest, 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("H5FD_mirror_xmit_encode_reply");
+    HDassert(dest && x);
+    HDbzero(dest, 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("H5FD_mirror_xmit_encode_set_eoa");
+    HDassert(dest && x);
+    n_writ += H5FD_mirror_xmit_encode_header(dest,
+            (const H5FD_mirror_xmit_t *)&(x->pub));
+    n_writ += H5FD__mirror_xmit_encode_uint8(&dest[n_writ], x->type);
+    n_writ += H5FD__mirror_xmit_encode_uint64(&dest[n_writ], x->eoa_addr);
+    HDassert(n_writ == H5FD_MIRROR_XMIT_EOA_SIZE);
+    return n_writ;
+} /* end H5FD_mirror_xmit_encode_set_eoa() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    H5FD_mirror_xmit_encode_write
+ *
+ * Purpose:     Encode a mirror_xmit_write_t to the bytes-buffer.
+ *
+ *              Fields will be packed into the buffer in a predictable manner,
+ *              any numbers stored in "network" (Big-Endian) word order.
+ *
+ *              The programmer must ensure that the destination buffer is
+ *              large enough to hold the expected data.
+ *
+ * Return:      The number of bytes written to the buffer.
+ * ---------------------------------------------------------------------------
+ */
+size_t
+H5FD_mirror_xmit_encode_write(unsigned char                  *dest,
+                              const H5FD_mirror_xmit_write_t *x)
+{
+    size_t n_writ = 0;
+    LOG_OP_CALL("H5FD_mirror_xmit_encode_write");
+    HDassert(dest && x);
+    n_writ += H5FD_mirror_xmit_encode_header(dest,
+            (const H5FD_mirror_xmit_t *)&(x->pub));
+    n_writ += H5FD__mirror_xmit_encode_uint8(&dest[n_writ], x->type);
+    n_writ += H5FD__mirror_xmit_encode_uint64(&dest[n_writ], x->offset);
+    n_writ += H5FD__mirror_xmit_encode_uint64(&dest[n_writ], x->size);
+    HDassert(n_writ == H5FD_MIRROR_XMIT_WRITE_SIZE);
+    return n_writ;
+} /* end H5FD_mirror_xmit_encode_write() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    H5FD_mirror_xmit_is_close
+ *
+ * Purpose:     Verify that a mirror_xmit_t is a valid CLOSE xmit.
+ *
+ *              Checks header validity and op code.
+ *
+ * Return:      TRUE if valid; else FALSE.
+ * ---------------------------------------------------------------------------
+ */
+hbool_t
+H5FD_mirror_xmit_is_close(const H5FD_mirror_xmit_t *xmit)
+{
+    LOG_OP_CALL("H5FD_mirror_xmit_is_close");
+    HDassert(xmit);
+    if ( (TRUE == H5FD_mirror_xmit_is_xmit(xmit)) &&
+         (H5FD_MIRROR_OP_CLOSE == xmit->op) )
+    {
+        return TRUE;
+    }
+    return FALSE;
+} /* end H5FD_mirror_xmit_is_close() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    H5FD_mirror_xmit_is_lock
+ *
+ * Purpose:     Verify that a mirror_xmit_lock_t is a valid LOCK xmit.
+ *
+ *              Checks header validity and op code.
+ *
+ * Return:      TRUE if valid; else FALSE.
+ * ---------------------------------------------------------------------------
+ */
+hbool_t
+H5FD_mirror_xmit_is_lock(const H5FD_mirror_xmit_lock_t *xmit)
+{
+    LOG_OP_CALL("H5FD_mirror_xmit_is_lock");
+    HDassert(xmit);
+    if ( (TRUE == H5FD_mirror_xmit_is_xmit(&(xmit->pub))) &&
+         (H5FD_MIRROR_OP_LOCK == xmit->pub.op) )
+    {
+        return TRUE;
+    }
+    return FALSE;
+} /* end H5FD_mirror_xmit_is_lock() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    H5FD_mirror_xmit_is_open
+ *
+ * Purpose:     Verify that a mirror_xmit_open_t is a valid OPEN xmit.
+ *
+ *              Checks header validity and op code.
+ *
+ * Return:      TRUE if valid; else FALSE.
+ * ---------------------------------------------------------------------------
+ */
+hbool_t
+H5FD_mirror_xmit_is_open(const H5FD_mirror_xmit_open_t *xmit)
+{
+    LOG_OP_CALL("H5FD_mirror_xmit_is_open");
+    HDassert(xmit);
+    if ( (TRUE == H5FD_mirror_xmit_is_xmit(&(xmit->pub))) &&
+         (H5FD_MIRROR_OP_OPEN == xmit->pub.op) )
+    {
+        return TRUE;
+    }
+    return FALSE;
+} /* end H5FD_mirror_xmit_is_open() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    H5FD_mirror_xmit_is_eoa
+ *
+ * Purpose:     Verify that a mirror_xmit_eoa_t is a valid SET-EOA xmit.
+ *
+ *              Checks header validity and op code.
+ *
+ * Return:      TRUE if valid; else FALSE.
+ * ---------------------------------------------------------------------------
+ */
+hbool_t
+H5FD_mirror_xmit_is_set_eoa(const H5FD_mirror_xmit_eoa_t *xmit)
+{
+    LOG_OP_CALL("H5FD_mirror_xmit_is_set_eoa");
+    HDassert(xmit);
+    if ( (TRUE == H5FD_mirror_xmit_is_xmit(&(xmit->pub))) &&
+         (H5FD_MIRROR_OP_SET_EOA == xmit->pub.op) )
+    {
+        return TRUE;
+    }
+    return FALSE;
+} /* end H5FD_mirror_xmit_is_eoa() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    H5FD_mirror_xmit_is_reply
+ *
+ * Purpose:     Verify that a mirror_xmit_reply_t is a valid REPLY xmit.
+ *
+ *              Checks header validity and op code.
+ *
+ * Return:      TRUE if valid; else FALSE.
+ * ---------------------------------------------------------------------------
+ */
+hbool_t
+H5FD_mirror_xmit_is_reply(const H5FD_mirror_xmit_reply_t *xmit)
+{
+    LOG_OP_CALL("H5FD_mirror_xmit_is_reply");
+    HDassert(xmit);
+    if ( (TRUE == H5FD_mirror_xmit_is_xmit(&(xmit->pub))) &&
+         (H5FD_MIRROR_OP_REPLY == xmit->pub.op) )
+    {
+        return TRUE;
+    }
+    return FALSE;
+} /* end H5FD_mirror_xmit_is_reply() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    H5FD_mirror_xmit_is_write
+ *
+ * Purpose:     Verify that a mirror_xmit_write_t is a valid WRITE xmit.
+ *
+ *              Checks header validity and op code.
+ *
+ * Return:      TRUE if valid; else FALSE.
+ * ---------------------------------------------------------------------------
+ */
+hbool_t
+H5FD_mirror_xmit_is_write(const H5FD_mirror_xmit_write_t *xmit)
+{
+    LOG_OP_CALL("H5FD_mirror_xmit_is_write");
+    HDassert(xmit);
+    if ( (TRUE == H5FD_mirror_xmit_is_xmit(&(xmit->pub))) &&
+         (H5FD_MIRROR_OP_WRITE == xmit->pub.op) )
+    {
+        return TRUE;
+    }
+    return FALSE;
+} /* end H5FD_mirror_xmit_is_write() */
+
+
+/* ---------------------------------------------------------------------------
+ * Function:    H5FD_mirror_xmit_is_xmit
+ *
+ * Purpose:     Verify that a mirror_xmit_t is well-formed.
+ *
+ *              Checks magic number and structure version.
+ *
+ * Return:      TRUE if valid; else FALSE.
+ * ---------------------------------------------------------------------------
+ */
+hbool_t
+H5FD_mirror_xmit_is_xmit(const H5FD_mirror_xmit_t *xmit)
+{
+    LOG_OP_CALL("H5FD_mirror_xmit_is_xmit");
+    HDassert(xmit);
+    if ( (H5FD_MIRROR_XMIT_MAGIC != xmit->magic) || 
+         (H5FD_MIRROR_XMIT_CURR_VERSION != xmit->version) )
+    {
+        return FALSE;
+    }
+    return TRUE;
+} /* end H5FD_mirror_xmit_is_xmit() */
+
+
+/* ----------------------------------------------------------------------------
+ * Function:    H5FD__mirror_verify_reply
+ *
+ * Purpose:     Wait for and read reply data from remote processes.
+ *              Sanity-check that a reply is well-formed and valid.
+ *              If all checks pass, inspect the reply contents and handle
+ *              reported error, if not an OK reply.
+ *
+ * Return:      SUCCEED if ok, else FAIL.
+ * ----------------------------------------------------------------------------
+ */
+herr_t
+H5FD__mirror_verify_reply(H5FD_mirror_t *file)
+{
+    char                                xmit_buf[H5FD_MIRROR_XMIT_REPLY_SIZE];
+    struct H5FD_mirror_xmit_reply_t     reply;
+    ssize_t                             read_ret = 0;
+    herr_t                              ret_value = SUCCEED;
+
+    FUNC_ENTER_NOAPI_NOINIT;
+
+    LOG_OP_CALL("H5FD__mirror_verify_reply");
+
+    HDassert(file && file->sock_fd);
+
+    read_ret = HDread(file->sock_fd, xmit_buf, H5FD_MIRROR_XMIT_REPLY_SIZE);
+    if (read_ret < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_READERROR, FAIL, "unable to read reply");
+    }
+    if (read_ret != H5FD_MIRROR_XMIT_REPLY_SIZE) {
+        HGOTO_ERROR(H5E_VFL, H5E_READERROR, FAIL, "unexpected read size");
+    }
+
+    LOG_XMIT_BYTES("reply", xmit_buf, read_ret);
+
+    if (H5FD_mirror_xmit_decode_reply(&reply, (const unsigned char *)xmit_buf)
+        != H5FD_MIRROR_XMIT_REPLY_SIZE)
+    {
+        HGOTO_ERROR(H5E_VFL, H5E_BADVALUE, FAIL, "unable to decode reply xmit");
+    }
+
+    if (H5FD_mirror_xmit_is_reply(&reply) != TRUE) {
+        HGOTO_ERROR(H5E_VFL, H5E_BADVALUE, FAIL, "xmit op code was not REPLY");
+    }
+
+    if (reply.pub.session_token != file->xmit.session_token) {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADVALUE, FAIL, "wrong session");
+    }
+
+    if (reply.pub.xmit_count != (file->xmit_i)++) {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADVALUE, FAIL, "xmit out of sync");
+    }
+
+    if (reply.status != H5FD_MIRROR_STATUS_OK) {
+        HGOTO_ERROR(H5E_VFL, H5E_BADVALUE, FAIL,
+                    "%s", (const char *)(reply.message));
+    }
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value);
+} /* end H5FD__mirror_verify_reply() */
+
+
+/* -------------------------------------------------------------------------
+ * Function:    H5FD_mirror_fapl_get
+ *
+ * Purpose:     Get the file access propety list which could be used to create
+ *              an identical file.
+ *
+ * Return:      Success: pointer to the new file access property list value.
+ *              Failure: NULL
+ * -------------------------------------------------------------------------
+ */
+static void *
+H5FD_mirror_fapl_get(H5FD_t *_file)
+{
+    H5FD_mirror_t      *file      = (H5FD_mirror_t *)_file;
+    H5FD_mirror_fapl_t *fa        = NULL;
+    void               *ret_value = NULL;
+
+    FUNC_ENTER_NOAPI_NOINIT
+
+    LOG_OP_CALL("H5FD_mirror_fapl_get");
+
+    fa = (H5FD_mirror_fapl_t *)H5MM_calloc(sizeof(H5FD_mirror_fapl_t));
+    if (NULL == fa) {
+        HGOTO_ERROR(H5E_RESOURCE, H5E_NOSPACE, NULL, "calloc failed");
+    }
+
+    HDmemcpy(fa, &(file->fa), sizeof(H5FD_mirror_fapl_t));
+
+    ret_value = fa;
+
+done:
+    if (ret_value == NULL) {
+        if (fa != NULL) {
+            H5MM_xfree(fa);
+        }
+    }
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD_mirror_fapl_get() */
+
+
+/* -------------------------------------------------------------------------
+ * Function:    H5FD_mirror_fapl_copy
+ *
+ * Purpose:     Copies the mirror vfd-specific file access properties.
+ *
+ * Return:      Success:        Pointer to a new property list
+ *              Failure:        NULL
+ * -------------------------------------------------------------------------
+ */
+static void *
+H5FD_mirror_fapl_copy(const void *_old_fa)
+{
+    const H5FD_mirror_fapl_t *old_fa    = (const H5FD_mirror_fapl_t *)_old_fa;
+    H5FD_mirror_fapl_t       *new_fa    = NULL;
+    void                     *ret_value = NULL;
+
+    FUNC_ENTER_NOAPI_NOINIT
+
+    LOG_OP_CALL("H5FD_mirror_fapl_copy");
+
+    new_fa = (H5FD_mirror_fapl_t *)H5MM_malloc(sizeof(H5FD_mirror_fapl_t));
+    if (new_fa == NULL) {
+        HGOTO_ERROR(H5E_RESOURCE, H5E_NOSPACE, NULL,
+                    "memory allocation failed");
+    }
+
+    HDmemcpy(new_fa, old_fa, sizeof(H5FD_mirror_fapl_t));
+    ret_value = new_fa;
+
+done:
+    if (ret_value == NULL) {
+        if (new_fa != NULL) {
+            H5MM_xfree(new_fa);
+        }
+    }
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD_mirror_fapl_copy() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_mirror_fapl_free
+ *
+ * Purpose:     Frees the mirror VFD-specific file access properties.
+ *
+ * Return:      SUCCEED (cannot fail)
+ *-------------------------------------------------------------------------
+ */
+static herr_t
+H5FD_mirror_fapl_free(void *_fa)
+{
+    H5FD_mirror_fapl_t *fa = (H5FD_mirror_fapl_t*)_fa;
+
+    FUNC_ENTER_NOAPI_NOINIT_NOERR
+
+    LOG_OP_CALL("H5FD_mirror_fapl_free");
+
+    /* sanity check */
+    HDassert(fa != NULL);
+    HDassert(fa->magic == H5FD_MIRROR_FAPL_MAGIC);
+
+    fa->magic += 1; /* invalidate */
+    H5MM_xfree(fa);
+
+    FUNC_LEAVE_NOAPI(SUCCEED)
+} /* end H5FD_mirror_fapl_free() */
+
+
+/* -------------------------------------------------------------------------
+ * Function:    H5Pget_fapl_mirror
+ *
+ * Purpose:     Get the configuration information for this fapl.
+ *              Data is memcopied into the fa_out pointer.
+ *
+ * Return:      SUCCEED/FAIL
+ * -------------------------------------------------------------------------
+ */
+herr_t
+H5Pget_fapl_mirror(hid_t fapl_id, H5FD_mirror_fapl_t *fa_out)
+{
+    const H5FD_mirror_fapl_t *fa        = NULL;
+    H5P_genplist_t           *plist     = NULL;
+    herr_t                    ret_value = SUCCEED;
+
+    FUNC_ENTER_API(FAIL)
+    H5TRACE2("e", "i*x", fapl_id, fa_out);
+
+    LOG_OP_CALL("H5Pget_fapl_mirror");
+
+    if (NULL == fa_out) {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADVALUE, FAIL, "fa_out is NULL");
+    }
+
+    plist = H5P_object_verify(fapl_id, H5P_FILE_ACCESS);
+    if (NULL == plist) {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADTYPE, FAIL,
+                    "not a file access property list");
+    }
+
+    if (H5P_peek_driver(plist) != H5FD_MIRROR) {
+        HGOTO_ERROR(H5E_PLIST, H5E_BADVALUE, FAIL, "incorrect VFL driver");
+    }
+
+    fa = (const H5FD_mirror_fapl_t *)H5P_peek_driver_info(plist);
+    if (NULL == fa) {
+        HGOTO_ERROR(H5E_PLIST, H5E_BADVALUE, FAIL, "bad VFL driver info");
+    }
+
+    HDassert(fa->magic == H5FD_MIRROR_FAPL_MAGIC); /* sanity check */
+
+    HDmemcpy(fa_out, fa, sizeof(H5FD_mirror_fapl_t));
+
+done:
+    FUNC_LEAVE_API(ret_value);
+} /* end H5Pget_fapl_mirror() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5Pset_fapl_mirror
+ *
+ * Purpose:     Modify the file access property list to use the mirror
+ *              driver (H5FD_MIRROR) defined in this source file.
+ *
+ * Return:      SUCCEED/FAIL
+ *-------------------------------------------------------------------------
+ */
+herr_t
+H5Pset_fapl_mirror(hid_t fapl_id, H5FD_mirror_fapl_t *fa)
+{
+    H5P_genplist_t *plist = NULL;
+    herr_t ret_value = FAIL;
+
+    FUNC_ENTER_API(FAIL)
+    H5TRACE2("e", "i*x", fapl_id, fa);
+
+    LOG_OP_CALL("H5Pset_fapl_mirror");
+
+    plist = H5P_object_verify(fapl_id, H5P_FILE_ACCESS);
+    if (NULL == plist) {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADTYPE, FAIL,
+                    "not a file access property list");
+    }
+    if (NULL == fa) {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADVALUE, FAIL, "null fapl_t pointer");
+    }
+    if (H5FD_MIRROR_FAPL_MAGIC != fa->magic) {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADVALUE, FAIL, "invalid fapl_t magic");
+    }
+    if (H5FD_MIRROR_CURR_FAPL_T_VERSION != fa->version) {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADVALUE, FAIL, "unknown fapl_t version");
+    }
+
+    ret_value = H5P_set_driver(plist, H5FD_MIRROR, (const void *)fa);
+
+done:
+    FUNC_LEAVE_API(ret_value)
+} /* end H5Pset_fapl_mirror() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_mirror_open
+ *
+ * Purpose:     Create and/or opens a file as an HDF5 file.
+ *
+ *              Initiate connection with remote Server/Writer.
+ *              If successful, the remote file is open.
+ *
+ * Return:      Success:    A pointer to a new file data structure. The
+ *                          public fields will be initialized by the
+ *                          caller, which is always H5FD_open().
+ *              Failure:    NULL
+ *-------------------------------------------------------------------------
+ */
+static H5FD_t *
+H5FD_mirror_open(const char *name,
+                 unsigned    flags,
+                 hid_t       fapl_id,
+                 haddr_t     maxaddr)
+{
+#define MIRR_OPEN_MAXBUF 16 /* local symbol to give meaning to magic number  */
+                            /* #defined because it is needed at compile time */
+                            /* Large enough to hold a port number string     */
+    int                      live_socket = -1;
+    struct sockaddr_in       target_addr;
+    socklen_t                addr_size;
+    char                     xmit_buf[H5FD_MIRROR_XMIT_OPEN_SIZE];
+    H5FD_mirror_fapl_t       fa;
+    H5FD_mirror_t           *file      = NULL;
+    H5FD_mirror_xmit_open_t  open_xmit;
+    H5FD_t                  *ret_value = NULL;
+
+    FUNC_ENTER_NOAPI_NOINIT
+
+    LOG_OP_CALL("H5FD_mirror_open");
+
+    /* --------------- */
+    /* Check arguments */
+    /* --------------- */
+
+    if (!name || !*name) {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADVALUE, NULL, "invalid file name");
+    }
+    if (HDstrlen(name) >= H5FD_MIRROR_XMIT_FILEPATH_MAX) {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADVALUE, NULL, "filename is too long");
+    }
+    if (0 == maxaddr || HADDR_UNDEF == maxaddr) {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADRANGE, NULL, "bogus maxaddr");
+    }
+    if (ADDR_OVERFLOW(maxaddr)) {
+        HGOTO_ERROR(H5E_ARGS, H5E_OVERFLOW, NULL, "bogus maxaddr");
+    }
+
+    if (H5Pget_fapl_mirror(fapl_id, &fa) == FAIL) {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADVALUE, NULL, "can't get config info");
+    }
+
+    if (H5FD_MIRROR_FAPL_MAGIC != fa.magic) {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADVALUE, NULL, "invalid fapl magic");
+    }
+
+    if (H5FD_MIRROR_CURR_FAPL_T_VERSION != fa.version) {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADVALUE, NULL, "invalid fapl version");
+    }
+
+    /* --------------------- */
+    /* Handshake with remote */
+    /* --------------------- */
+
+    live_socket = HDsocket(AF_INET, SOCK_STREAM, 0);
+    if (live_socket < 0) {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADVALUE, NULL, "can't create socket");
+    }
+
+    target_addr.sin_family = AF_INET;
+    target_addr.sin_port = HDhtons((uint16_t)fa.handshake_port);
+    target_addr.sin_addr.s_addr = HDinet_addr(fa.remote_ip);
+    HDmemset(target_addr.sin_zero, '\0', sizeof target_addr.sin_zero);
+
+    addr_size = sizeof(target_addr);
+    if (HDconnect(live_socket, (struct sockaddr *)&target_addr, addr_size) < 0)
+    {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADVALUE, NULL,
+                    "can't connect to remote server");
+    }
+
+    /* ------------- */
+    /* Open the file */
+    /* ------------- */
+
+    file = (H5FD_mirror_t *)H5FL_CALLOC(H5FD_mirror_t);
+    if (NULL == file) {
+        HGOTO_ERROR(H5E_RESOURCE, H5E_NOSPACE, NULL,
+                    "unable to allocate file struct");
+    }
+
+    file->sock_fd = live_socket;
+    file->xmit_i = 0;
+
+    file->xmit.magic = H5FD_MIRROR_XMIT_MAGIC;
+    file->xmit.version = H5FD_MIRROR_XMIT_CURR_VERSION;
+    file->xmit.xmit_count = file->xmit_i++;
+    file->xmit.session_token = (uint32_t)(0x01020304 ^ file->sock_fd); /* TODO: hashing? */
+    /* int --> uint32_t may truncate on some systems... shouldn't matter? */
+
+    file->xmit.op = H5FD_MIRROR_OP_OPEN;
+    open_xmit.pub = file->xmit;
+    open_xmit.flags = (uint32_t)flags;
+    open_xmit.maxaddr = (uint64_t)maxaddr;
+    open_xmit.size_t_blob = (uint64_t)((size_t)(-1));
+    HDsnprintf(open_xmit.filename, H5FD_MIRROR_XMIT_FILEPATH_MAX-1, "%s", name);
+
+    if (H5FD_mirror_xmit_encode_open((unsigned char *)xmit_buf,
+            (const H5FD_mirror_xmit_open_t *)&open_xmit)
+        != H5FD_MIRROR_XMIT_OPEN_SIZE)
+    {
+        HGOTO_ERROR(H5E_VFL, H5E_WRITEERROR, NULL, "unable to encode open");
+    }
+
+    LOG_XMIT_BYTES("open", xmit_buf, H5FD_MIRROR_XMIT_OPEN_SIZE);
+
+    if (HDwrite(file->sock_fd, xmit_buf, H5FD_MIRROR_XMIT_OPEN_SIZE) < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_WRITEERROR, NULL, "unable to transmit open");
+    }
+
+    if (H5FD__mirror_verify_reply(file) == FAIL) {
+        HGOTO_ERROR(H5E_VFL, H5E_BADVALUE, NULL, "invalid reply");
+    }
+
+    ret_value = (H5FD_t *)file;
+
+done:
+    if (NULL == ret_value) {
+        if (file) {
+            file = H5FL_FREE(H5FD_mirror_t, file);
+        }
+        if (live_socket >= 0 && HDclose(live_socket) < 0) {
+            HDONE_ERROR(H5E_VFL, H5E_CANTCLOSEFILE, NULL,
+                        "can't close socket");
+        }
+    }
+    FUNC_LEAVE_NOAPI(ret_value)
+#undef MIRR_OPEN_MAXBUF
+} /* end H5FD_mirror_open() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_mirror_close
+ *
+ * Purpose:     Closes the HDF5 file.
+ *
+ *              Tries to send a CLOSE op to the remote Writer and expects
+ *              a valid reply, then closes the socket.
+ *              In error, attempts to send a deliberately invalid xmit to the
+ *              Writer to get it to close/abort, then attempts to close the
+ *              socket.
+ *
+ * Return:      Success:    SUCCEED
+ *              Failure:    FAIL, file possibly not closed but resources freed.
+ *-------------------------------------------------------------------------
+ */
+static herr_t
+H5FD_mirror_close(H5FD_t *_file)
+{
+    H5FD_mirror_t *file = (H5FD_mirror_t *)_file;
+    unsigned char  xmit_buf[H5FD_MIRROR_XMIT_HEADER_SIZE];
+    int xmit_encoded = 0; /* monitor point of failure */
+    herr_t         ret_value = SUCCEED;
+
+    FUNC_ENTER_NOAPI_NOINIT
+
+    LOG_OP_CALL("H5FD_mirror_close");
+
+    /* Sanity check */
+    HDassert(file);
+    HDassert(file->sock_fd >= 0);
+
+    file->xmit.xmit_count = (file->xmit_i)++;
+    file->xmit.op = H5FD_MIRROR_OP_CLOSE;
+
+    if (H5FD_mirror_xmit_encode_header(xmit_buf,
+            (const H5FD_mirror_xmit_t *)&(file->xmit))
+        != H5FD_MIRROR_XMIT_HEADER_SIZE)
+    {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADVALUE, FAIL, "unable to encode close");
+    }
+    xmit_encoded = 1;
+
+    LOG_XMIT_BYTES("close", xmit_buf, H5FD_MIRROR_XMIT_HEADER_SIZE);
+
+    if (HDwrite(file->sock_fd, xmit_buf, H5FD_MIRROR_XMIT_HEADER_SIZE) < 0) {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADVALUE, FAIL, "unable to transmit close");
+    }
+
+    if (H5FD__mirror_verify_reply(file) == FAIL) {
+        HGOTO_ERROR(H5E_VFL, H5E_BADVALUE, FAIL, "invalid reply");
+    }
+
+    if (HDclose(file->sock_fd) < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_CANTCLOSEFILE, FAIL, "can't close socket");
+    }
+
+done:
+    if (ret_value == FAIL) {
+        if (xmit_encoded == 0) {
+            /* Encode failed; send GOODBYE to force writer halt.
+             * We can ignore any response from the writer, if we receive
+             * any reply at all.
+             */
+            if (HDwrite(file->sock_fd, "GOODBYE", HDstrlen("GOODBYE")) < 0) {
+                HDONE_ERROR(H5E_ARGS, H5E_BADVALUE, FAIL,
+                            "unable to transmit close");
+                if (HDclose(file->sock_fd) < 0) {
+                    HDONE_ERROR(H5E_VFL, H5E_CANTCLOSEFILE, FAIL,
+                                "can't close socket");
+                }
+                file->sock_fd = -1; /* invalidate for later */
+            } /* end if problem writing goodbye; go down hard */
+            else
+            if (HDshutdown(file->sock_fd, SHUT_WR) < 0) {
+                HDONE_ERROR(H5E_VFL, H5E_BADVALUE, FAIL,
+                            "can't shutdown socket write: %s",
+                            HDstrerror(errno));
+            } /* end else-if problem shutting down socket */
+        } /* end if xmit encode failed */
+
+        if (file->sock_fd >= 0) {
+            if (HDclose(file->sock_fd) < 0) {
+                HDONE_ERROR(H5E_VFL, H5E_CANTCLOSEFILE, FAIL,
+                            "can't close socket");
+            }
+        } /* end if socket not closed by going down hard */
+    } /* end if error */
+
+    file = H5FL_FREE(H5FD_mirror_t, file); /* always release resources */
+
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD_mirror_close() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_mirror_query
+ *
+ * Purpose:     Get the driver feature flags implemented by the driver.
+ *
+ * Return:      SUCCEED (non-negative) (can't fail)
+ *-------------------------------------------------------------------------
+ */
+static herr_t
+H5FD_mirror_query(const H5FD_t H5_ATTR_UNUSED *_file, unsigned long *flags)
+{
+    FUNC_ENTER_NOAPI_NOINIT_NOERR;
+
+    LOG_OP_CALL("H5FD_mirror_query");
+
+    /* Notice: the Mirror VFD Writer currently uses only the Sec2 driver as
+     * the underying driver -- as such, the Mirror VFD implementation copies
+     * the Sec2 feature flags as its own.
+     *
+     * File pointer is always NULL/unused -- the H5FD_FEAT_IGNORE_DRVRINFO flag
+     * is never included.
+     * -- JOS 2020-01-13
+     */
+    if (flags) {
+        *flags = 0                                      \
+               | H5FD_FEAT_AGGREGATE_METADATA           \
+               | H5FD_FEAT_ACCUMULATE_METADATA          \
+               | H5FD_FEAT_DATA_SIEVE                   \
+               | H5FD_FEAT_AGGREGATE_SMALLDATA          \
+               | H5FD_FEAT_POSIX_COMPAT_HANDLE          \
+               | H5FD_FEAT_SUPPORTS_SWMR_IO             \
+               | H5FD_FEAT_DEFAULT_VFD_COMPATIBLE;
+    }
+
+    FUNC_LEAVE_NOAPI(SUCCEED);
+} /* end H5FD_mirror_query() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_mirror_get_eoa
+ *
+ * Purpose:     Gets the end-of-address marker for the file. The EOA marker
+ *              is the first address past the last byte allocated in the
+ *              format address space.
+ *
+ *              Required to register the driver.
+ *
+ * Return:      The end-of-address marker.
+ *-------------------------------------------------------------------------
+ */
+static haddr_t
+H5FD_mirror_get_eoa(const H5FD_t *_file, H5FD_mem_t H5_ATTR_UNUSED type)
+{
+    const H5FD_mirror_t *file = (const H5FD_mirror_t *)_file;
+
+    FUNC_ENTER_NOAPI_NOINIT_NOERR
+
+    LOG_OP_CALL("H5FD_mirror_get_eoa");
+
+    HDassert(file);
+
+    FUNC_LEAVE_NOAPI(file->eoa)
+} /* end H5FD_mirror_get_eoa() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_mirror_set_eoa
+ *
+ * Purpose:     Set the end-of-address marker for the file. This function is
+ *              called shortly after an existing HDF5 file is opened in order
+ *              to tell the driver where the end of the HDF5 data is located.
+ *
+ * Return:      SUCCEED / FAIL
+ *-------------------------------------------------------------------------
+ */
+static herr_t
+H5FD_mirror_set_eoa(H5FD_t *_file, H5FD_mem_t type, haddr_t addr)
+{
+    H5FD_mirror_xmit_eoa_t      xmit_eoa;
+    unsigned char       xmit_buf[H5FD_MIRROR_XMIT_EOA_SIZE];
+    H5FD_mirror_t      *file      = (H5FD_mirror_t *)_file;
+    herr_t              ret_value = SUCCEED;
+
+    FUNC_ENTER_NOAPI_NOINIT
+
+    LOG_OP_CALL("H5FD_mirror_set_eoa");
+
+    HDassert(file);
+
+    file->eoa = addr; /* local copy */
+
+    file->xmit.xmit_count = (file->xmit_i)++;
+    file->xmit.op = H5FD_MIRROR_OP_SET_EOA;
+
+    xmit_eoa.pub = file->xmit;
+    xmit_eoa.type = (uint8_t)type;
+    xmit_eoa.eoa_addr = (uint64_t)addr;
+
+    if (H5FD_mirror_xmit_encode_set_eoa(xmit_buf,
+            (const H5FD_mirror_xmit_eoa_t *)&xmit_eoa)
+        != H5FD_MIRROR_XMIT_EOA_SIZE)
+    {
+        HGOTO_ERROR(H5E_VFL, H5E_WRITEERROR, FAIL, "unable to encode set-eoa");
+    }
+
+    LOG_XMIT_BYTES("set-eoa", xmit_buf, H5FD_MIRROR_XMIT_EOA_SIZE);
+
+    if (HDwrite(file->sock_fd, xmit_buf, H5FD_MIRROR_XMIT_EOA_SIZE) < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_WRITEERROR, FAIL,
+                    "unable to transmit set-eoa");
+    }
+
+    if (H5FD__mirror_verify_reply(file) == FAIL) {
+        HGOTO_ERROR(H5E_VFL, H5E_BADVALUE, FAIL, "invalid reply");
+    }
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD_mirror_set_eoa() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_mirror_get_eof
+ *
+ * Purpose:     Returns the end-of-file marker, which is the greater of
+ *              either the filesystem end-of-file or the HDF5 end-of-address
+ *              markers.
+ *
+ *              Required to register the driver.
+ *
+ * Return:      End of file address, the first address past the end of the
+ *              "file", either the filesystem file or the HDF5 file.
+ *-------------------------------------------------------------------------
+ */
+static haddr_t
+H5FD_mirror_get_eof(const H5FD_t *_file, H5FD_mem_t H5_ATTR_UNUSED type)
+{
+    const H5FD_mirror_t *file = (const H5FD_mirror_t *)_file;
+
+    FUNC_ENTER_NOAPI_NOINIT_NOERR
+
+    LOG_OP_CALL("H5FD_mirror_get_eof");
+
+    HDassert(file);
+
+    FUNC_LEAVE_NOAPI(file->eof)
+} /* end H5FD_mirror_get_eof() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_mirror_read
+ *
+ * Purpose:     Required to register the driver.
+ *              If called, MUST fail.
+ *
+ * Return:      FAIL
+ *-------------------------------------------------------------------------
+ */
+static herr_t
+H5FD_mirror_read(H5FD_t     H5_ATTR_UNUSED *_file,
+                 H5FD_mem_t H5_ATTR_UNUSED  type,
+                 hid_t      H5_ATTR_UNUSED  fapl_id,
+                 haddr_t    H5_ATTR_UNUSED  addr,
+                 size_t     H5_ATTR_UNUSED  size,
+                 void       H5_ATTR_UNUSED *buf)
+{
+    herr_t ret_value = FAIL;
+
+    FUNC_ENTER_NOAPI_NOINIT_NOERR
+
+    LOG_OP_CALL("H5FD_mirror_read");
+
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD_mirror_read() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_mirror_write
+ *
+ * Purpose:     Writes SIZE bytes of data to FILE beginning at address ADDR
+ *              from buffer BUF according to data transfer properties in
+ *              DXPL_ID.
+ *
+ *              Send metadata regarding the write (location, size) to the
+ *              remote Writer, then separately transmits the data.
+ *              Both transmission expect an OK reply from the Writer.
+ *              This two-exchange approach incurs significant overhead,
+ *              but is a simple and modular approach.
+ *              Start optimizations here.
+ *
+ * Return:      SUCCEED/FAIL
+ *-------------------------------------------------------------------------
+ */
+static herr_t
+H5FD_mirror_write(H5FD_t               *_file,
+                  H5FD_mem_t            type,
+                  hid_t H5_ATTR_UNUSED  dxpl_id,
+                  haddr_t               addr,
+                  size_t                size,
+                  const void           *buf)
+{
+    H5FD_mirror_xmit_write_t    xmit_write;
+    unsigned char               xmit_buf[H5FD_MIRROR_XMIT_WRITE_SIZE];
+    H5FD_mirror_t              *file        = (H5FD_mirror_t *)_file;
+    herr_t                      ret_value   = SUCCEED;
+
+    FUNC_ENTER_NOAPI_NOINIT
+
+    LOG_OP_CALL("H5FD_mirror_write");
+
+    HDassert(file);
+    HDassert(buf);
+
+    file->xmit.xmit_count = (file->xmit_i)++;
+    file->xmit.op = H5FD_MIRROR_OP_WRITE;
+
+    xmit_write.pub = file->xmit;
+    xmit_write.size = (uint64_t)size;
+    xmit_write.offset = (uint64_t)addr;
+    xmit_write.type = (uint8_t)type;
+
+
+    /* Notify Writer of incoming data to write. */
+    if (H5FD_mirror_xmit_encode_write(xmit_buf,
+            (const H5FD_mirror_xmit_write_t *)&xmit_write)
+        != H5FD_MIRROR_XMIT_WRITE_SIZE)
+    {
+        HGOTO_ERROR(H5E_VFL, H5E_WRITEERROR, FAIL, "unable to encode write");
+    }
+
+    LOG_XMIT_BYTES("write", xmit_buf, H5FD_MIRROR_XMIT_WRITE_SIZE);
+
+    if (HDwrite(file->sock_fd, xmit_buf, H5FD_MIRROR_XMIT_WRITE_SIZE) < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_WRITEERROR, FAIL, "unable to transmit write");
+    }
+
+    /* Check that our write xmission was received */
+    if (H5FD__mirror_verify_reply(file) == FAIL) {
+        HGOTO_ERROR(H5E_VFL, H5E_BADVALUE, FAIL, "invalid reply");
+    }
+
+    /* Send the data to be written */
+    if (HDwrite(file->sock_fd, buf, size) < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_WRITEERROR, FAIL, "unable to transmit data");
+    }
+
+    /* Writer should reply that it got the data and is still okay/ready */
+    if (H5FD__mirror_verify_reply(file) == FAIL) {
+        HGOTO_ERROR(H5E_VFL, H5E_BADVALUE, FAIL, "invalid reply");
+    }
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD_mirror_write() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_mirror_truncate
+ *
+ * Purpose:     Makes sure that the true file size is the same (or larger)
+ *              than the end-of-address.
+ *
+ * Return:      SUCCEED/FAIL
+ *-------------------------------------------------------------------------
+ */
+static herr_t
+H5FD_mirror_truncate(H5FD_t                 *_file,
+                     hid_t   H5_ATTR_UNUSED  dxpl_id,
+                     hbool_t H5_ATTR_UNUSED  closing)
+{
+    unsigned char       xmit_buf[H5FD_MIRROR_XMIT_HEADER_SIZE];
+    H5FD_mirror_t      *file = (H5FD_mirror_t *)_file;
+    herr_t      ret_value = SUCCEED;
+
+    FUNC_ENTER_NOAPI_NOINIT
+
+    LOG_OP_CALL("H5FD_mirror_truncate");
+
+    file->xmit.xmit_count = (file->xmit_i)++;
+    file->xmit.op = H5FD_MIRROR_OP_TRUNCATE;
+
+    if (H5FD_mirror_xmit_encode_header(xmit_buf, &(file->xmit))
+        != H5FD_MIRROR_XMIT_HEADER_SIZE)
+    {
+        HGOTO_ERROR(H5E_VFL, H5E_WRITEERROR, FAIL, "unable to encode truncate");
+    }
+
+    LOG_XMIT_BYTES("truncate", xmit_buf, H5FD_MIRROR_XMIT_HEADER_SIZE);
+
+    if (HDwrite(file->sock_fd, xmit_buf, H5FD_MIRROR_XMIT_HEADER_SIZE) < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_WRITEERROR, FAIL,
+                    "unable to transmit truncate");
+    }
+
+    if (H5FD__mirror_verify_reply(file) == FAIL) {
+        HGOTO_ERROR(H5E_VFL, H5E_BADVALUE, FAIL, "invalid reply");
+    }
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD_mirror_truncate() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_mirror_lock
+ *
+ * Purpose:     To place an advisory lock on a file.
+ *              The lock type to apply depends on the parameter "rw":
+ *                      TRUE--opens for write: an exclusive lock
+ *                      FALSE--opens for read: a shared lock
+ *
+ * Return:      SUCCEED/FAIL
+ *-------------------------------------------------------------------------
+ */
+static herr_t
+H5FD_mirror_lock(H5FD_t *_file, hbool_t rw)
+{
+    H5FD_mirror_xmit_lock_t xmit_lock;
+    unsigned char       xmit_buf[H5FD_MIRROR_XMIT_LOCK_SIZE];
+    H5FD_mirror_t      *file = (H5FD_mirror_t *)_file;
+    herr_t ret_value = SUCCEED;
+
+    FUNC_ENTER_NOAPI_NOINIT;
+
+    LOG_OP_CALL("H5FD_mirror_lock");
+
+    file->xmit.xmit_count = (file->xmit_i)++;
+    file->xmit.op = H5FD_MIRROR_OP_LOCK;
+
+    xmit_lock.pub = file->xmit;
+    xmit_lock.rw = (uint64_t)rw;
+
+    if (H5FD_mirror_xmit_encode_lock(xmit_buf,
+            (const H5FD_mirror_xmit_lock_t *)&xmit_lock)
+        != H5FD_MIRROR_XMIT_LOCK_SIZE)
+    {
+        HGOTO_ERROR(H5E_VFL, H5E_WRITEERROR, FAIL, "unable to encode lock");
+    }
+
+    LOG_XMIT_BYTES("lock", xmit_buf, H5FD_MIRROR_XMIT_LOCK_SIZE);
+
+    if (HDwrite(file->sock_fd, xmit_buf, H5FD_MIRROR_XMIT_LOCK_SIZE) < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_WRITEERROR, FAIL, "unable to transmit lock");
+    }
+
+    if (H5FD__mirror_verify_reply(file) == FAIL) {
+        HGOTO_ERROR(H5E_VFL, H5E_BADVALUE, FAIL, "invalid reply");
+    }
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value);
+} /* end H5FD_mirror_lock */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_mirror_unlock
+ *
+ * Purpose:     Remove the existing lock on the file.
+ *
+ * Return:      SUCCEED/FAIL
+ *-------------------------------------------------------------------------
+ */
+static herr_t
+H5FD_mirror_unlock(H5FD_t *_file)
+{
+    unsigned char       xmit_buf[H5FD_MIRROR_XMIT_HEADER_SIZE];
+    H5FD_mirror_t      *file = (H5FD_mirror_t *)_file;
+    herr_t ret_value = SUCCEED;
+
+    FUNC_ENTER_NOAPI_NOINIT;
+
+    LOG_OP_CALL("H5FD_mirror_unlock");
+
+    file->xmit.xmit_count = (file->xmit_i)++;
+    file->xmit.op = H5FD_MIRROR_OP_UNLOCK;
+
+    if (H5FD_mirror_xmit_encode_header(xmit_buf, &(file->xmit))
+        != H5FD_MIRROR_XMIT_HEADER_SIZE)
+    {
+        HGOTO_ERROR(H5E_VFL, H5E_WRITEERROR, FAIL, "unable to encode unlock");
+    }
+
+    LOG_XMIT_BYTES("unlock", xmit_buf, H5FD_MIRROR_XMIT_HEADER_SIZE);
+
+    if (HDwrite(file->sock_fd, xmit_buf, H5FD_MIRROR_XMIT_HEADER_SIZE) < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_WRITEERROR, FAIL, "unable to transmit unlock");
+    }
+
+    if (H5FD__mirror_verify_reply(file) == FAIL) {
+        HGOTO_ERROR(H5E_VFL, H5E_BADVALUE, FAIL, "invalid reply");
+    }
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value);
+} /* end H5FD_mirror_unlock */
+
+#endif /* H5_HAVE_MIRROR_VFD */
+
diff --git a/src/H5FDmirror.h b/src/H5FDmirror.h
new file mode 100644
index 0000000..fb66b7b
--- /dev/null
+++ b/src/H5FDmirror.h
@@ -0,0 +1,371 @@
+/* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
+ * Copyright by The HDF Group.                                               *
+ * All rights reserved.                                                      *
+ *                                                                           *
+ * This file is part of HDF5.  The full HDF5 copyright notice, including     *
+ * terms governing use, modification, and redistribution, is contained in    *
+ * the COPYING file, which can be found at the root of the source code       *
+ * distribution tree, or in https://support.hdfgroup.org/ftp/HDF5/releases.  *
+ * If you do not have access to either file, you may request a copy from     *
+ * help@hdfgroup.org.                                                        *
+ * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */
+
+/*
+ * Purpose: Public, shared definitions for Mirror VFD & remote Writer.
+ */
+
+#ifndef H5FDmirror_H
+#define H5FDmirror_H
+
+#ifdef H5_HAVE_MIRROR_VFD
+
+#define H5FD_MIRROR (H5FD_mirror_init())
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/* ============================================================================
+ * Mirror VFD use and operation.
+ * ============================================================================
+ */
+
+/* ---------------------------------------------------------------------------
+ * Structure:   H5FD_mirror_fapl_t
+ *
+ * Used to pass configuraiton information to the Mirror VFD.
+ * Populate components as appropriate and pass structure pointer to
+ * `H5Pset_fapl_mirror()`.
+ *
+ * `magic` (uint32_t)
+ *      Semi-unique number to sanity-check pointers to this structure type.
+ *      MUST equal H5FD_MIRROR_FAPL_MAGIC to be considered valid.
+ *
+ * `version` (uint32_t)
+ *      Indicates expected components of the structure.
+ *
+ * `handshake_port (int)
+ *      Port number to expect to reach the "Mirror Server" on the remote host.
+ *
+ * `remote_ip` (char[])
+ *      IP address string of "Mirror Server" remote host.
+ * ---------------------------------------------------------------------------
+ */
+#define H5FD_MIRROR_FAPL_MAGIC 0xF8DD514C
+#define H5FD_MIRROR_CURR_FAPL_T_VERSION 1
+#define H5FD_MIRROR_MAX_IP_LEN 32
+typedef struct H5FD_mirror_fapl_t {
+    uint32_t magic;
+    uint32_t version;
+    int     handshake_port;
+    char    remote_ip[H5FD_MIRROR_MAX_IP_LEN + 1];
+} H5FD_mirror_fapl_t;
+
+H5_DLL hid_t  H5FD_mirror_init(void);
+H5_DLL herr_t H5Pget_fapl_mirror(hid_t fapl_id, H5FD_mirror_fapl_t *fa_out);
+H5_DLL herr_t H5Pset_fapl_mirror(hid_t fapl_id, H5FD_mirror_fapl_t *fa);
+
+/* = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = =
+ * IPC - Mirror VFD and Remote Worker application.
+ * = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = =
+ */
+
+
+/* The maximum allowed size for a receiving buffer when accepting bytes to
+ * write. Writes larger than this size are performed by multiple accept-write
+ * steps by the Writer. */
+#define H5FD_MIRROR_DATA_BUFFER_MAX H5_GB /* 1 Gigabyte */
+
+#define H5FD_MIRROR_XMIT_CURR_VERSION 1
+#define H5FD_MIRROR_XMIT_MAGIC 0x87F8005B
+
+#define H5FD_MIRROR_OP_OPEN     1
+#define H5FD_MIRROR_OP_CLOSE    2
+#define H5FD_MIRROR_OP_WRITE    3
+#define H5FD_MIRROR_OP_TRUNCATE 4
+#define H5FD_MIRROR_OP_REPLY    5
+#define H5FD_MIRROR_OP_SET_EOA  6
+#define H5FD_MIRROR_OP_LOCK     7
+#define H5FD_MIRROR_OP_UNLOCK   8
+
+#define H5FD_MIRROR_STATUS_OK 0
+#define H5FD_MIRROR_STATUS_ERROR 1
+#define H5FD_MIRROR_STATUS_MESSAGE_MAX 256 /* Dedicated error message size */
+
+/* Maximum length of a path/filename string, including the NULL-terminator.
+ * Must not be smaller than H5FD_SPLITTER_PATH_MAX. */
+#define H5FD_MIRROR_XMIT_FILEPATH_MAX 4097
+
+/* Define the exact sizes of the various xmit blobs as sent over the wire.
+ * This is used to minimize the number of bytes transmitted as well as to
+ * sanity-check received bytes.
+ * Any modifications to the xmit structures and/or the encode/decode functions
+ * must be reflected here.
+ * */
+#define H5FD_MIRROR_XMIT_HEADER_SIZE 14
+#define H5FD_MIRROR_XMIT_EOA_SIZE (H5FD_MIRROR_XMIT_HEADER_SIZE + 9)
+#define H5FD_MIRROR_XMIT_LOCK_SIZE (H5FD_MIRROR_XMIT_HEADER_SIZE + 8)
+#define H5FD_MIRROR_XMIT_OPEN_SIZE (H5FD_MIRROR_XMIT_HEADER_SIZE + 20 + H5FD_MIRROR_XMIT_FILEPATH_MAX)
+#define H5FD_MIRROR_XMIT_REPLY_SIZE (H5FD_MIRROR_XMIT_HEADER_SIZE + 4 + H5FD_MIRROR_STATUS_MESSAGE_MAX)
+#define H5FD_MIRROR_XMIT_WRITE_SIZE (H5FD_MIRROR_XMIT_HEADER_SIZE + 17)
+
+/* Maximum length of any xmit. */
+#define H5FD_MIRROR_XMIT_BUFFER_MAX MAX2( MAX3(H5FD_MIRROR_XMIT_HEADER_SIZE,  \
+                                               H5FD_MIRROR_XMIT_EOA_SIZE,     \
+                                               H5FD_MIRROR_XMIT_LOCK_SIZE),   \
+                                          MAX3(H5FD_MIRROR_XMIT_OPEN_SIZE,    \
+                                               H5FD_MIRROR_XMIT_REPLY_SIZE,   \
+                                               H5FD_MIRROR_XMIT_WRITE_SIZE) ) \
+
+/* ---------------------------------------------------------------------------
+ * Structure:   H5FD_mirror_xmit_t
+ *
+ * Common structure 'header' for all mirror VFD/worker IPC.
+ * Must be the first component of a derived operation xmit structure,
+ * such as file-open or write command.
+ *
+ * `magic` (uint32_t)
+ *      A "unique" number identifying the structure and endianness of
+ *      transmitting maching.
+ *      Must be set to H5FD_MIRROR_XMIT_MAGIC native to the VFD "sender".
+ *
+ * `version` (uint8_t)
+ *      Number used to identify the structure membership.
+ *      Allows sane modifications to this structure in the future.
+ *      Must be set to H5FD_MIRROR_XMIT_CURR_VERSION.
+ *
+ * `session_token` (uint32_t)
+ *      A "unique" number identifying the session between VFD sender and
+ *      remote receiver/worker/writer. Exists to help sanity-check.
+ *
+ * `xmit_count` (uint32_t)
+ *      Which transmission this is since the session began.
+ *      Used to sanity-check transmission errors.
+ *      First xmit (file-open) must be 0.
+ *
+ * `op` (uint8_t)
+ *      Number identifying which operation to perform.
+ *      Corresponds with the extended structure outside of this xmit header.
+ *      Possible values are all defined H5FD_MIRROR_OP_* constants.
+ *
+ * ---------------------------------------------------------------------------
+ */
+typedef struct H5FD_mirror_xmit_t {
+    uint32_t magic;
+    uint8_t  version;
+    uint32_t session_token;
+    uint32_t xmit_count;
+    uint8_t  op;
+} H5FD_mirror_xmit_t;
+
+/* ---------------------------------------------------------------------------
+ * Structure:   H5FD_mirror_xmit_eoa_t
+ *
+ * Structure containing eoa-set information from VFD sender.
+ *
+ * `pub` (H5FD_mirror_xmit_t)
+ *      Common transmission header, containing session information.
+ *      Must be first.
+ *
+ * `type` (uint8_t)
+ *      System-independent alias for H5F[D]_mem_t.
+ *      Specifies datatype to be written.
+ *
+ * `eoa_addr` (uint64_t)
+ *      New address for eoa.
+ *      (Natively 'haddr_t', always a 64-bit field)
+ *
+ * ---------------------------------------------------------------------------
+ */
+typedef struct H5FD_mirror_xmit_eoa_t {
+    H5FD_mirror_xmit_t pub;
+    uint8_t            type;
+    uint64_t           eoa_addr;
+} H5FD_mirror_xmit_eoa_t;
+
+/* ---------------------------------------------------------------------------
+ * Structure:   H5FD_mirror_xmit_lock_t
+ *
+ * Structure containing eoa-set information from VFD sender.
+ *
+ * `pub` (H5FD_mirror_xmit_t)
+ *      Common transmission header, containing session information.
+ *      Must be first.
+ *
+ * `rw` (uint64_t)
+ *      The Read/Write mode flag passed into H5FDlock().
+ *      (Natively `hbool_t`, an 'int') TODO: native int may be 64-bit?
+ *
+ * ---------------------------------------------------------------------------
+ */
+typedef struct H5FD_mirror_xmit_lock_t {
+    H5FD_mirror_xmit_t pub;
+    uint64_t           rw;
+} H5FD_mirror_xmit_lock_t;
+
+/* ---------------------------------------------------------------------------
+ * Structure:   H5FD_mirror_xmit_open_t
+ *
+ * Structure containing file-open information from the VFD sender.
+ *
+ * `pub` (H5FD_mirror_xmit_t)
+ *      Common transmission header, containing session information.
+ *      Must be first.
+ *
+ * `flags` (uint32_t)
+ *      VFL-layer file-open flags passed directly to H5FDopen().
+ *      (Natively 'unsigned [int]') TODO: native int may be 64-bit?
+ *
+ * `maxaddr` (uint64_t)
+ *      VFL-layer maximum allowed address space for the file to open passed
+ *      directly to H5FDopen().
+ *      (Natively 'haddr_t', always a 64-bit field)
+ *
+ * `size_t_blob` (uint64_t)
+ *      A number indicating how large a size_t is on the sending system.
+ *      Must be set to (uint64_t)((size_t)(-1))
+ *          (maximum possible value of size_t, cast to uint64_t).
+ *      The receiving system inspects this value -- if the local (remote)
+ *      size_t is smaller than that of the Sender, issues a warning.
+ *      Not an error, as:
+ *      1. It is assumed that underlying file systems/drivers have become
+ *         smart enough to handle file sizes that otherwise might be
+ *         constrained.
+ *      2. The Mirror Writer ingests bytes to write multiple 'slices' if the
+ *         size is greater than H5FD_MIRROR_DATA_BUFFER_MAX, regardless of
+ *         any size_t storage size disparity.
+ *
+ * `filename` (char[])
+ *      String giving the filename and path of file to open.
+ *
+ * ---------------------------------------------------------------------------
+ */
+typedef struct H5FD_mirror_xmit_open_t {
+    H5FD_mirror_xmit_t pub;
+    uint32_t           flags;
+    uint64_t           maxaddr;
+    uint64_t           size_t_blob;
+    char               filename[H5FD_MIRROR_XMIT_FILEPATH_MAX];
+} H5FD_mirror_xmit_open_t;
+
+/* ---------------------------------------------------------------------------
+ * Structure: H5FD_mirror_xmit_reply_t
+ *
+ * Structure used by the remote receiver/worker/writer to respond to
+ * a command from the VFD sender.
+ *
+ * `pub` (H5FD_mirror_xmit_t)
+ *      Common transmission header, containing session information.
+ *      Must be first.
+ *
+ * `status` (uint32_t)
+ *      Number indicating whether the command was successful or if an
+ *      occured.
+ *      Allowed values are H5FD_MIRROR_STATUS_OK and
+ *      H5FD_MIRROR_STATUS_ERROR.
+ *
+ * `message` (char[])
+ *      Error message. Populated if and only if there was a problem.
+ *      It is possible that a message may reach the end of the alloted
+ *      space without a NULL terminator -- the onus is on the programmer to
+ *      handle this situation.
+ *
+ * ---------------------------------------------------------------------------
+ */
+typedef struct H5FD_mirror_xmit_reply_t {
+    H5FD_mirror_xmit_t pub;
+    uint32_t           status;
+    char               message[H5FD_MIRROR_STATUS_MESSAGE_MAX];
+} H5FD_mirror_xmit_reply_t;
+
+/* ---------------------------------------------------------------------------
+ * Structure:   H5FD_mirror_xmit_write_t
+ *
+ * Structure containing data-write information from VFD sender.
+ *
+ * The data to be written is transmitted in subsequent, packets
+ * and may be broken up into more than one transmission buffer.
+ * The VFD sender and remote receiver/worker/writer must coordinate
+ * the receipt of data.
+ *
+ * `pub` (H5FD_mirror_xmit_t)
+ *      Common transmission header, containing session information.
+ *      Must be first.
+ *
+ * `type` (uint8_t)
+ *      Specifies datatype to be written.
+ *      (Natively 'H5FD_mem_t', an enumerated type in H5Fpublic.h)
+ *
+ * `offset` (uint64_t)
+ *      Start location of write in file.
+ *      (Natively 'haddr_t', always a 64-bit field)
+ *
+ * `size` (uint64_t)
+ *      Size of the data to be written, in bytes.
+ *      (Natively 'size_t', accommodate the largest possible as 64-bits)
+ *
+ * ---------------------------------------------------------------------------
+ */
+typedef struct H5FD_mirror_xmit_write_t {
+    H5FD_mirror_xmit_t  pub;
+    uint8_t             type;
+    uint64_t            offset;
+    uint64_t            size;
+} H5FD_mirror_xmit_write_t;
+
+
+
+/* Encode/decode routines are required to "pack" the xmit data into a known
+ * byte format for transmission over the wire.
+ *
+ * All component numbers must be stored in "network" word order (Big-Endian).
+ *
+ * All components must be packed in the order given in the structure definition.
+ *
+ * All components must be packed with zero padding between.
+ */
+
+H5_DLL size_t H5FD__mirror_xmit_decode_uint16(uint16_t *out, const unsigned char *buf);
+H5_DLL size_t H5FD__mirror_xmit_decode_uint32(uint32_t *out, const unsigned char *buf);
+H5_DLL size_t H5FD__mirror_xmit_decode_uint64(uint64_t *out, const unsigned char *buf);
+H5_DLL size_t H5FD__mirror_xmit_decode_uint8(uint8_t *out, const unsigned char *buf);
+H5_DLL size_t H5FD__mirror_xmit_encode_uint16(unsigned char *dest, uint16_t v);
+H5_DLL size_t H5FD__mirror_xmit_encode_uint32(unsigned char *dest, uint32_t v);
+H5_DLL size_t H5FD__mirror_xmit_encode_uint64(unsigned char *dest, uint64_t v);
+H5_DLL size_t H5FD__mirror_xmit_encode_uint8(unsigned char *dest, uint8_t v);
+
+H5_DLL size_t H5FD_mirror_xmit_decode_header(H5FD_mirror_xmit_t *out, const unsigned char *buf);
+H5_DLL size_t H5FD_mirror_xmit_decode_lock(H5FD_mirror_xmit_lock_t *out, const unsigned char *buf);
+H5_DLL size_t H5FD_mirror_xmit_decode_open(H5FD_mirror_xmit_open_t *out, const unsigned char *buf);
+H5_DLL size_t H5FD_mirror_xmit_decode_reply(H5FD_mirror_xmit_reply_t *out, const unsigned char *buf);
+H5_DLL size_t H5FD_mirror_xmit_decode_set_eoa(H5FD_mirror_xmit_eoa_t *out, const unsigned char *buf);
+H5_DLL size_t H5FD_mirror_xmit_decode_write(H5FD_mirror_xmit_write_t *out, const unsigned char *buf);
+
+H5_DLL size_t H5FD_mirror_xmit_encode_header(unsigned char *dest, const H5FD_mirror_xmit_t *x);
+H5_DLL size_t H5FD_mirror_xmit_encode_lock(unsigned char *dest, const H5FD_mirror_xmit_lock_t *x);
+H5_DLL size_t H5FD_mirror_xmit_encode_open(unsigned char *dest, const H5FD_mirror_xmit_open_t *x);
+H5_DLL size_t H5FD_mirror_xmit_encode_reply(unsigned char *dest, const H5FD_mirror_xmit_reply_t *x);
+H5_DLL size_t H5FD_mirror_xmit_encode_set_eoa(unsigned char *dest, const H5FD_mirror_xmit_eoa_t *x);
+H5_DLL size_t H5FD_mirror_xmit_encode_write(unsigned char *dest, const H5FD_mirror_xmit_write_t *x);
+
+H5_DLL hbool_t H5FD_mirror_xmit_is_close(const H5FD_mirror_xmit_t *xmit);
+H5_DLL hbool_t H5FD_mirror_xmit_is_lock(const H5FD_mirror_xmit_lock_t *xmit);
+H5_DLL hbool_t H5FD_mirror_xmit_is_open(const H5FD_mirror_xmit_open_t *xmit);
+H5_DLL hbool_t H5FD_mirror_xmit_is_reply(const H5FD_mirror_xmit_reply_t *xmit);
+H5_DLL hbool_t H5FD_mirror_xmit_is_set_eoa(const H5FD_mirror_xmit_eoa_t *xmit);
+H5_DLL hbool_t H5FD_mirror_xmit_is_write(const H5FD_mirror_xmit_write_t *xmit);
+H5_DLL hbool_t H5FD_mirror_xmit_is_xmit(const H5FD_mirror_xmit_t *xmit);
+
+#ifdef __cplusplus
+}
+#endif
+
+#else /* H5_HAVE_MIRROR_VFD */
+
+#define H5FD_MIRROR (H5I_INAVLID_HID)
+
+#endif /* H5_HAVE_MIRROR_VFD */
+
+#endif /* H5FDmirror_H */
+
+
diff --git a/src/H5FDpublic.h b/src/H5FDpublic.h
index 514d1bf..124bac6 100644
--- a/src/H5FDpublic.h
+++ b/src/H5FDpublic.h
@@ -255,6 +255,8 @@ typedef enum H5F_mem_t	H5FD_mem_t;
      * that creates a file which is compatible with the default VFD.
      * Generally, this means that the VFD creates a single file that follows
      * the canonical HDF5 file format.
+     * Regarding the Splitter VFD specifically, only drivers with this flag
+     * enabled may be used as the Write-Only (W/O) channel driver.
      */
 #define H5FD_FEAT_DEFAULT_VFD_COMPATIBLE        0x00008000
 
diff --git a/src/H5FDsec2.c b/src/H5FDsec2.c
index a393490..d718a93 100644
--- a/src/H5FDsec2.c
+++ b/src/H5FDsec2.c
@@ -521,6 +521,12 @@ H5FD_sec2_query(const H5FD_t *_file, unsigned long *flags /* out */)
     FUNC_ENTER_NOAPI_NOINIT_NOERR
 
     /* Set the VFL feature flags that this driver supports */
+    /* Notice: the Mirror VFD Writer currently uses only the Sec2 driver as
+     * the underying driver -- as such, the Mirror VFD implementation copies
+     * these feature flags as its own. Any modifications made here must be
+     * reflected in H5FDmirror.c
+     * -- JOS 2020-01-13
+     */
     if(flags) {
         *flags = 0;
         *flags |= H5FD_FEAT_AGGREGATE_METADATA;     /* OK to aggregate metadata allocations                             */
diff --git a/src/H5FDsplitter.c b/src/H5FDsplitter.c
new file mode 100644
index 0000000..13816a5
--- /dev/null
+++ b/src/H5FDsplitter.c
@@ -0,0 +1,1467 @@
+/* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
+ * Copyright by The HDF Group.                                               *
+ * All rights reserved.                                                      *
+ *                                                                           *
+ * This file is part of HDF5.  The full HDF5 copyright notice, including     *
+ * terms governing use, modification, and redistribution, is contained in    *
+ * the COPYING file, which can be found at the root of the source code       *
+ * distribution tree, or in https://support.hdfgroup.org/ftp/HDF5/releases.  *
+ * If you do not have access to either file, you may request a copy from     *
+ * help@hdfgroup.org.                                                        *
+ * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */
+
+/*
+ * Purpose:     The Splitter VFD implements a file driver which relays all the
+ *              VFD calls to an underlying VFD, and send all the write calls to
+ *              another underlying VFD. Maintains two files simultaneously.
+ */
+
+/* This source code file is part of the H5FD driver module */
+#include "H5FDdrvr_module.h"
+
+#include "H5private.h"      /* Generic Functions        */
+#include "H5Eprivate.h"     /* Error handling           */
+#include "H5Fprivate.h"     /* File access              */
+#include "H5FDprivate.h"    /* File drivers             */
+#include "H5FDsplitter.h"   /* Splitter file driver     */
+#include "H5FLprivate.h"    /* Free Lists               */
+#include "H5Iprivate.h"     /* IDs                      */
+#include "H5MMprivate.h"    /* Memory management        */
+#include "H5FDsec2.h"       /* Generic Functions        */
+#include "H5FDstdio.h"      /* Generic Functions        */
+#include "H5Pprivate.h"     /* Property lists           */
+
+/* The driver identification number, initialized at runtime */
+static hid_t H5FD_SPLITTER_g = 0;
+
+/* Driver-specific file access properties */
+typedef struct H5FD_splitter_fapl_t {
+    hid_t rw_fapl_id;   /* fapl for the R/W channel       */
+    hid_t wo_fapl_id;   /* fapl for the W/O channel       */
+    char wo_path[H5FD_SPLITTER_PATH_MAX + 1]; /* file name for the W/O channel */
+    char log_file_path[H5FD_SPLITTER_PATH_MAX + 1]; /* file to record errors reported by the W/O channel */
+    hbool_t ignore_wo_errs; /* TRUE to ignore errors on the W/O channel */
+} H5FD_splitter_fapl_t;
+
+/* The information of this splitter */
+typedef struct H5FD_splitter_t {
+    H5FD_t pub;         /* public stuff, must be first    */
+    unsigned version;   /* version of the H5FD_splitter_vfd_config_t structure used */
+    H5FD_splitter_fapl_t fa; /* driver-specific file access properties */
+    H5FD_t  *rw_file;   /* pointer of R/W channel */
+    H5FD_t  *wo_file;   /* pointer of W/O channel */
+    FILE    *logfp;     /* Log file pointer */
+} H5FD_splitter_t;
+
+/*
+ * These macros check for overflow of various quantities.  These macros
+ * assume that HDoff_t is signed and haddr_t and size_t are unsigned.
+ *
+ * ADDR_OVERFLOW:   Checks whether a file address of type `haddr_t'
+ *                  is too large to be represented by the second argument
+ *                  of the file seek function.
+ *
+ * SIZE_OVERFLOW:   Checks whether a buffer size of type `hsize_t' is too
+ *                  large to be represented by the `size_t' type.
+ *
+ * REGION_OVERFLOW: Checks whether an address and size pair describe data
+ *                  which can be addressed entirely by the second
+ *                  argument of the file seek function.
+ */
+#define MAXADDR (((haddr_t)1<<(8*sizeof(HDoff_t)-1))-1)
+#define ADDR_OVERFLOW(A)    (HADDR_UNDEF==(A) || ((A) & ~(haddr_t)MAXADDR))
+#define SIZE_OVERFLOW(Z)    ((Z) & ~(hsize_t)MAXADDR)
+#define REGION_OVERFLOW(A,Z)    (ADDR_OVERFLOW(A) || SIZE_OVERFLOW(Z) ||    \
+                                 HADDR_UNDEF==(A)+(Z) ||                    \
+                                (HDoff_t)((A)+(Z))<(HDoff_t)(A))
+
+/* This macro provides a wrapper for shared fail-log-ignore behavior
+ * for errors arising in the splitter's W/O channel.
+ * Logs an error entry in a log file, if the file exists.
+ * If not set to ignore errors, registers an error with the library.
+ */
+#define H5FD_SPLITTER_WO_ERROR(file, funcname, errmajor, errminor, ret, mesg) \
+{                                                                             \
+    H5FD__splitter_log_error((file), (funcname), (mesg));                     \
+    if (FALSE == (file)->fa.ignore_wo_errs) {                                 \
+        HGOTO_ERROR((errmajor), (errminor), (ret), (mesg))                    \
+    }                                                                         \
+}
+
+#define H5FD_SPLITTER_DEBUG_OP_CALLS 0 /* debugging print toggle; 0 disables */
+
+#if H5FD_SPLITTER_DEBUG_OP_CALLS
+#define H5FD_SPLITTER_LOG_CALL(name) do { \
+    HDprintf("called %s()\n", (name));    \
+    fflush(stdout);                       \
+} while (0)
+#else
+#define H5FD_SPLITTER_LOG_CALL(name) /* no-op */
+#endif /* H5FD_SPLITTER_DEBUG_OP_CALLS */
+
+/* Private functions */
+
+/* Print error messages from W/O channel to log file */
+static herr_t H5FD__splitter_log_error(const H5FD_splitter_t *file, const char *atfunc, const char *msg);
+
+static int H5FD__copy_plist(hid_t fapl_id, hid_t *id_out_ptr);
+
+/* Prototypes */
+static herr_t H5FD_splitter_term(void);
+static hsize_t H5FD_splitter_sb_size(H5FD_t *_file);
+static herr_t H5FD_splitter_sb_encode(H5FD_t *_file, char *name/*out*/, unsigned char *buf/*out*/);
+static herr_t H5FD_splitter_sb_decode(H5FD_t *_file, const char *name, const unsigned char *buf);
+static void *H5FD_splitter_fapl_get(H5FD_t *_file);
+static void *H5FD_splitter_fapl_copy(const void *_old_fa);
+static herr_t H5FD_splitter_fapl_free(void *_fapl);
+static H5FD_t *H5FD_splitter_open(const char *name, unsigned flags, hid_t fapl_id, haddr_t maxaddr);
+static herr_t H5FD_splitter_close(H5FD_t *_file);
+static int H5FD_splitter_cmp(const H5FD_t *_f1, const H5FD_t *_f2);
+static herr_t H5FD_splitter_query(const H5FD_t *_file, unsigned long *flags /* out */);
+static herr_t H5FD_splitter_get_type_map(const H5FD_t *_file, H5FD_mem_t *type_map);
+static haddr_t H5FD_splitter_alloc(H5FD_t *file, H5FD_mem_t type, hid_t dxpl_id, hsize_t size);
+static herr_t H5FD_splitter_free(H5FD_t *_file, H5FD_mem_t type, hid_t dxpl_id, haddr_t addr, hsize_t size);
+static haddr_t H5FD_splitter_get_eoa(const H5FD_t *_file, H5FD_mem_t H5_ATTR_UNUSED type);
+static herr_t H5FD_splitter_set_eoa(H5FD_t *_file, H5FD_mem_t H5_ATTR_UNUSED type, haddr_t addr);
+static haddr_t H5FD_splitter_get_eof(const H5FD_t *_file, H5FD_mem_t H5_ATTR_UNUSED type);
+static herr_t  H5FD_splitter_get_handle(H5FD_t *_file, hid_t H5_ATTR_UNUSED fapl, void** file_handle);
+static herr_t H5FD_splitter_read(H5FD_t *_file, H5FD_mem_t type, hid_t dxpl_id, haddr_t addr, size_t size, void *buf);
+static herr_t H5FD_splitter_write(H5FD_t *_file, H5FD_mem_t type, hid_t dxpl_id, haddr_t addr, size_t size, const void *buf);
+static herr_t H5FD_splitter_flush(H5FD_t *_file, hid_t dxpl_id, hbool_t closing);
+static herr_t H5FD_splitter_truncate(H5FD_t *_file, hid_t dxpl_id, hbool_t closing);
+static herr_t H5FD_splitter_lock(H5FD_t *_file, hbool_t rw);
+static herr_t H5FD_splitter_unlock(H5FD_t *_file);
+
+static const H5FD_class_t H5FD_splitter_g = {
+    "splitter",                 /* name                 */
+    MAXADDR,                    /* maxaddr              */
+    H5F_CLOSE_WEAK,             /* fc_degree            */
+    H5FD_splitter_term,         /* terminate            */
+    H5FD_splitter_sb_size,      /* sb_size              */
+    H5FD_splitter_sb_encode,    /* sb_encode            */
+    H5FD_splitter_sb_decode,    /* sb_decode            */
+    sizeof(H5FD_splitter_fapl_t), /* fapl_size          */
+    H5FD_splitter_fapl_get,     /* fapl_get             */
+    H5FD_splitter_fapl_copy,    /* fapl_copy            */
+    H5FD_splitter_fapl_free,    /* fapl_free            */
+    0,                          /* dxpl_size            */
+    NULL,                       /* dxpl_copy            */
+    NULL,                       /* dxpl_free            */
+    H5FD_splitter_open,         /* open                 */
+    H5FD_splitter_close,        /* close                */
+    H5FD_splitter_cmp,          /* cmp                  */
+    H5FD_splitter_query,        /* query                */
+    H5FD_splitter_get_type_map, /* get_type_map         */
+    H5FD_splitter_alloc,        /* alloc                */
+    H5FD_splitter_free,         /* free                 */
+    H5FD_splitter_get_eoa,      /* get_eoa              */
+    H5FD_splitter_set_eoa,      /* set_eoa              */
+    H5FD_splitter_get_eof,      /* get_eof              */
+    H5FD_splitter_get_handle,   /* get_handle           */
+    H5FD_splitter_read,         /* read                 */
+    H5FD_splitter_write,        /* write                */
+    H5FD_splitter_flush,        /* flush                */
+    H5FD_splitter_truncate,     /* truncate             */
+    H5FD_splitter_lock,         /* lock                 */
+    H5FD_splitter_unlock,       /* unlock               */
+    H5FD_FLMAP_DICHOTOMY        /* fl_map               */
+};
+
+/* Declare a free list to manage the H5FD_splitter_t struct */
+H5FL_DEFINE_STATIC(H5FD_splitter_t);
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD__init_package
+ *
+ * Purpose:     Initializes any interface-specific data or routines.
+ *
+ * Return:      SUCCEED/FAIL
+ *-------------------------------------------------------------------------
+ */
+static herr_t
+H5FD__init_package(void)
+{
+    herr_t ret_value = SUCCEED;
+
+    FUNC_ENTER_STATIC
+
+    H5FD_SPLITTER_LOG_CALL("H5FD__init_package");
+
+    if (H5FD_splitter_init() < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_CANTINIT, FAIL, "unable to initialize splitter VFD")
+    }
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* H5FD__init_package() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_splitter_init
+ *
+ * Purpose:     Initialize the splitter driver by registering it with the
+ *              library.
+ *
+ * Return:      Success:    The driver ID for the splitter driver.
+ *              Failure:    Negative
+ *-------------------------------------------------------------------------
+ */
+hid_t
+H5FD_splitter_init(void)
+{
+    hid_t ret_value = H5I_INVALID_HID;
+
+    FUNC_ENTER_NOAPI(FAIL)
+
+    H5FD_SPLITTER_LOG_CALL("H5FD_splitter_init");
+
+    if (H5I_VFL != H5I_get_type(H5FD_SPLITTER_g)) {
+        H5FD_SPLITTER_g = H5FDregister(&H5FD_splitter_g);
+    }
+
+    ret_value = H5FD_SPLITTER_g;
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD_splitter_init() */
+
+
+/*---------------------------------------------------------------------------
+ * Function:    H5FD_splitter_term
+ *
+ * Purpose:     Shut down the splitter VFD.
+ *
+ * Returns:     SUCCEED (Can't fail)
+ *---------------------------------------------------------------------------
+ */
+static herr_t
+H5FD_splitter_term(void)
+{
+    FUNC_ENTER_NOAPI_NOINIT_NOERR
+
+    H5FD_SPLITTER_LOG_CALL("H5FD_splitter_term");
+
+    /* Reset VFL ID */
+    H5FD_SPLITTER_g = 0;
+
+    FUNC_LEAVE_NOAPI(SUCCEED)
+} /* end H5FD_splitter_term() */
+
+
+ /*-------------------------------------------------------------------------
+ * Function:    H5FD__copy_plist
+ *
+ * Purpose:     Sanity-wrapped H5P_copy_plist() for each channel.
+ *              Utility function for operation in multiple locations.
+ *
+ * Return:      0 on success, -1 on error.
+ *-------------------------------------------------------------------------
+ */
+static int
+H5FD__copy_plist(hid_t   fapl_id,
+                 hid_t  *id_out_ptr)
+{
+    int                 ret_value = 0;
+    H5P_genplist_t     *plist_ptr = NULL;
+
+    FUNC_ENTER_NOAPI_NOINIT
+
+    H5FD_SPLITTER_LOG_CALL("H5FD__copy_plist");
+
+    HDassert(id_out_ptr != NULL);
+
+    if (FALSE == H5P_isa_class(fapl_id, H5P_FILE_ACCESS)) {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADTYPE, -1, "not a file access property list");
+    }
+
+    plist_ptr = (H5P_genplist_t *)H5I_object(fapl_id);
+    if (NULL == plist_ptr) {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADTYPE, -1, "unable to get property list");
+    }
+
+    *id_out_ptr = H5P_copy_plist(plist_ptr, FALSE);
+    if (H5I_INVALID_HID == *id_out_ptr) {
+        HGOTO_ERROR(H5E_VFL, H5E_BADTYPE, -1, "unable to copy file access property list");
+    }
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value);
+} /* end H5FD__copy_plist() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5Pset_fapl_splitter
+ *
+ * Purpose:     Sets the file access property list to use the
+ *              splitter driver.
+ *
+ * Return:      SUCCEED/FAIL
+ *-------------------------------------------------------------------------
+ */
+herr_t
+H5Pset_fapl_splitter(hid_t fapl_id, H5FD_splitter_vfd_config_t *vfd_config)
+{
+    H5FD_splitter_fapl_t  info;
+    H5P_genplist_t       *plist_ptr = NULL;
+    herr_t                ret_value = SUCCEED;
+
+    H5Eclear2(H5E_DEFAULT);
+
+    FUNC_ENTER_API(FAIL)
+    H5TRACE2("e", "i*Dr", fapl_id, vfd_config);
+
+    H5FD_SPLITTER_LOG_CALL("H5Pset_fapl_splitter");
+
+    if (H5FD_SPLITTER_MAGIC != vfd_config->magic) {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADVALUE, FAIL, "invalid configuration (magic number mismatch)")
+    }
+    if (H5FD_CURR_SPLITTER_VFD_CONFIG_VERSION != vfd_config->version) {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADVALUE, FAIL, "invlaid config (version number mismatch)")
+    }
+
+    if (NULL == (plist_ptr = (H5P_genplist_t *)H5I_object(fapl_id))) {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADTYPE, FAIL, "not a valid property list")
+    }
+
+
+    /* Make sure that the W/O channel supports write-only capability.
+     * Some drivers (e.g. family or multi) do revision of the superblock
+     * in-memory, causing problems in that channel.
+     * Uses the feature flag H5FD_FEAT_DEFAULT_VFD_COMPATIBLE as the
+     * determining attribute.
+     */
+    if (H5P_DEFAULT != vfd_config->wo_fapl_id) {
+        H5FD_class_t       *wo_driver = NULL;
+        H5FD_driver_prop_t  wo_driver_prop;
+        H5P_genplist_t     *wo_plist_ptr = NULL;
+        unsigned long       wo_driver_flags = 0;
+
+        wo_plist_ptr = (H5P_genplist_t *)H5I_object(vfd_config->wo_fapl_id);
+        if (NULL == wo_plist_ptr) {
+            HGOTO_ERROR(H5E_ARGS, H5E_BADTYPE, FAIL, "not a file access property list")
+        }
+        if (H5P_peek(wo_plist_ptr, H5F_ACS_FILE_DRV_NAME, &wo_driver_prop) < 0) {
+            HGOTO_ERROR(H5E_PLIST, H5E_CANTGET, FAIL, "can't get driver ID & info")
+        }
+        wo_driver = (H5FD_class_t *)H5I_object(wo_driver_prop.driver_id);
+        if (NULL == wo_driver) {
+            HGOTO_ERROR(H5E_VFL, H5E_BADVALUE, FAIL, "invalid driver ID in file access property list")
+        }
+        if (H5FD_driver_query(wo_driver, &wo_driver_flags) < 0) {
+            HGOTO_ERROR(H5E_VFL, H5E_BADVALUE, FAIL, "can't query VFD flags")
+        }
+        if (0 == (H5FD_FEAT_DEFAULT_VFD_COMPATIBLE & wo_driver_flags)) {
+            HGOTO_ERROR(H5E_VFL, H5E_BADVALUE, FAIL, "unsuitable W/O driver")
+        }
+    } /* end if W/O VFD is non-default */
+
+
+    info.ignore_wo_errs = vfd_config->ignore_wo_errs;
+    HDstrncpy(info.wo_path, vfd_config->wo_path, H5FD_SPLITTER_PATH_MAX);
+    HDstrncpy(info.log_file_path, vfd_config->log_file_path, H5FD_SPLITTER_PATH_MAX);
+    info.rw_fapl_id = H5P_FILE_ACCESS_DEFAULT; /* pre-set value */
+    info.wo_fapl_id = H5P_FILE_ACCESS_DEFAULT; /* pre-set value */
+
+    /* Set non-default channel FAPL IDs in splitter configuration info */
+    if (H5P_DEFAULT != vfd_config->rw_fapl_id) {
+        if (FALSE == H5P_isa_class(vfd_config->rw_fapl_id, H5P_FILE_ACCESS)) {
+            HGOTO_ERROR(H5E_ARGS, H5E_BADTYPE, FAIL, "not a file access list")
+        }
+        info.rw_fapl_id = vfd_config->rw_fapl_id;
+    }
+    if (H5P_DEFAULT != vfd_config->wo_fapl_id) {
+        if (FALSE == H5P_isa_class(vfd_config->wo_fapl_id, H5P_FILE_ACCESS)) {
+            HGOTO_ERROR(H5E_ARGS, H5E_BADTYPE, FAIL, "not a file access list")
+        }
+        info.wo_fapl_id = vfd_config->wo_fapl_id;
+    }
+
+    ret_value = H5P_set_driver(plist_ptr, H5FD_SPLITTER, &info);
+
+done:
+    FUNC_LEAVE_API(ret_value)
+} /* end H5Pset_fapl_splitter() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5Pget_fapl_splitter
+ *
+ * Purpose:     Returns information about the splitter file access property
+ *              list through the structure config_out.
+ *
+ *              Will fail if config_out is received without pre-set valid
+ *              magic and version information.
+ *
+ * Return:      SUCCEED/FAIL
+ *-------------------------------------------------------------------------
+ */
+herr_t
+H5Pget_fapl_splitter(hid_t fapl_id, H5FD_splitter_vfd_config_t *config_out)
+{
+    const H5FD_splitter_fapl_t *fapl_ptr  = NULL;
+    H5P_genplist_t             *plist_ptr = NULL;
+    herr_t                      ret_value = SUCCEED;
+
+    FUNC_ENTER_API(FAIL)
+    H5TRACE2("e", "i*Dr", fapl_id, config_out);
+
+    H5FD_SPLITTER_LOG_CALL("H5Pget_fapl_splitter");
+
+    /* Check arguments */
+    if (TRUE != H5P_isa_class(fapl_id, H5P_FILE_ACCESS)) {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADTYPE, FAIL, "not a file access property list")
+    }
+    if (config_out == NULL) {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADVALUE, FAIL, "config_out pointer is null")
+    }
+    if (H5FD_SPLITTER_MAGIC != config_out->magic) {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADVALUE, FAIL, "info-out pointer invalid (magic number mismatch)")
+    }
+    if (H5FD_CURR_SPLITTER_VFD_CONFIG_VERSION != config_out->version) {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADVALUE, FAIL, "info-out pointer invalid (version unsafe)")
+    }
+
+    /* Pre-set out FAPL IDs with intent to replace these values */
+    config_out->rw_fapl_id = H5I_INVALID_HID;
+    config_out->wo_fapl_id = H5I_INVALID_HID;
+
+    /* Check and get the splitter fapl */
+    if (NULL == (plist_ptr = H5P_object_verify(fapl_id, H5P_FILE_ACCESS))) {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADTYPE, FAIL, "not a file access property list")
+    }
+    if (H5FD_SPLITTER != H5P_peek_driver(plist_ptr)) {
+        HGOTO_ERROR(H5E_PLIST, H5E_BADVALUE, FAIL, "incorrect VFL driver")
+    }
+    if (NULL == (fapl_ptr = (const H5FD_splitter_fapl_t *)H5P_peek_driver_info(plist_ptr))) {
+        HGOTO_ERROR(H5E_PLIST, H5E_BADVALUE, FAIL, "unable to get specific-driver info")
+    }
+
+    HDstrncpy(config_out->wo_path, fapl_ptr->wo_path, H5FD_SPLITTER_PATH_MAX);
+    HDstrncpy(config_out->log_file_path, fapl_ptr->log_file_path, H5FD_SPLITTER_PATH_MAX);
+    config_out->ignore_wo_errs = fapl_ptr->ignore_wo_errs;
+
+    /* Copy R/W and W/O FAPLs */
+    if (H5FD__copy_plist(fapl_ptr->rw_fapl_id, &(config_out->rw_fapl_id)) < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_BADVALUE, FAIL, "can't copy R/W FAPL");
+    }
+    if (H5FD__copy_plist(fapl_ptr->wo_fapl_id, &(config_out->wo_fapl_id)) < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_BADVALUE, FAIL, "can't copy W/O FAPL");
+    }
+
+done:
+    FUNC_LEAVE_API(ret_value)
+} /* end H5Pget_fapl_splitter() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_splitter_flush
+ *
+ * Purpose:     Flushes all data to disk for both channels.
+ *
+ * Return:      SUCCEED/FAIL
+ *-------------------------------------------------------------------------
+ */
+static herr_t
+H5FD_splitter_flush(H5FD_t *_file, hid_t H5_ATTR_UNUSED dxpl_id, hbool_t closing)
+{
+    H5FD_splitter_t *file = (H5FD_splitter_t *)_file;
+    herr_t      ret_value = SUCCEED;       /* Return value */
+
+    FUNC_ENTER_NOAPI_NOINIT
+
+    H5FD_SPLITTER_LOG_CALL("H5FD_splitter_flush");
+
+    /* Public API for dxpl "context" */
+    if (H5FDflush(file->rw_file, dxpl_id, closing) < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_CANTFLUSH, FAIL, "unable to flush R/W file")
+    }
+    if (H5FDflush(file->wo_file, dxpl_id, closing) < 0) {
+       H5FD_SPLITTER_WO_ERROR(file, "H5FD_splitter_flush",
+                              H5E_VFL, H5E_CANTFLUSH, FAIL,
+                              "unable to flush W/O file")
+    }
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD_splitter_flush() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_splitter_read
+ *
+ * Purpose:     Reads SIZE bytes of data from the R/W channel, beginning at
+ *              address ADDR into buffer BUF according to data transfer
+ *              properties in DXPL_ID.
+ *
+ * Return:      Success:    SUCCEED
+ *                          The read result is written into the BUF buffer
+ *                          which should be allocated by the caller.
+ *              Failure:    FAIL
+ *                          The contents of BUF are undefined.
+ *-------------------------------------------------------------------------
+ */
+static herr_t
+H5FD_splitter_read(
+        H5FD_t     *_file,
+        H5FD_mem_t  H5_ATTR_UNUSED type,
+        hid_t       H5_ATTR_UNUSED dxpl_id,
+        haddr_t     addr,
+        size_t      size,
+        void       *buf /*out*/)
+{
+    H5FD_splitter_t *file       = (H5FD_splitter_t *)_file;
+    herr_t          ret_value   = SUCCEED;
+
+    FUNC_ENTER_NOAPI_NOINIT
+
+    H5FD_SPLITTER_LOG_CALL("H5FD_splitter_read");
+
+    HDassert(file && file->pub.cls);
+    HDassert(buf);
+
+    /* Check for overflow conditions */
+    if (!H5F_addr_defined(addr)) {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADVALUE, FAIL, "addr undefined, addr = %llu", (unsigned long long)addr)
+    }
+    if (REGION_OVERFLOW(addr, size)) {
+        HGOTO_ERROR(H5E_ARGS, H5E_OVERFLOW, FAIL, "addr overflow, addr = %llu", (unsigned long long)addr)
+    }
+
+    /* Only read from R/W channel */
+    /* Public API for dxpl "context" */
+    if (H5FDread(file->rw_file, type, dxpl_id, addr, size, buf) < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_READERROR, FAIL, "Reading from R/W channel failed")
+    }
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD_splitter_read() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_splitter_write
+ *
+ * Purpose:     Writes SIZE bytes of data to R/W and W/O channels, beginning
+ *              at address ADDR from buffer BUF according to data transfer
+ *              properties in DXPL_ID.
+ *
+ * Return:      SUCCEED/FAIL
+ *-------------------------------------------------------------------------
+ */
+static herr_t
+H5FD_splitter_write(H5FD_t *_file, H5FD_mem_t type, hid_t dxpl_id, haddr_t addr, size_t size, const void *buf)
+{
+    H5FD_splitter_t *file      = (H5FD_splitter_t *)_file;
+    H5P_genplist_t  *plist_ptr     = NULL;
+    herr_t           ret_value = SUCCEED;
+
+    FUNC_ENTER_NOAPI_NOINIT
+
+    H5FD_SPLITTER_LOG_CALL("H5FD_splitter_write");
+
+    if (NULL == (plist_ptr = (H5P_genplist_t *)H5I_object(dxpl_id))) {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADTYPE, FAIL, "not a property list")
+    }
+
+    /* Write to each file */
+    /* Public API for dxpl "context" */
+    if (H5FDwrite(file->rw_file, type, dxpl_id, addr, size, buf) < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_WRITEERROR, FAIL, "R/W file write failed")
+    }
+    if (H5FDwrite(file->wo_file, type, dxpl_id, addr, size, buf) < 0) {
+       H5FD_SPLITTER_WO_ERROR(file, "H5FD_splitter_write",
+                              H5E_VFL, H5E_WRITEERROR, FAIL,
+                              "unable to write W/O file")
+    }
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD_splitter_write() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_splitter_fapl_get
+ *
+ * Purpose:     Returns a file access property list which indicates how the
+ *              specified file is being accessed. The return list could be
+ *              used to access another file the same way.
+ *
+ * Return:      Success:    Ptr to new file access property list with all
+ *                          members copied from the file struct.
+ *              Failure:    NULL
+ *-------------------------------------------------------------------------
+ */
+static void *
+H5FD_splitter_fapl_get(H5FD_t *_file)
+{
+    H5FD_splitter_t *file      = (H5FD_splitter_t *)_file;
+    void            *ret_value = NULL;
+
+    FUNC_ENTER_NOAPI_NOINIT_NOERR
+
+    H5FD_SPLITTER_LOG_CALL("H5FD_splitter_fapl_get");
+
+    ret_value = H5FD_splitter_fapl_copy(&(file->fa));
+
+    FUNC_LEAVE_NOAPI(ret_value)
+}
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_splitter_fapl_copy
+ *
+ * Purpose:     Copies the file access properties.
+ *
+ * Return:      Success:    Pointer to a new property list info structure.
+ *              Failure:    NULL
+ *-------------------------------------------------------------------------
+ */
+static void *
+H5FD_splitter_fapl_copy(const void *_old_fa)
+{
+    const H5FD_splitter_fapl_t *old_fa_ptr = (const H5FD_splitter_fapl_t *)_old_fa;
+    H5FD_splitter_fapl_t       *new_fa_ptr = NULL;
+    void                       *ret_value  = NULL;
+
+    FUNC_ENTER_NOAPI_NOINIT
+
+    H5FD_SPLITTER_LOG_CALL("H5FD_splitter_fapl_copy");
+
+    HDassert(old_fa_ptr);
+
+    new_fa_ptr = (H5FD_splitter_fapl_t *)H5MM_calloc(sizeof(H5FD_splitter_fapl_t));
+    if (NULL == new_fa_ptr) {
+        HGOTO_ERROR(H5E_VFL, H5E_CANTALLOC, NULL, "unable to allocate log file FAPL")
+    }
+
+    if (HDmemcpy(new_fa_ptr, old_fa_ptr, sizeof(H5FD_splitter_fapl_t)) == NULL) {
+        HGOTO_ERROR(H5E_ARGS, H5E_CANTCOPY, NULL, "unable to shallow-copy info")
+    }
+    if (HDstrncpy(new_fa_ptr->wo_path, old_fa_ptr->wo_path, H5FD_SPLITTER_PATH_MAX) == NULL) {
+        HGOTO_ERROR(H5E_ARGS, H5E_CANTCOPY, NULL, "unable to copy write-only channel file path")
+    }
+    if (HDstrncpy(new_fa_ptr->log_file_path, old_fa_ptr->log_file_path, H5FD_SPLITTER_PATH_MAX) == NULL) {
+        HGOTO_ERROR(H5E_ARGS, H5E_CANTCOPY, NULL, "unable to copy log file path")
+    }
+
+    /* Copy R/W and W/O FAPLs */
+    if (H5FD__copy_plist(old_fa_ptr->rw_fapl_id, &(new_fa_ptr->rw_fapl_id)) < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_BADVALUE, NULL, "can't copy R/W FAPL");
+    }
+    if (H5FD__copy_plist(old_fa_ptr->wo_fapl_id, &(new_fa_ptr->wo_fapl_id)) < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_BADVALUE, NULL, "can't copy W/O FAPL");
+    }
+
+    ret_value = (void *)new_fa_ptr;
+
+done:
+    if (NULL == ret_value) {
+        if (new_fa_ptr) {
+            H5MM_free(new_fa_ptr);
+        }
+    }
+
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD_splitter_fapl_copy() */
+
+
+/*--------------------------------------------------------------------------
+ * Function:    H5FD_splitter_fapl_free
+ *
+ * Purpose:     Releases the file access lists
+ *
+ * Return:      SUCCEED/FAIL
+ *--------------------------------------------------------------------------
+ */
+static herr_t
+H5FD_splitter_fapl_free(void *_fapl)
+{
+    H5FD_splitter_fapl_t *fapl      = (H5FD_splitter_fapl_t*)_fapl;
+    herr_t                ret_value = SUCCEED;
+
+    FUNC_ENTER_NOAPI_NOINIT
+
+    H5FD_SPLITTER_LOG_CALL("H5FD_splitter_fapl_free");
+
+    /* Check arguments */
+    HDassert(fapl);
+
+    if (H5I_dec_ref(fapl->rw_fapl_id) < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_CANTDEC, FAIL, "can't close R/W FAPL ID")
+    }
+    if (H5I_dec_ref(fapl->wo_fapl_id) < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_CANTDEC, FAIL, "can't close W/O FAPL ID")
+    }
+
+    /* Free the property list */
+    H5MM_free(fapl);
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD_splitter_fapl_free() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_splitter_open
+ *
+ * Purpose:     Create and/or opens a file as an HDF5 file.
+ *
+ * Return:      Success:    A pointer to a new file data structure. The
+ *                          public fields will be initialized by the
+ *                          caller, which is always H5FD_open().
+ *              Failure:    NULL
+ *-------------------------------------------------------------------------
+ */
+static H5FD_t *
+H5FD_splitter_open(const char *name, unsigned flags, hid_t splitter_fapl_id, haddr_t maxaddr)
+{
+    H5FD_splitter_t            *file_ptr  = NULL; /* Splitter VFD info */
+    const H5FD_splitter_fapl_t *fapl_ptr  = NULL; /* Driver-specific property list */
+    H5P_genplist_t     *plist_ptr = NULL;
+    H5FD_t     *ret_value = NULL;
+
+    FUNC_ENTER_NOAPI_NOINIT
+
+    H5FD_SPLITTER_LOG_CALL("H5FD_splitter_open");
+
+    /* Check arguments */
+    if (!name || !*name) {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADVALUE, NULL, "invalid file name")
+    }
+    if (0 == maxaddr || HADDR_UNDEF == maxaddr) {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADRANGE, NULL, "bogus maxaddr")
+    }
+    if (ADDR_OVERFLOW(maxaddr)) {
+        HGOTO_ERROR(H5E_ARGS, H5E_OVERFLOW, NULL, "bogus maxaddr")
+    }
+    if ( (H5P_FILE_ACCESS_DEFAULT == splitter_fapl_id) ||
+         (H5FD_SPLITTER != H5Pget_driver(splitter_fapl_id)) )
+    {
+        /* presupposes that H5P_FILE_ACCESS_DEFAULT is not a splitter */
+        HGOTO_ERROR(H5E_ARGS, H5E_BADVALUE, NULL, "driver is not splitter")
+    }
+
+    file_ptr = (H5FD_splitter_t *)H5FL_CALLOC(H5FD_splitter_t);
+    if (NULL == file_ptr) {
+        HGOTO_ERROR(H5E_RESOURCE, H5E_NOSPACE, NULL, "unable to allocate file struct")
+    }
+    file_ptr->fa.rw_fapl_id = H5I_INVALID_HID;
+    file_ptr->fa.wo_fapl_id = H5I_INVALID_HID;
+
+    /* Get the driver-specific file access properties */
+    plist_ptr = (H5P_genplist_t *)H5I_object(splitter_fapl_id);
+    if (NULL == plist_ptr) {
+        HGOTO_ERROR(H5E_ARGS, H5E_BADTYPE, NULL, "not a file access property list")
+    }
+    fapl_ptr = (const H5FD_splitter_fapl_t *)H5P_peek_driver_info(plist_ptr);
+    if (NULL == fapl_ptr) {
+        HGOTO_ERROR(H5E_PLIST, H5E_CANTGET, NULL, "unable to get VFL driver info")
+    }
+
+    /* Copy simpler info */
+    if (HDstrncpy(file_ptr->fa.wo_path, fapl_ptr->wo_path, H5FD_SPLITTER_PATH_MAX) == NULL) {
+        HGOTO_ERROR(H5E_VFL, H5E_CANTCOPY, NULL, "unable to copy write-only path")
+    }
+    if (HDstrncpy(file_ptr->fa.log_file_path, fapl_ptr->log_file_path, H5FD_SPLITTER_PATH_MAX) == NULL) {
+        HGOTO_ERROR(H5E_VFL, H5E_CANTCOPY, NULL, "unable to copy logfile path")
+    }
+    file_ptr->fa.ignore_wo_errs = fapl_ptr->ignore_wo_errs;
+
+    /* Copy R/W and W/O channel FAPLs. */
+    if (H5FD__copy_plist(fapl_ptr->rw_fapl_id, &(file_ptr->fa.rw_fapl_id)) < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_BADVALUE, NULL, "can't copy R/W FAPL");
+    }
+    if (H5FD__copy_plist(fapl_ptr->wo_fapl_id, &(file_ptr->fa.wo_fapl_id)) < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_BADVALUE, NULL, "can't copy W/O FAPL");
+    }
+
+    /* Prepare log file if necessary.
+     * If application wants to ignore the errors from W/O channel and
+     * provided a name for the log file, then open it
+     */
+    if (!file_ptr->logfp) {
+        if (file_ptr->fa.log_file_path[0] != '\0') {
+            file_ptr->logfp = HDfopen(file_ptr->fa.log_file_path, "w");
+            if (file_ptr->logfp == NULL) {
+                HGOTO_ERROR(H5E_VFL, H5E_CANTOPENFILE, NULL, "unable to open log file")
+            }
+        } /* end if logfile path given */
+    } /* end if logfile pointer/handle does not exist */
+
+    file_ptr->rw_file = H5FD_open(name, flags, fapl_ptr->rw_fapl_id, HADDR_UNDEF);
+    if (!file_ptr->rw_file) {
+        HGOTO_ERROR(H5E_VFL, H5E_CANTOPENFILE, NULL, "unable to open R/W file")
+    }
+
+    file_ptr->wo_file = H5FD_open(fapl_ptr->wo_path, flags, fapl_ptr->wo_fapl_id, HADDR_UNDEF);
+    if (!file_ptr->wo_file) {
+       H5FD_SPLITTER_WO_ERROR(file_ptr, "H5FD_splitter_open",
+                              H5E_VFL, H5E_CANTOPENFILE, NULL,
+                              "unable to open W/O file")
+    }
+
+    ret_value = (H5FD_t*)file_ptr;
+
+done:
+    if (NULL == ret_value) {
+        if (file_ptr) {
+            if (H5I_INVALID_HID != file_ptr->fa.rw_fapl_id) {
+                H5I_dec_ref(file_ptr->fa.rw_fapl_id);
+            }
+            if (H5I_INVALID_HID != file_ptr->fa.wo_fapl_id) {
+                H5I_dec_ref(file_ptr->fa.wo_fapl_id);
+            }
+            if (file_ptr->rw_file) {
+                H5FD_close(file_ptr->rw_file);
+            }
+            if (file_ptr->wo_file) {
+                H5FD_close(file_ptr->wo_file);
+            }
+            H5FL_FREE(H5FD_splitter_t, file_ptr);
+        }
+    } /* end if error */
+
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD_splitter_open() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_splitter_close
+ *
+ * Purpose:     Closes files on both read-write and write-only channels.
+ *
+ * Return:      Success:    SUCCEED
+ *              Failure:    FAIL, file not closed.
+ *-------------------------------------------------------------------------
+ */
+static herr_t
+H5FD_splitter_close(H5FD_t *_file)
+{
+    H5FD_splitter_t *file      = (H5FD_splitter_t *)_file;
+    herr_t           ret_value = SUCCEED;
+
+    FUNC_ENTER_NOAPI_NOINIT
+
+    H5FD_SPLITTER_LOG_CALL("H5FD_splitter_close");
+
+    /* Sanity check */
+    HDassert(file);
+
+    if (H5I_dec_ref(file->fa.rw_fapl_id) < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_ARGS, FAIL, "can't close R/W FAPL")
+    }
+    if (H5I_dec_ref(file->fa.wo_fapl_id) < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_ARGS, FAIL, "can't close W/O FAPL")
+    }
+
+    if (file->rw_file) {
+        if (H5FD_close(file->rw_file) == FAIL) {
+            HGOTO_ERROR(H5E_VFL, H5E_CANTCLOSEFILE, FAIL, "unable to close R/W file")
+        }
+    }
+    if (file->wo_file) {
+        if (H5FD_close(file->wo_file) == FAIL) {
+            H5FD_SPLITTER_WO_ERROR(file, "H5FD_splitter_close",
+                              H5E_VFL, H5E_CANTCLOSEFILE, FAIL,
+                              "unable to close W/O file")
+        }
+    }
+
+    if (file->logfp) {
+        HDfclose(file->logfp);
+        file->logfp = NULL;
+    }
+
+    /* Release the file info */
+    file = H5FL_FREE(H5FD_splitter_t, file);
+    file = NULL;
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD_splitter_close() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_splitter_get_eoa
+ *
+ * Purpose:     Returns the end-of-address marker for the file. The EOA
+ *              marker is the first address past the last byte allocated in
+ *              the format address space.
+ *
+ * Return:      Success:    The end-of-address-marker
+ *
+ *              Failure:    HADDR_UNDEF
+ *-------------------------------------------------------------------------
+ */
+static haddr_t
+H5FD_splitter_get_eoa(const H5FD_t *_file, H5FD_mem_t H5_ATTR_UNUSED type)
+{
+    const H5FD_splitter_t *file      = (const H5FD_splitter_t *)_file;
+    haddr_t                ret_value = HADDR_UNDEF;
+
+    FUNC_ENTER_NOAPI_NOINIT
+
+    H5FD_SPLITTER_LOG_CALL("H5FD_splitter_get_eoa");
+
+    /* Sanity check */
+    HDassert(file);
+    HDassert(file->rw_file);
+
+    if ((ret_value = H5FD_get_eoa(file->rw_file, type)) == HADDR_UNDEF) {
+        HGOTO_ERROR(H5E_VFL, H5E_BADVALUE, HADDR_UNDEF, "unable to get eoa")
+    }
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD_splitter_get_eoa */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_splitter_set_eoa
+ *
+ * Purpose:     Set the end-of-address marker for the file. This function is
+ *              called shortly after an existing HDF5 file is opened in order
+ *              to tell the driver where the end of the HDF5 data is located.
+ *
+ * Return:      SUCCEED/FAIL
+ *-------------------------------------------------------------------------
+ */
+static herr_t
+H5FD_splitter_set_eoa(H5FD_t *_file, H5FD_mem_t H5_ATTR_UNUSED type, haddr_t addr)
+{
+    H5FD_splitter_t *file = (H5FD_splitter_t *)_file;
+    herr_t      ret_value = SUCCEED;                /* Return value */
+
+    FUNC_ENTER_NOAPI_NOINIT
+
+    H5FD_SPLITTER_LOG_CALL("H5FD_splitter_set_eoa";)
+
+    /* Sanity check */
+    HDassert(file);
+    HDassert(file->rw_file);
+    HDassert(file->wo_file);
+
+    if (H5FD_set_eoa(file->rw_file, type, addr) < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_CANTSET, FAIL, "H5FDset_eoa failed for R/W file")
+    }
+
+    if (H5FD_set_eoa(file->wo_file, type, addr) < 0) {
+       H5FD_SPLITTER_WO_ERROR(file, "H5FD_splitter_set_eoa",
+                              H5E_VFL, H5E_CANTSET, FAIL,
+                              "unable to set EOA for W/O file")
+    }
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD_splitter_set_eoa() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_splitter_get_eof
+ *
+ * Purpose:     Returns the end-of-address marker for the file. The EOA
+ *              marker is the first address past the last byte allocated in
+ *              the format address space.
+ *
+ * Return:      Success:    The end-of-address-marker
+ *
+ *              Failure:    HADDR_UNDEF
+ *-------------------------------------------------------------------------
+ */
+static haddr_t
+H5FD_splitter_get_eof(const H5FD_t *_file, H5FD_mem_t H5_ATTR_UNUSED type)
+{
+    const H5FD_splitter_t *file = (const H5FD_splitter_t *)_file;
+    haddr_t ret_value = HADDR_UNDEF;        /* Return value */
+
+    FUNC_ENTER_NOAPI_NOINIT
+
+    H5FD_SPLITTER_LOG_CALL("H5FD_splitter_get_eof");
+
+    /* Sanity check */
+    HDassert(file);
+    HDassert(file->rw_file);
+
+    if (HADDR_UNDEF == (ret_value = H5FD_get_eof(file->rw_file, type))) {
+        HGOTO_ERROR(H5E_VFL, H5E_CANTGET, HADDR_UNDEF, "unable to get eof")
+    }
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD_splitter_get_eof */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_splitter_truncate
+ *
+ * Purpose:     Notify driver to truncate the file back to the allocated size.
+ *
+ * Return:      SUCCEED/FAIL
+ *-------------------------------------------------------------------------
+ */
+static herr_t
+H5FD_splitter_truncate(H5FD_t *_file, hid_t dxpl_id, hbool_t closing)
+{
+    H5FD_splitter_t *file = (H5FD_splitter_t *)_file;
+    herr_t           ret_value = SUCCEED;       /* Return value */
+
+    FUNC_ENTER_NOAPI_NOINIT
+
+    H5FD_SPLITTER_LOG_CALL("H5FD_splitter_truncate");
+
+    HDassert(file);
+    HDassert(file->rw_file);
+    HDassert(file->wo_file);
+
+    if (H5FDtruncate(file->rw_file, dxpl_id, closing) < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_CANTUPDATE, FAIL, "unable to truncate R/W file")
+    }
+
+    if (H5FDtruncate(file->wo_file, dxpl_id, closing) < 0) {
+       H5FD_SPLITTER_WO_ERROR(file, "H5FD_splitter_truncate",
+                              H5E_VFL, H5E_CANTUPDATE, FAIL,
+                              "unable to truncate W/O file")
+    }
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD_splitter_truncate */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_splitter_sb_size
+ *
+ * Purpose:     Obtains the number of bytes required to store the driver file
+ *              access data in the HDF5 superblock.
+ *
+ * Return:      Success:    Number of bytes required.
+ *
+ *              Failure:    0 if an error occurs or if the driver has no
+ *                          data to store in the superblock.
+ *
+ * NOTE: no public API for H5FD_sb_size, it needs to be added
+ *-------------------------------------------------------------------------
+ */
+static hsize_t
+H5FD_splitter_sb_size(H5FD_t *_file)
+{
+    H5FD_splitter_t *file      = (H5FD_splitter_t *)_file;
+    hsize_t          ret_value = 0;
+
+    FUNC_ENTER_NOAPI_NOINIT_NOERR
+
+    H5FD_SPLITTER_LOG_CALL("H5FD_splitter_sb_size");
+
+    /* Sanity check */
+    HDassert(file);
+    HDassert(file->rw_file);
+
+    if (file->rw_file) {
+        ret_value = H5FD_sb_size(file->rw_file);
+    }
+
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD_splitter_sb_size */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_splitter_sb_encode
+ *
+ * Purpose:     Encode driver-specific data into the output arguments.
+ *
+ * Return:      SUCCEED/FAIL
+ *-------------------------------------------------------------------------
+ */
+static herr_t
+H5FD_splitter_sb_encode(H5FD_t *_file, char *name/*out*/, unsigned char *buf/*out*/)
+{
+    H5FD_splitter_t *file = (H5FD_splitter_t *)_file;
+    herr_t           ret_value = SUCCEED;       /* Return value */
+
+    FUNC_ENTER_NOAPI_NOINIT
+
+    H5FD_SPLITTER_LOG_CALL("H5FD_splitter_sb_encode");
+
+    /* Sanity check */
+    HDassert(file);
+    HDassert(file->rw_file);
+
+    if (file->rw_file && H5FD_sb_encode(file->rw_file, name, buf) < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_CANTENCODE, FAIL, "unable to encode the superblock in R/W file")
+    }
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD_splitter_sb_encode */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_splitter_sb_decode
+ *
+ * Purpose:     Decodes the driver information block.
+ *
+ * Return:      SUCCEED/FAIL
+ *
+ * NOTE: no public API for H5FD_sb_size, need to add
+ *-------------------------------------------------------------------------
+ */
+static herr_t
+H5FD_splitter_sb_decode(H5FD_t *_file, const char *name, const unsigned char *buf)
+{
+    H5FD_splitter_t *file = (H5FD_splitter_t *)_file;
+    herr_t           ret_value = SUCCEED;       /* Return value */
+
+    FUNC_ENTER_NOAPI_NOINIT
+
+    H5FD_SPLITTER_LOG_CALL("H5FD_splitter_sb_decode");
+
+    /* Sanity check */
+    HDassert(file);
+    HDassert(file->rw_file);
+
+    if (H5FD_sb_load(file->rw_file, name, buf) < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_CANTDECODE, FAIL, "unable to decode the superblock in R/W file")
+    }
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD_splitter_sb_decode */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_splitter_cmp
+ *
+ * Purpose:     Compare the keys of two files.
+ *
+ * Return:      Success:    A value like strcmp()
+ *              Failure:    Must never fail
+ *-------------------------------------------------------------------------
+ */
+static int
+H5FD_splitter_cmp(const H5FD_t *_f1, const H5FD_t *_f2)
+{
+    const H5FD_splitter_t  *f1 = (const H5FD_splitter_t *)_f1;
+    const H5FD_splitter_t  *f2 = (const H5FD_splitter_t *)_f2;
+    herr_t ret_value = 0;   /* Return value */
+
+    FUNC_ENTER_NOAPI_NOINIT_NOERR
+
+    H5FD_SPLITTER_LOG_CALL("H5FD_splitter_cmp");
+
+    HDassert(f1);
+    HDassert(f2);
+
+    ret_value = H5FD_cmp(f1->rw_file, f2->rw_file);
+
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD_splitter_cmp */
+
+
+/*--------------------------------------------------------------------------
+ * Function:    H5FD_splitter_get_handle
+ *
+ * Purpose:     Returns a pointer to the file handle of low-level virtual
+ *              file driver.
+ *
+ * Return:      SUCCEED/FAIL
+ *--------------------------------------------------------------------------
+ */
+static herr_t
+H5FD_splitter_get_handle(
+        H5FD_t                *_file,
+        hid_t H5_ATTR_UNUSED   fapl,
+        void                 **file_handle)
+{
+    H5FD_splitter_t  *file = (H5FD_splitter_t*)_file;
+    herr_t ret_value = SUCCEED;   /* Return value */
+
+    FUNC_ENTER_NOAPI_NOINIT
+
+    H5FD_SPLITTER_LOG_CALL("H5FD_splitter_get_handle");
+
+    /* Check arguments */
+    HDassert(file);
+    HDassert(file->rw_file);
+    HDassert(file_handle);
+
+    /* Only do for R/W channel */
+    if (H5FD_get_vfd_handle(file->rw_file, file->fa.rw_fapl_id, file_handle) < 0)
+    {
+        HGOTO_ERROR(H5E_VFL, H5E_CANTGET, FAIL, "unable to get handle of R/W file")
+    }
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD_splitter_get_handle */
+
+
+/*--------------------------------------------------------------------------
+ * Function:    H5FD_splitter_lock
+ *
+ * Purpose:     Sets a file lock.
+ *
+ * Return:      SUCCEED/FAIL
+ *--------------------------------------------------------------------------
+ */
+static herr_t
+H5FD_splitter_lock(H5FD_t *_file, hbool_t rw)
+{
+    H5FD_splitter_t *file = (H5FD_splitter_t *)_file;   /* VFD file struct */
+    herr_t ret_value = SUCCEED;         /* Return value */
+
+    FUNC_ENTER_NOAPI_NOINIT
+
+    H5FD_SPLITTER_LOG_CALL("H5FD_splitter_lock");
+
+    HDassert(file);
+    HDassert(file->rw_file);
+
+    /* Place the lock on each file */
+    if (H5FD_lock(file->rw_file, rw) < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_CANTLOCK, FAIL, "unable to lock R/W file")
+    }
+    if (file->wo_file != NULL) {
+        if (H5FD_lock(file->wo_file, rw) < 0) {
+           H5FD_SPLITTER_WO_ERROR(file, "H5FD_splitter_lock",
+                                  H5E_VFL, H5E_CANTLOCK, FAIL,
+                                  "unable to lock W/O file")
+        }
+    }
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD_splitter_lock */
+
+
+/*--------------------------------------------------------------------------
+ * Function:    H5FD_splitter_unlock
+ *
+ * Purpose:     Removes a file lock.
+ *
+ * Return:      SUCCEED/FAIL
+ *--------------------------------------------------------------------------
+ */
+static herr_t
+H5FD_splitter_unlock(H5FD_t *_file)
+{
+    H5FD_splitter_t *file = (H5FD_splitter_t *)_file;   /* VFD file struct */
+    herr_t ret_value = SUCCEED;         /* Return value */
+
+    FUNC_ENTER_NOAPI_NOINIT
+
+    H5FD_SPLITTER_LOG_CALL("H5FD_splitter_unlock");
+
+    /* Check arguments */
+    HDassert(file);
+    HDassert(file->rw_file);
+
+    /* Remove the lock on each file */
+    if (H5FD_unlock(file->rw_file) < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_CANTUNLOCK, FAIL, "unable to unlock R/W file")
+    }
+    if (file->wo_file != NULL) {
+        if (H5FD_unlock(file->wo_file) < 0) {
+            HGOTO_ERROR(H5E_VFL, H5E_CANTUNLOCK, FAIL,
+                        "unable to unlock W/O file")
+        }
+    }
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD_splitter_unlock */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_splitter_query
+ *
+ * Purpose:     Set the flags that this VFL driver is capable of supporting.
+ *              (listed in H5FDpublic.h)
+ *
+ * Return:      SUCCEED/FAIL
+ *-------------------------------------------------------------------------
+ */
+static herr_t
+H5FD_splitter_query(const H5FD_t *_file, unsigned long *flags /* out */)
+{
+    const H5FD_splitter_t *file      = (const H5FD_splitter_t *)_file;
+    herr_t                 ret_value = SUCCEED;
+
+    FUNC_ENTER_NOAPI_NOINIT
+
+    H5FD_SPLITTER_LOG_CALL("H5FD_splitter_query");
+
+    if (file) {
+        HDassert(file);
+        HDassert(file->rw_file);
+
+        if (H5FDquery(file->rw_file, flags) < 0) {
+            HGOTO_ERROR(H5E_VFL, H5E_CANTLOCK, FAIL,
+                        "unable to query R/W file");
+        }
+    }
+    else {
+        /* There is no file. Because this is a pure passthrough VFD,
+         * it has no features of its own.
+         */
+        if (flags) {
+            *flags = 0;
+        }
+    }
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD_splitter_query() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_splitter_alloc
+ *
+ * Purpose:     Allocate file memory.
+ *
+ * Return:      Address of allocated space (HADDR_UNDEF if error).
+ *-------------------------------------------------------------------------
+ */
+static haddr_t
+H5FD_splitter_alloc(H5FD_t *_file, H5FD_mem_t type, hid_t dxpl_id, hsize_t size)
+{
+    H5FD_splitter_t *file = (H5FD_splitter_t *)_file;   /* VFD file struct */
+    haddr_t ret_value = HADDR_UNDEF;         /* Return value */
+
+    FUNC_ENTER_NOAPI_NOINIT
+
+    H5FD_SPLITTER_LOG_CALL("H5FD_splitter_alloc");
+
+    /* Check arguments */
+    HDassert(file);
+    HDassert(file->rw_file);
+
+    /* Allocate memory for each file, only return the return value for R/W file. */
+    if ((ret_value = H5FDalloc(file->rw_file, type, dxpl_id, size)) == HADDR_UNDEF) {
+        HGOTO_ERROR(H5E_VFL, H5E_CANTINIT, HADDR_UNDEF, "unable to allocate for R/W file")
+    }
+
+    if (H5FDalloc(file->wo_file, type, dxpl_id, size) == HADDR_UNDEF) {
+       H5FD_SPLITTER_WO_ERROR(file, "H5FD_splitter_alloc",
+                              H5E_VFL, H5E_CANTINIT, HADDR_UNDEF,
+                              "unable to alloc for W/O file")
+    }
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD_splitter_alloc() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_splitter_get_type_map
+ *
+ * Purpose:     Retrieve the memory type mapping for this file
+ *
+ * Return:      SUCCEED/FAIL
+ *-------------------------------------------------------------------------
+ */
+static herr_t
+H5FD_splitter_get_type_map(const H5FD_t *_file, H5FD_mem_t *type_map)
+{
+    const H5FD_splitter_t *file      = (const H5FD_splitter_t *)_file;
+    herr_t                 ret_value = SUCCEED;
+
+    FUNC_ENTER_NOAPI_NOINIT
+
+    H5FD_SPLITTER_LOG_CALL("H5FD_splitter_get_type_map");
+
+    /* Check arguments */
+    HDassert(file);
+    HDassert(file->rw_file);
+
+    /* Retrieve memory type mapping for R/W channel only */
+    if (H5FD_get_fs_type_map(file->rw_file, type_map) < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_CANTGET, FAIL, "unable to allocate for R/W file")
+    }
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD_splitter_get_type_map() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD_splitter_free
+ *
+ * Purpose:     Free the resources for the splitter VFD.
+ *
+ * Return:      SUCCEED/FAIL
+ *-------------------------------------------------------------------------
+ */
+static herr_t
+H5FD_splitter_free(H5FD_t *_file, H5FD_mem_t type, hid_t dxpl_id, haddr_t addr, hsize_t size)
+{
+    H5FD_splitter_t *file = (H5FD_splitter_t *)_file;   /* VFD file struct */
+    herr_t ret_value = SUCCEED;         /* Return value */
+
+    FUNC_ENTER_NOAPI_NOINIT
+
+    H5FD_SPLITTER_LOG_CALL("H5FD_splitter_free");
+
+    /* Check arguments */
+    HDassert(file);
+    HDassert(file->rw_file);
+
+    if (H5FDfree(file->rw_file, type, dxpl_id, addr, size) < 0) {
+        HGOTO_ERROR(H5E_VFL, H5E_CANTFREE, FAIL, "unable to free for R/W file")
+    }
+
+    if (H5FDfree(file->wo_file, type, dxpl_id, addr, size) < 0) {
+       H5FD_SPLITTER_WO_ERROR(file, "H5FD_splitter_free",
+                              H5E_VFL, H5E_CANTINIT, FAIL,
+                              "unable to free for W/O file")
+    }
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD_splitter_free() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5FD__splitter_log_error
+ *
+ * Purpose:     Log an error from the W/O channel appropriately.
+ *
+ * Return:      SUCCEED/FAIL
+ *-------------------------------------------------------------------------
+ */
+static herr_t
+H5FD__splitter_log_error(const H5FD_splitter_t *file, const char *atfunc, const char *msg)
+{
+    size_t size = 0;
+    char  *s = NULL;
+    herr_t ret_value = SUCCEED;
+
+    FUNC_ENTER_STATIC_NOERR
+
+    H5FD_SPLITTER_LOG_CALL("H5FD__splitter_log_error");
+
+    /* Check arguments */
+    HDassert(file);
+    HDassert(atfunc && *atfunc);
+    HDassert(msg && *msg);
+
+    if (file->logfp != NULL) {
+        size = strlen(atfunc) + strlen(msg) + 3; /* ':', ' ', '\n' */
+        s = (char *)malloc(sizeof(char) * (size+1));
+        if (NULL == s) {
+            ret_value = FAIL;
+        }
+        else
+        if (size < (size_t)HDsnprintf(s, size+1, "%s: %s\n", atfunc, msg)) {
+            ret_value = FAIL;
+        }
+        else
+        if (size != HDfwrite(s, 1, size, file->logfp)) {
+            ret_value = FAIL;
+        }
+        HDfree(s);
+    }
+
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5FD__splitter_log_error() */
+
diff --git a/src/H5FDsplitter.h b/src/H5FDsplitter.h
new file mode 100644
index 0000000..5a5ef29
--- /dev/null
+++ b/src/H5FDsplitter.h
@@ -0,0 +1,99 @@
+/* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
+ * Copyright by The HDF Group.                                               *
+ * All rights reserved.                                                      *
+ *                                                                           *
+ * This file is part of HDF5.  The full HDF5 copyright notice, including     *
+ * terms governing use, modification, and redistribution, is contained in    *
+ * the COPYING file, which can be found at the root of the source code       *
+ * distribution tree, or in https://support.hdfgroup.org/ftp/HDF5/releases.  *
+ * If you do not have access to either file, you may request a copy from     *
+ * help@hdfgroup.org.                                                        *
+ * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */
+
+/*
+ * Purpose:	The public header file for the "splitter" driver.
+ */
+
+#ifndef H5FDsplitter_H
+#define H5FDsplitter_H
+
+#define H5FD_SPLITTER (H5FD_splitter_init())
+
+/* The version of the H5FD_splitter_vfd_config_t structure used */
+#define H5FD_CURR_SPLITTER_VFD_CONFIG_VERSION 1
+
+/* Maximum length of a filename/path string in the Write-Only channel,
+ * including the NULL-terminator.
+ */
+#define H5FD_SPLITTER_PATH_MAX 4096
+
+/* Semi-unique constant used to help identify structure pointers */
+#define H5FD_SPLITTER_MAGIC 0x2B916880
+
+/* ----------------------------------------------------------------------------
+ * Structure:   H5FD_spliiter_vfd_config_t
+ *
+ * One-stop shopping for configuring a Splitter VFD (rather than many
+ * paramaters passed into H5Pset/get functions).
+ *
+ * magic (int32_t)
+ *      Semi-unique number, used to sanity-check that a given pointer is
+ *      likely (or not) to be this structure type. MUST be first.
+ *      If magic is not H5FD_SPLITTER_MAGIC, the structure (and/or pointer to)
+ *      must be considered invalid.
+ *
+ * version (unsigned int)
+ *      Version number of this structure -- informs component membership.
+ *      If not H5FD_CURR_SPLITTER_VFD_CONFIG_VERSION, the structure (and/or
+ *      pointer to) must be considered invalid.
+ *
+ * rw_fapl_id (hid_t)
+ *      Library-given identification number of the Read/Write channel driver
+ *      File Access Property List.
+ *      The driver must support read/write access.
+ *      Must be set to H5P_DEFAULT or a valid FAPL ID.
+ *
+ * wo_fapl_id (hid_t)
+ *      Library-given identification number of the Read/Write channel driver
+ *      File Access Property List.
+ *      The driver feature flags must include H5FD_FEAT_DEFAULT_VFD_COMPAITBLE.
+ *      Must be set to H5P_DEFAULT or a valid FAPL ID.
+ *
+ * wo_file_path (char[H5FD_SPLITTER_PATH_MAX + 1])
+ *      String buffer for the Write-Only channel target file.
+ *      Must be null-terminated, cannot be empty.
+ *
+ * log_file_path (char[H5FD_SPLITTER_PATH_MAX + 1])
+ *      String buffer for the Splitter VFD logging output.
+ *      Must be null-terminated.
+ *      If null, no logfile is created.
+ *
+ * ignore_wo_errors (hbool_t)
+ *      Toggle flag for how judiciously to respond to errors on the Write-Only
+ *      channel.
+ *
+ * ----------------------------------------------------------------------------
+ */
+typedef struct H5FD_splitter_vfd_config_t {
+    int32_t magic;
+    unsigned int version;
+    hid_t rw_fapl_id;
+    hid_t wo_fapl_id;
+    char wo_path[H5FD_SPLITTER_PATH_MAX + 1];
+    char log_file_path[H5FD_SPLITTER_PATH_MAX + 1];
+    hbool_t ignore_wo_errs;
+} H5FD_splitter_vfd_config_t;
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+H5_DLL hid_t H5FD_splitter_init(void);
+H5_DLL herr_t H5Pset_fapl_splitter(hid_t fapl_id, H5FD_splitter_vfd_config_t *config_ptr);
+H5_DLL herr_t H5Pget_fapl_splitter(hid_t fapl_id, H5FD_splitter_vfd_config_t *config_ptr);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif
+
diff --git a/src/H5private.h b/src/H5private.h
index f570723..f7fb40b 100644
--- a/src/H5private.h
+++ b/src/H5private.h
@@ -365,6 +365,22 @@
 #endif /* __cplusplus */
 
 /*
+ * Networking headers used by the mirror VFD and related tests and utilities.
+ */
+#ifdef H5_HAVE_ARPA_INET_H
+#   include <arpa/inet.h>
+#endif
+#ifdef H5_HAVE_NETDB_H
+#   include <netdb.h>
+#endif
+#ifdef H5_HAVE_NETINET_IN_H
+#   include <netinet/in.h>
+#endif
+#ifdef H5_HAVE_SYS_SOCKET_H
+#   include <sys/socket.h>
+#endif
+
+/*
  * Status return values for the `herr_t' type.
  * Since some unix/c routines use 0 and -1 (or more precisely, non-negative
  * vs. negative) as their return code, and some assumption had been made in
@@ -646,6 +662,9 @@ typedef struct {
 #ifndef HDabs
     #define HDabs(X)    abs(X)
 #endif /* HDabs */
+#ifndef HDaccept
+    #define HDaccept(A,B,C)    accept((A),(B),(C)) /* mirror VFD */
+#endif /* HDaccept */
 #ifndef HDaccess
     #define HDaccess(F,M)    access(F, M)
 #endif /* HDaccess */
@@ -692,9 +711,15 @@ 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 */
+#ifndef HDbzero
+    #define HDbzero(A,B)   bzero((A),(B)) /* mirror VFD */
+#endif /* HDbzero */
 #ifndef HDcalloc
     #define HDcalloc(N,Z)    calloc(N,Z)
 #endif /* HDcalloc */
@@ -734,6 +759,9 @@ typedef struct {
 #ifndef HDclosedir
     #define HDclosedir(D)    closedir(D)
 #endif /* HDclosedir */
+#ifndef HDconnect
+    #define HDconnect(A,B,C)    connect((A),(B),(C)) /* mirror VFD */
+#endif /* HDconnect */
 #ifndef HDcos
     #define HDcos(X)    cos(X)
 #endif /* HDcos */
@@ -978,9 +1006,12 @@ typedef off_t               h5_stat_size_t;
 #ifndef HDgetgroups
     #define HDgetgroups(Z,G)  getgroups(Z,G)
 #endif /* HDgetgroups */
+#ifndef HDgethostbyaddr
+    #define HDgethostbyaddr(A,B,C)  gethostbyaddr((A),(B),(C)) /* mirror VFD */
+#endif /* HDgethostbyaddr */
 #ifndef HDgethostname
     #define HDgethostname(N,L)    gethostname(N,L)
-#endif /* HDgetlogin */
+#endif /* HDgethostname */
 #ifndef HDgetlogin
     #define HDgetlogin()    getlogin()
 #endif /* HDgetlogin */
@@ -1014,6 +1045,18 @@ typedef off_t               h5_stat_size_t;
 #ifndef HDgmtime
     #define HDgmtime(T)    gmtime(T)
 #endif /* HDgmtime */
+#ifndef HDhtonl
+    #define HDhtonl(X)    htonl((X)) /* mirror VFD */
+#endif /* HDhtonl */
+#ifndef HDhtons
+    #define HDhtons(X)    htons((X)) /* mirror VFD */
+#endif /* HDhtons */
+#ifndef HDinet_addr
+    #define HDinet_addr(C)    inet_addr((C)) /* mirror VFD */
+#endif /* HDinet_addr */
+#ifndef HDinet_ntoa
+    #define HDinet_ntoa(C)    inet_ntoa((C)) /* mirror VFD */
+#endif /* HDinet_ntoa */
 #ifndef HDisalnum
     #define HDisalnum(C)    isalnum((int)(C)) /*cast for solaris warning*/
 #endif /* HDisalnum */
@@ -1068,6 +1111,9 @@ typedef off_t               h5_stat_size_t;
 #ifndef HDlink
     #define HDlink(OLD,NEW)    link(OLD,NEW)
 #endif /* HDlink */
+#ifndef HDlisten
+    #define HDlisten(A,B)    listen((A),(B)) /* mirror VFD */
+#endif /* HDlisten */
 #ifndef HDllround
     #define HDllround(V)     llround(V)
 #endif /* HDround */
@@ -1149,6 +1195,12 @@ typedef off_t               h5_stat_size_t;
 #ifndef HDnanosleep
     #define HDnanosleep(N, O)    nanosleep(N, O)
 #endif /* HDnanosleep */
+#ifndef HDntohl
+    #define HDntohl(A)    ntohl((A)) /* mirror VFD */
+#endif /* HDntohl */
+#ifndef HDntohs
+    #define HDntohs(A)    ntohs((A)) /* mirror VFD */
+#endif /* HDntohs */
 #ifndef HDopen
     #define HDopen(F,...)    open(F,__VA_ARGS__)
 #endif /* HDopen */
@@ -1296,12 +1348,21 @@ typedef off_t               h5_stat_size_t;
 #ifndef HDsetsid
     #define HDsetsid()    setsid()
 #endif /* HDsetsid */
+#ifndef HDsetsockopt
+    #define HDsetsockopt(A,B,C,D,E)    setsockopt((A),(B),(C),(D),(E)) /* mirror VFD */
+#endif /* HDsetsockopt */
 #ifndef HDsetuid
     #define HDsetuid(U)    setuid(U)
 #endif /* HDsetuid */
 #ifndef HDsetvbuf
     #define HDsetvbuf(F,S,M,Z)  setvbuf(F,S,M,Z)
 #endif /* HDsetvbuf */
+#ifndef HDshutdown
+    #define HDshutdown(A, B)    shutdown((A),(B)) /* mirror VFD */
+#endif /* HDshutdown */
+#ifndef HDsigaction
+    #define HDsigaction(S,A,O)  sigaction((S),(A),(O))
+#endif /* HDsigaction */
 #ifndef HDsigaddset
     #define HDsigaddset(S,N)  sigaddset(S,N)
 #endif /* HDsigaddset */
@@ -1347,6 +1408,9 @@ typedef off_t               h5_stat_size_t;
 #ifndef HDsnprintf
     #define HDsnprintf    snprintf /*varargs*/
 #endif /* HDsnprintf */
+#ifndef HDsocket
+    #define HDsocket(A,B,C)    socket((A),(B),(C)) /* mirror VFD */
+#endif /* HDsocket */
 #ifndef HDsprintf
     #define HDsprintf    sprintf /*varargs*/
 #endif /* HDsprintf */
diff --git a/src/Makefile.am b/src/Makefile.am
index 0c07c1b..4f40e60 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -62,7 +62,8 @@ libhdf5_la_SOURCES= H5.c H5checksum.c H5dbg.c H5lib_settings.c H5system.c \
         H5FA.c H5FAcache.c H5FAdbg.c H5FAdblock.c H5FAdblkpage.c H5FAhdr.c \
         H5FAint.c H5FAstat.c H5FAtest.c \
         H5FD.c H5FDcore.c H5FDfamily.c H5FDhdfs.c H5FDint.c H5FDlog.c \
-        H5FDmulti.c H5FDsec2.c H5FDspace.c H5FDstdio.c H5FDtest.c \
+        H5FDmirror.c H5FDmulti.c H5FDros3.c H5FDsec2.c H5FDspace.c \
+        H5FDsplitter.c H5FDstdio.c H5FDtest.c \
         H5FL.c H5FO.c H5FS.c H5FScache.c H5FSdbg.c H5FSint.c H5FSsection.c \
         H5FSstat.c H5FStest.c \
         H5G.c H5Gbtree2.c H5Gcache.c H5Gcompact.c H5Gdense.c H5Gdeprec.c \
@@ -135,9 +136,9 @@ include_HEADERS = hdf5.h H5api_adpt.h H5overflow.h H5pubconf.h H5public.h H5vers
         H5Apublic.h H5ACpublic.h \
         H5Cpublic.h H5Dpublic.h \
         H5Epubgen.h H5Epublic.h H5ESpublic.h H5Fpublic.h \
-        H5FDpublic.h H5FDcore.h H5FDdirect.h \
-        H5FDfamily.h H5FDhdfs.h H5FDlog.h H5FDmpi.h H5FDmpio.h \
-        H5FDmulti.h H5FDros3.h H5FDsec2.h H5FDstdio.h H5FDwindows.h \
+        H5FDpublic.h H5FDcore.h H5FDdirect.h  H5FDfamily.h H5FDhdfs.h \
+        H5FDlog.h H5FDmirror.h H5FDmpi.h H5FDmpio.h  H5FDmulti.h H5FDros3.h \
+        H5FDsec2.h H5FDsplitter.h H5FDstdio.h H5FDwindows.h \
         H5Gpublic.h  H5Ipublic.h H5Lpublic.h \
         H5Mpublic.h H5MMpublic.h H5Opublic.h H5Ppublic.h \
         H5PLextern.h H5PLpublic.h \
diff --git a/src/hdf5.h b/src/hdf5.h
index 8367122..eaaf8ae 100644
--- a/src/hdf5.h
+++ b/src/hdf5.h
@@ -46,10 +46,12 @@
 #include "H5FDfamily.h"         /* File families                            */
 #include "H5FDhdfs.h"           /* Hadoop HDFS                              */
 #include "H5FDlog.h"            /* sec2 driver with I/O logging (for debugging) */
+#include "H5FDmirror.h"         /* Mirror VFD and IPC definitions           */
 #include "H5FDmpi.h"            /* MPI-based file drivers                   */
 #include "H5FDmulti.h"          /* Usage-partitioned file family            */
 #include "H5FDros3.h"           /* R/O S3 "file" I/O                        */
 #include "H5FDsec2.h"           /* POSIX unbuffered file I/O                */
+#include "H5FDsplitter.h"       /* Twin-channel (R/W & R/O) I/O passthrough */
 #include "H5FDstdio.h"          /* Standard C buffered I/O                  */
 #ifdef H5_HAVE_WINDOWS
 #include "H5FDwindows.h"        /* Win32 I/O                                */
diff --git a/src/libhdf5.settings.in b/src/libhdf5.settings.in
index 1591bed..df7ddf2 100644
--- a/src/libhdf5.settings.in
+++ b/src/libhdf5.settings.in
@@ -80,6 +80,7 @@ Parallel Filtered Dataset Writes: @PARALLEL_FILTERED_WRITES@
                              MPE: @MPE@
                    Map (H5M) API: @MAP_API@
                       Direct VFD: @DIRECT_VFD@
+                      Mirror VFD: @MIRROR_VFD@
               (Read-Only) S3 VFD: @ROS3_VFD@
             (Read-Only) HDFS VFD: @HAVE_LIBHDFS@
                          dmalloc: @HAVE_DMALLOC@