[svn-r20064] Purpose: Implement external file cache

Description:
Implements a cache of files opened through external links.  Adds the public
functions H5Pset_elink_file_cache_size(), H5Pget_elink_file_cache_size(), and
H5Frelease_file_cache().

Tested: jam, amani, heiwa (h5committest), fedora 64.

11 files changed, 1147 insertions, 39 deletions

diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index 3330b8b..93370c0 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -145,6 +145,7 @@ SET (H5F_SRCS
     ${HDF5_SRC_DIR}/H5Faccum.c
     ${HDF5_SRC_DIR}/H5Fdbg.c
     ${HDF5_SRC_DIR}/H5Fdeprec.c
+    ${HDF5_SRC_DIR}/H5Fefc.c
     ${HDF5_SRC_DIR}/H5Ffake.c
     ${HDF5_SRC_DIR}/H5Fio.c
     ${HDF5_SRC_DIR}/H5Fmount.c
diff --git a/src/H5F.c b/src/H5F.c
index 46d9407..f84dea2 100644
--- a/src/H5F.c
+++ b/src/H5F.c
@@ -292,6 +292,7 @@ H5F_get_access_plist(H5F_t *f, hbool_t app_ref)
     H5P_genplist_t *new_plist;              /* New property list */
     H5P_genplist_t *old_plist;              /* Old property list */
     void		*driver_info=NULL;
+    unsigned            efc_size = 0;
     hid_t		ret_value = SUCCEED;
 
     FUNC_ENTER_NOAPI(H5F_get_access_plist, FAIL)
@@ -330,6 +331,10 @@ H5F_get_access_plist(H5F_t *f, hbool_t app_ref)
         HGOTO_ERROR(H5E_PLIST, H5E_CANTSET, FAIL, "can't set 'small data' cache size")
     if(H5P_set(new_plist, H5F_ACS_LATEST_FORMAT_NAME, &(f->shared->latest_format)) < 0)
         HGOTO_ERROR(H5E_PLIST, H5E_CANTSET, FAIL, "can't set 'latest format' flag")
+    if(f->shared->efc)
+        efc_size = H5F_efc_max_nfiles(f->shared->efc);
+    if(H5P_set(new_plist, H5F_ACS_EFC_SIZE_NAME, &efc_size) < 0)
+        HGOTO_ERROR(H5E_PLIST, H5E_CANTGET, FAIL, "can't set elink file cache size")
 
     /*
      * Since we're resetting the driver ID and info, close them if they
@@ -840,6 +845,7 @@ H5F_new(H5F_file_t *shared, hid_t fcpl_id, hid_t fapl_id, H5FD_t *lf)
     } /* end if */
     else {
         H5P_genplist_t *plist;          /* Property list */
+        unsigned        efc_size;       /* External file cache size */
         size_t u;                       /* Local index variable */
 
         HDassert(lf != NULL);
@@ -902,6 +908,11 @@ H5F_new(H5F_file_t *shared, hid_t fcpl_id, hid_t fapl_id, H5FD_t *lf)
         if(H5P_get(plist, H5F_ACS_SDATA_BLOCK_SIZE_NAME, &(f->shared->sdata_aggr.alloc_size)) < 0)
             HGOTO_ERROR(H5E_PLIST, H5E_CANTGET, NULL, "can't get 'small data' cache size")
         f->shared->sdata_aggr.feature_flag = H5FD_FEAT_AGGREGATE_SMALLDATA;
+        if(H5P_get(plist, H5F_ACS_EFC_SIZE_NAME, &efc_size) < 0)
+            HGOTO_ERROR(H5E_PLIST, H5E_CANTGET, NULL, "can't get elink file cache size")
+        if(efc_size > 0)
+            if(NULL == (f->shared->efc = H5F_efc_create(efc_size)))
+                HGOTO_ERROR(H5E_FILE, H5E_CANTINIT, NULL, "can't create external file cache")
 
         /* Get the VFD values to cache */
         f->shared->maxaddr = H5FD_get_maxaddr(lf);
@@ -998,6 +1009,13 @@ H5F_dest(H5F_t *f, hid_t dxpl_id, hbool_t flush)
             if(H5F_flush(f, dxpl_id) < 0)
                 HDONE_ERROR(H5E_CACHE, H5E_CANTFLUSH, FAIL, "unable to flush cache")
 
+        /* Release the external file cache */
+        if(f->shared->efc) {
+            if(H5F_efc_destroy(f->shared->efc) < 0)
+                HDONE_ERROR(H5E_FILE, H5E_CANTRELEASE, FAIL, "can't destroy external file cache")
+            f->shared->efc = NULL;
+        } /* end if */
+
         /* Release objects that depend on the superblock being initialized */
         if(f->shared->sblock) {
             /* Shutdown file free space manager(s) */
@@ -1903,6 +1921,13 @@ H5F_try_close(H5F_t *f)
     if(H5F_close_mounts(f) < 0)
         HGOTO_ERROR(H5E_FILE, H5E_CANTCLOSEFILE, FAIL, "can't unmount child files")
 
+    /* If there is more than one reference to the shared file struct and the
+     * file has an external file cache, we should see if it can be closed.  This
+     * can happen if a cycle is formed with external file caches */
+    if(f->shared->efc && (f->shared->nrefs > 1))
+        if(H5F_efc_try_close(f) < 0)
+            HGOTO_ERROR(H5E_FILE, H5E_CANTRELEASE, FAIL, "can't attempt to close EFC")
+
     /* Delay flush until the shared file struct is closed, in H5F_dest.  If the
      * application called H5Fclose, it would have been flushed in that function
      * (unless it will have been flushed in H5F_dest anyways). */
@@ -2929,3 +2954,40 @@ done:
     FUNC_LEAVE_API(ret_value)
 } /* end H5Fget_free_sections() */
 
+
+/*-------------------------------------------------------------------------
+ * Function:    H5Frelease_file_cache
+ *
+ * Purpose:     Releases the external file cache associated with the
+ *              provided file, potentially closing any cached files
+ *              unless they are held open from somewhere\ else.
+ *
+ * Return:      Success:        non-negative
+ *              Failure:        negative
+ *
+ * Programmer:  Neil Fortner; December 30, 2010
+ *
+ *-------------------------------------------------------------------------
+ */
+herr_t
+H5Frelease_file_cache(hid_t file_id)
+{
+    H5F_t         *file;        /* File */
+    herr_t        ret_value = SUCCEED; /* Return value */
+
+    FUNC_ENTER_API(H5Frelease_file_cache, FAIL)
+    H5TRACE1("e", "i", file_id);
+
+    /* Check args */
+    if(NULL == (file = (H5F_t *)H5I_object_verify(file_id, H5I_FILE)))
+        HGOTO_ERROR(H5E_ARGS, H5E_BADVALUE, FAIL, "not a file ID")
+
+    /* Release the EFC */
+    if(file->shared->efc)
+        if(H5F_efc_release(file->shared->efc) < 0)
+            HGOTO_ERROR(H5E_FILE, H5E_CANTRELEASE, FAIL, "can't release external file cache")
+
+done:
+    FUNC_LEAVE_API(ret_value)
+} /* end H5Frelease_file_cache() */
+
diff --git a/src/H5Fefc.c b/src/H5Fefc.c
new file mode 100644
index 0000000..eebf143
--- /dev/null
+++ b/src/H5Fefc.c
@@ -0,0 +1,951 @@
+/* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
+ * Copyright by The HDF Group.                                               *
+ * Copyright by the Board of Trustees of the University of Illinois.         *
+ * 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 files COPYING and Copyright.html.  COPYING can be found at the root   *
+ * of the source code distribution tree; Copyright.html can be found at the  *
+ * root level of an installed copy of the electronic HDF5 document set and   *
+ * is linked from the top-level documents page.  It can also be found at     *
+ * http://hdfgroup.org/HDF5/doc/Copyright.html.  If you do not have          *
+ * access to either file, you may request a copy from help@hdfgroup.org.     *
+ * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */
+
+/*-------------------------------------------------------------------------
+ *
+ * Created:             H5Defc.c
+ *                      December 13, 2010
+ *                      Neil Fortner <nfortne2@hdfgroup.org>
+ *
+ * Purpose:             External file caching routines - implements a
+ *                      cache of external files to minimize the number of
+ *                      file opens and closes.
+ *
+ *-------------------------------------------------------------------------
+ */
+
+#define H5F_PACKAGE             /*suppress error about including H5Fpkg   */
+
+
+/* Packages needed by this file... */
+#include "H5private.h"          /* Generic Functions                    */
+#include "H5Eprivate.h"         /* Error handling                       */
+#include "H5Fpkg.h"             /* File access                          */
+#include "H5MMprivate.h"        /* Memory management                    */
+#include "H5Pprivate.h"         /* Property lists                       */
+
+
+/* Special values for the "tag" field below */
+#define H5F_EFC_TAG_DEFAULT     -1
+#define H5F_EFC_TAG_LOCK        -2
+#define H5F_EFC_TAG_CLOSE       -3
+#define H5F_EFC_TAG_DONTCLOSE   -4
+
+/* Structure for each entry in a file's external file cache */
+typedef struct H5F_efc_ent_t {
+    char                *name;          /* Name of the file */
+    H5F_t               *file;          /* File object */
+    struct H5F_efc_ent_t *LRU_next;     /* Next item in LRU list */
+    struct H5F_efc_ent_t *LRU_prev;     /* Previous item in LRU list */
+    unsigned            nopen;          /* Number of times this file is currently opened by an EFC client */
+} H5F_efc_ent_t;
+
+/* Structure for a shared file struct's external file cache */
+struct H5F_efc_t {
+    H5SL_t              *slist;         /* Skip list of cached external files */
+    H5F_efc_ent_t       *LRU_head;      /* Head of LRU list.  This is the least recently used file */
+    H5F_efc_ent_t       *LRU_tail;      /* Tail of LRU list.  This is the most recently used file */
+    unsigned            nfiles;         /* Size of the external file cache */
+    unsigned            max_nfiles;     /* Maximum size of the external file cache */
+    unsigned            nrefs;          /* Number of times this file appears in another file's EFC */
+    int                 tag;            /* Temporary variable used by H5F_efc_try_close() */
+    H5F_file_t          *tmp_next;      /* Next file in temporary list used by H5F_efc_try_close() */
+};
+
+/* Private prototypes */
+static herr_t H5F_efc_remove_ent(H5F_efc_t *efc, H5F_efc_ent_t *ent);
+static void H5F_efc_try_close_tag1(H5F_file_t *sf, H5F_file_t **tail);
+static void H5F_efc_try_close_tag2(H5F_file_t *sf, H5F_file_t **tail);
+
+/* Free lists */
+H5FL_DEFINE_STATIC(H5F_efc_ent_t);
+H5FL_DEFINE_STATIC(H5F_efc_t);
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5F_efc_create
+ *
+ * Purpose:     Allocate and initialize a new external file cache object,
+ *              which can the be used to cache open external files.
+ *              the object must be freed with H5F_efc_destroy.
+ *
+ * Return:      Pointer to new external file cache object on success
+ *              NULL on failure
+ *
+ * Programmer:  Neil Fortner
+ *              Tuesday, December 14, 2010
+ *
+ *-------------------------------------------------------------------------
+ */
+H5F_efc_t *
+H5F_efc_create(unsigned max_nfiles)
+{
+    H5F_efc_t   *efc = NULL;            /* EFC object */
+    H5F_efc_t   *ret_value;             /* Return value */
+
+    FUNC_ENTER_NOAPI(H5F_efc_create, NULL)
+
+    /* Sanity checks */
+    HDassert(max_nfiles > 0);
+
+    /* Allocate EFC struct */
+    if(NULL == (efc = H5FL_CALLOC(H5F_efc_t)))
+        HGOTO_ERROR(H5E_RESOURCE, H5E_NOSPACE, NULL, "memory allocation failed")
+
+    /* Initialize maximum number of files */
+    efc->max_nfiles = max_nfiles;
+
+    /* Initialize temporary ref count */
+    efc->tag = H5F_EFC_TAG_DEFAULT;
+
+    /* Set the return value */
+    ret_value = efc;
+
+done:
+    if(ret_value == NULL && efc)
+        efc = H5FL_FREE(H5F_efc_t, efc);
+
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5F_efc_create() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5F_efc_open
+ *
+ * Purpose:     Opens a file using the external file cache.  The target
+ *              file is added to the external file cache of the parent
+ *              if it is not already present.  If the target file is in
+ *              the parent's EFC, simply returns the target file.  When
+ *              the file object is no longer in use, it should be closed
+ *              with H5F_efc_close (will not actually close the file
+ *              until it is evicted from the EFC).
+ *
+ * Return:      Pointer to open file on success
+ *              NULL on failure
+ *
+ * Programmer:  Neil Fortner
+ *              Tuesday, December 14, 2010
+ *
+ *-------------------------------------------------------------------------
+ */
+H5F_t *
+H5F_efc_open(H5F_t *parent, const char *name, unsigned flags, hid_t fcpl_id,
+    hid_t fapl_id, hid_t dxpl_id)
+{
+    H5F_efc_t   *efc = NULL;    /* External file cache for parent file */
+    H5F_efc_ent_t *ent = NULL;  /* Entry for target file in efc */
+    hbool_t     open_file = FALSE; /* Whether ent->file needs to be closed in case of error */
+    H5F_t       *ret_value;     /* Return value */
+
+    FUNC_ENTER_NOAPI_NOINIT(H5F_efc_open)
+
+    /* Sanity checks */
+    HDassert(parent);
+    HDassert(parent->shared);
+    HDassert(name);
+
+    /* Get external file cache */
+    efc = parent->shared->efc;
+
+    /* Check if the EFC exists.  If it does not, just call H5F_open().  We
+     * support this so clients do not have to make 2 different calls depending
+     * on the state of the efc. */
+    if(!efc) {
+        if(NULL == (ret_value = H5F_open(name, flags, fcpl_id, fapl_id,
+                dxpl_id)))
+            HGOTO_ERROR(H5E_FILE, H5E_CANTOPENFILE, NULL, "can't open file")
+
+        /* Increment the number of open objects to prevent the file from being
+         * closed out from under us - "simulate" having an open file id.  Note
+         * that this behaviour replaces the calls to H5F_incr_nopen_objs() and
+         * H5F_decr_nopen_objs() in H5L_extern_traverse(). */
+        ret_value->nopen_objs++;
+
+        HGOTO_DONE(ret_value)
+    } /* end if */
+
+    /* Search the skip list for name if the skip list exists, create the skip
+     * list otherwise */
+    if(efc->slist) {
+        if(efc->nfiles > 0)
+            ent = (H5F_efc_ent_t *)H5SL_search(efc->slist, name);
+    } /* end if */
+    else {
+        HDassert(efc->nfiles == 0);
+        if(NULL == (efc->slist = H5SL_create(H5SL_TYPE_STR)))
+            HGOTO_ERROR(H5E_FILE, H5E_CANTCREATE, NULL, "can't create skip list")
+    } /* end else */
+
+    /* If we found the file update the LRU list and return the cached file,
+     * otherwise open the file and cache it */
+    if(ent) {
+        HDassert(efc->LRU_head);
+        HDassert(efc->LRU_tail);
+
+        /* Move ent to the head of the LRU list, if it is not already there */
+        if(ent->LRU_prev) {
+            HDassert(efc->LRU_head != ent);
+
+            /* Remove from current position.  Note that once we touch the LRU
+             * list we cannot revert to the previous state.  Make sure there can
+             * be no errors between when we first touch the LRU list and when
+             * the cache is in a consistent state! */
+            if(ent->LRU_next)
+                ent->LRU_next->LRU_prev = ent->LRU_prev;
+            else {
+                HDassert(efc->LRU_tail == ent);
+                efc->LRU_tail = ent->LRU_prev;
+            } /* end else */
+            ent->LRU_prev->LRU_next = ent->LRU_next;
+
+            /* Add to head of LRU list */
+            ent->LRU_next = efc->LRU_head;
+            ent->LRU_next->LRU_prev = ent;
+            ent->LRU_prev = NULL;
+            efc->LRU_head = ent;
+        } /* end if */
+
+        /* Mark the file as open */
+        ent->nopen++;
+    } /* end if */
+    else {
+        /* Check if we need to evict something */
+        if(efc->nfiles == efc->max_nfiles) {
+            /* Search for an unopened file from the tail */
+            for(ent = efc->LRU_tail; ent && ent->nopen; ent = ent->LRU_prev);
+
+            /* Evict the file if found, otherwise just open the target file and
+             * do not add it to cache */
+            if(ent) {
+                if(H5F_efc_remove_ent(efc, ent) < 0)
+                    HGOTO_ERROR(H5E_FILE, H5E_CANTREMOVE, NULL, "can't remove entry from external file cache")
+
+                /* Do not free ent, we will recycle it below */
+            } /* end if */
+            else {
+                /* Cannot cache file, just open file and return */
+                if(NULL == (ret_value = H5F_open(name, flags, fcpl_id, fapl_id,
+                        dxpl_id)))
+                    HGOTO_ERROR(H5E_FILE, H5E_CANTOPENFILE, NULL, "can't open file")
+
+                /* Increment the number of open objects to prevent the file from
+                 * being closed out from under us - "simulate" having an open
+                 * file id */
+                ret_value->nopen_objs++;
+
+                HGOTO_DONE(ret_value)
+            } /* end else */
+        } /* end if */
+        else
+            /* Allocate new entry */
+            if(NULL == (ent = H5FL_MALLOC(H5F_efc_ent_t)))
+                HGOTO_ERROR(H5E_RESOURCE, H5E_NOSPACE, NULL, "memory allocation failed")
+
+        /* Build new entry */
+        if(NULL == (ent->name = H5MM_strdup(name)))
+            HGOTO_ERROR(H5E_RESOURCE, H5E_NOSPACE, NULL, "memory allocation failed")
+
+        /* Open the file */
+        if(NULL == (ent->file = H5F_open(name, flags, fcpl_id, fapl_id,
+                dxpl_id)))
+            HGOTO_ERROR(H5E_FILE, H5E_CANTOPENFILE, NULL, "can't open file")
+        open_file = TRUE;
+
+        /* Increment the number of open objects to prevent the file from being
+         * closed out from under us - "simulate" having an open file id */
+        ent->file->nopen_objs++;
+
+        /* Add the file to the cache */
+        /* Skip list */
+        if(H5SL_insert(efc->slist, ent, ent->name) < 0)
+            HGOTO_ERROR(H5E_FILE, H5E_CANTINSERT, NULL, "can't insert entry into skip list")
+
+        /* Add to head of LRU list and update tail if necessary */
+        ent->LRU_next = efc->LRU_head;
+        if(ent->LRU_next)
+            ent->LRU_next->LRU_prev = ent;
+        ent->LRU_prev = NULL;
+        efc->LRU_head = ent;
+        if(!efc->LRU_tail) {
+            HDassert(!ent->LRU_next);
+            efc->LRU_tail = ent;
+        } /* end if */
+
+        /* Mark the file as open */
+        ent->nopen = 1;
+
+        /* Update nfiles and nrefs */
+        efc->nfiles++;
+        if(ent->file->shared->efc)
+            ent->file->shared->efc->nrefs++;
+    } /* end else */
+
+    HDassert(ent);
+    HDassert(ent->file);
+    HDassert(ent->name);
+    HDassert(ent->nopen);
+
+    /* Set the return value */
+    ret_value = ent->file;
+
+done:
+    if(!ret_value)
+        if(ent) {
+            if(open_file) {
+                ent->file->nopen_objs--;
+                if(H5F_try_close(ent->file) < 0)
+                    HDONE_ERROR(H5E_FILE, H5E_CANTCLOSEFILE, NULL, "can't close external file")
+            } /* end if */
+            ent->name = (char *)H5MM_xfree(ent->name);
+            ent = H5FL_FREE(H5F_efc_ent_t, ent);
+        } /* end if */
+
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5F_efc_open() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5F_efc_close
+ *
+ * Purpose:     Closes (unlocks) a file opened using the external file
+ *              cache.  The target file is not immediately closed unless
+ *              there is no external file cache for the parent file.
+ *
+ * Return:      Non-negative on success
+ *              Negative on failure
+ *
+ * Programmer:  Neil Fortner
+ *              Wednesday, December 15, 2010
+ *
+ *-------------------------------------------------------------------------
+ */
+herr_t
+H5F_efc_close(H5F_t *parent, H5F_t *file)
+{
+    H5F_efc_t   *efc = NULL;    /* External file cache for parent file */
+    H5F_efc_ent_t *ent = NULL;  /* Entry for target file in efc */
+    herr_t      ret_value = SUCCEED; /* Return value */
+
+    FUNC_ENTER_NOAPI_NOINIT(H5F_efc_close)
+
+    /* Sanity checks */
+    HDassert(parent);
+    HDassert(parent->shared);
+    HDassert(file);
+    HDassert(file->shared);
+
+    /* Get external file cache */
+    efc = parent->shared->efc;
+
+    /* Check if the EFC exists.  If it does not, just call H5F_try_close().  We
+     * support this so clients do not have to make 2 different calls depending
+     * on the state of the efc. */
+    if(!efc) {
+        file->nopen_objs--;
+        if(H5F_try_close(file) < 0)
+            HGOTO_ERROR(H5E_FILE, H5E_CANTCLOSEFILE, FAIL, "can't close external file")
+
+        HGOTO_DONE(SUCCEED)
+    } /* end if */
+
+    /* Scan the parent's LRU list from the head to file file.  We do this
+     * instead of a skip list lookup because the file will almost always be at
+     * the head.  In the unlikely case that the file is not found, just call
+     * H5F_try_close().  This could happen if the EFC was full of open files
+     * when the file was opened. */
+    for(ent = efc->LRU_head; ent && ent->file != file; ent = ent->LRU_next);
+    if(!ent) {
+        file->nopen_objs--;
+        if(H5F_try_close(file) < 0)
+            HGOTO_ERROR(H5E_FILE, H5E_CANTCLOSEFILE, FAIL, "can't close external file")
+    } /* end if */
+    else
+        /* Reduce the open count on this entry */
+        ent->nopen--;
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5F_efc_close() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5F_efc_max_nfiles
+ *
+ * Purpose:     Returns the maximum number of files in the provided
+ *              external file cache.
+ *
+ * Return:      Maximum number of files (never fails)
+ *
+ * Programmer:  Neil Fortner
+ *              Wednesday, December 15, 2010
+ *
+ *-------------------------------------------------------------------------
+ */
+unsigned
+H5F_efc_max_nfiles(H5F_efc_t *efc)
+{
+    FUNC_ENTER_NOAPI_NOINIT_NOFUNC(H5F_efc_max_nfiles)
+
+    HDassert(efc);
+    HDassert(efc->max_nfiles > 0);
+
+    FUNC_LEAVE_NOAPI(efc->max_nfiles)
+} /* end H5F_efc_max_nfiles */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5F_efc_release
+ *
+ * Purpose:     Releases the external file cache, potentially closing any
+ *              cached files unless they are held open from somewhere
+ *              else (or are currently opened by a client).
+ *
+ * Return:      Non-negative on success
+ *              Negative on failure
+ *
+ * Programmer:  Neil Fortner
+ *              Wednesday, December 15, 2010
+ *
+ *-------------------------------------------------------------------------
+ */
+herr_t
+H5F_efc_release(H5F_efc_t *efc)
+{
+    H5F_efc_ent_t *ent = NULL;          /* EFC entry */
+    H5F_efc_ent_t *prev_ent = NULL;     /* Previous EFC entry */
+    herr_t      ret_value = SUCCEED;    /* Return value */
+
+    FUNC_ENTER_NOAPI_NOINIT(H5F_efc_release)
+
+    /* Sanity checks */
+    HDassert(efc);
+
+    /* Lock the EFC to prevent manipulation of the EFC wile we are releasing it.
+     * The EFC should never be locked when we enter this function because that
+     * would require a cycle, a cycle would necessarily invoke
+     * H5F_efc_try_close(), and that function checks the status of the lock
+     * before calling this one. */
+    HDassert((efc->tag == H5F_EFC_TAG_DEFAULT)
+            || (efc->tag == H5F_EFC_TAG_CLOSE));
+    efc->tag = H5F_EFC_TAG_LOCK;
+
+    /* Walk down the LRU list, releasing any files that are not opened by an EFC
+     * client */
+    ent = efc->LRU_head;
+    while(ent)
+        if(!ent->nopen) {
+            if(H5F_efc_remove_ent(efc, ent) < 0)
+                HGOTO_ERROR(H5E_FILE, H5E_CANTREMOVE, FAIL, "can't remove entry from external file cache")
+
+            /* Free the entry and move to next entry in LRU list */
+            prev_ent = ent;
+            ent = ent->LRU_next;
+            prev_ent = H5FL_FREE(H5F_efc_ent_t, prev_ent);
+        } /* end if */
+        else
+            /* Can't release file because it's open; just advance the pointer */
+            ent = ent->LRU_next;
+
+    /* Reset tag.  No need to reset to CLOSE if that was the original tag, as in
+     * that case the file must be getting closed anyways. */
+    efc->tag = H5F_EFC_TAG_DEFAULT;
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value);
+} /* end H5F_efc_release() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5F_efc_destroy
+ *
+ * Purpose:     Frees an external file cache object, releasing it first
+ *              if necessary.  If it cannot be fully released, for example
+ *              if there are open files, returns an error.
+ *
+ * Return:      Non-negative on success
+ *              Negative on failure
+ *
+ * Programmer:  Neil Fortner
+ *              Wednesday, December 15, 2010
+ *
+ *-------------------------------------------------------------------------
+ */
+herr_t
+H5F_efc_destroy(H5F_efc_t *efc)
+{
+    herr_t      ret_value = SUCCEED;    /* Return value */
+
+    FUNC_ENTER_NOAPI_NOINIT(H5F_efc_destroy)
+
+    /* Sanity checks */
+    HDassert(efc);
+
+    if(efc->nfiles > 0) {
+        /* Release (clear) the efc */
+        if(H5F_efc_release(efc) < 0)
+            HGOTO_ERROR(H5E_FILE, H5E_CANTRELEASE, FAIL, "can't release external file cache")
+
+        /* If there are still cached files, return an error */
+        if(efc->nfiles > 0)
+            HGOTO_ERROR(H5E_FILE, H5E_CANTFREE, FAIL, "can't destroy EFC after incomplete release")
+    } /* end if */
+
+    HDassert(efc->nfiles == 0);
+    HDassert(efc->LRU_head == NULL);
+    HDassert(efc->LRU_tail == NULL);
+
+    /* Close skip list */
+    if(efc->slist)
+        if(H5SL_close(efc->slist) < 0)
+            HGOTO_ERROR(H5E_FILE, H5E_CANTFREE, FAIL, "can't close skip list")
+
+    /* Free EFC object */
+    (void)H5FL_FREE(H5F_efc_t, efc);
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value);
+} /* end H5F_efc_destroy() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5F_efc_remove_ent
+ *
+ * Purpose:     Removes the specified entry from the specified EFC,
+ *              closing the file if requested.  Does not free the entry.
+ *
+ * Return:      Non-negative on success
+ *              Negative on failure
+ *
+ * Programmer:  Neil Fortner
+ *              Wednesday, December 15, 2010
+ *
+ *-------------------------------------------------------------------------
+ */
+static herr_t
+H5F_efc_remove_ent(H5F_efc_t *efc, H5F_efc_ent_t *ent)
+{
+    herr_t      ret_value = SUCCEED;    /* Return value */
+
+    FUNC_ENTER_NOAPI_NOINIT(H5F_efc_remove_ent)
+
+    /* Sanity checks */
+    HDassert(efc);
+    HDassert(efc->slist);
+    HDassert(ent);
+
+    /* Remove from skip list */
+    if(ent != H5SL_remove(efc->slist, ent->name))
+        HGOTO_ERROR(H5E_FILE, H5E_CANTDELETE, FAIL, "can't delete entry from skip list")
+
+    /* Remove from LRU list */
+    if(ent->LRU_next)
+        ent->LRU_next->LRU_prev = ent->LRU_prev;
+    else {
+        HDassert(efc->LRU_tail == ent);
+        efc->LRU_tail = ent->LRU_prev;
+    } /* end else */
+    if(ent->LRU_prev)
+        ent->LRU_prev->LRU_next = ent->LRU_next;
+    else {
+        HDassert(efc->LRU_head == ent);
+        efc->LRU_head = ent->LRU_next;
+    } /* end else */
+
+    /* Update nfiles and nrefs */
+    efc->nfiles--;
+    if(ent->file->shared->efc)
+        ent->file->shared->efc->nrefs--;
+
+    /* Free the name */
+    ent->name = (char *)H5MM_xfree(ent->name);
+
+    /* Close the file.  Note that since H5F_t structs returned from H5F_open()
+     * are *always* unique, there is no need to reference count this struct.
+     * However we must still manipulate the nopen_objs field to prevent the file
+     * from being closed out from under us. */
+    ent->file->nopen_objs--;
+    if(H5F_try_close(ent->file) < 0)
+        HGOTO_ERROR(H5E_FILE, H5E_CANTCLOSEFILE, FAIL, "can't close external file")
+    ent->file = NULL;
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5F_efc_remove_ent() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5F_efc_try_close_tag1
+ *
+ * Purpose:     Recursively traverse the EFC tree, keeping a temporary
+ *              reference count on each file that assumes all reachable
+ *              files will eventually be closed.
+ *
+ * Return:      void (never fails)
+ *
+ * Programmer:  Neil Fortner
+ *              Monday, January 10, 2011
+ *
+ *-------------------------------------------------------------------------
+ */
+static void
+H5F_efc_try_close_tag1(H5F_file_t *sf, H5F_file_t **tail)
+{
+    H5F_efc_ent_t       *ent = NULL;    /* EFC entry */
+    H5F_file_t          *esf;           /* Convenience pointer to ent->file->shared */
+
+    FUNC_ENTER_NOAPI_NOINIT_NOFUNC(H5F_efc_try_close_tag1)
+
+    /* Sanity checks */
+    HDassert(sf);
+    HDassert(sf->efc);
+    HDassert((sf->efc->tag > 0) || (sf->nrefs == sf->efc->nrefs));
+    HDassert(sf->efc->tag != H5F_EFC_TAG_LOCK);
+    HDassert(tail);
+    HDassert(*tail);
+
+    /* Recurse into this file's cached files */
+    for(ent = sf->efc->LRU_head; ent; ent = ent->LRU_next) {
+        esf = ent->file->shared;
+
+        if(esf->efc) {
+            /* If tag were 0, that would mean there are more actual references
+             * than are counted by nrefs */
+            HDassert(esf->efc->tag != 0);
+
+            /* If tag has been set, we have already visited this file so just
+             * decrement tag and continue */
+            if(esf->efc->tag > 0)
+                esf->efc->tag--;
+            /* If there are references that are not from an EFC, it will never
+             * be possible to close the file.  Just continue.  Also continue if
+             * the EFC is locked or the file is open (through the EFC).  Note
+             * that the reference counts will never match for the root file, but
+             * that's ok because the root file will always have a tag and enter
+             * the branch above. */
+            else if((esf->nrefs == esf->efc->nrefs)
+                    && (esf->efc->tag != H5F_EFC_TAG_LOCK) && !(ent->nopen)) {
+                /* If we get here, this file's "tmp_next" pointer must be NULL
+                 */
+                HDassert(esf->efc->tmp_next == NULL);
+
+                /* If nrefs > 1, Add this file to the list of files with nrefs >
+                 * 1 and initialize tag to the number of references (except this
+                 * one) */
+                if(esf->nrefs > 1) {
+                    (*tail)->efc->tmp_next = esf;
+                    *tail = esf;
+                    esf->efc->tag = (int)esf->nrefs - 1;
+                } /* end if */
+
+                /* Recurse into the entry */
+                H5F_efc_try_close_tag1(ent->file->shared, tail);
+            } /* end if */
+        } /* end if */
+    } /* end for */
+
+    FUNC_LEAVE_NOAPI_VOID
+} /* end H5F_efc_try_close_tag1() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5F_efc_try_close_tag2
+ *
+ * Purpose:     Recuresively mark all files reachable through this one as
+ *              uncloseable, and add newly uncloseable files to the tail
+ *              of the provided linked list.
+ *
+ * Return:      void (never fails)
+ *
+ * Programmer:  Neil Fortner
+ *              Monday, January 10, 2011
+ *
+ *-------------------------------------------------------------------------
+ */
+static void
+H5F_efc_try_close_tag2(H5F_file_t *sf, H5F_file_t **tail)
+{
+    H5F_efc_ent_t       *ent = NULL;    /* EFC entry */
+    H5F_file_t          *esf;           /* Convenience pointer to ent->file->shared */
+
+    FUNC_ENTER_NOAPI_NOINIT_NOFUNC(H5F_efc_try_close_tag2)
+
+    /* Sanity checks */
+    HDassert(sf);
+    HDassert(sf->efc);
+
+    /* Recurse into this file's cached files */
+    for(ent = sf->efc->LRU_head; ent; ent = ent->LRU_next) {
+        esf = ent->file->shared;
+
+        /* Only recurse if the file is tagged CLOSE or DEFAULT.  If it is tagged
+         * DONTCLOSE, we have already visited this file *or* it will be the
+         * start point of another iteration.  No files should be tagged with a
+         * nonegative value at this point.  If it is tagged as DEFAULT, we must
+         * apply the same conditions as in cb1 above for recursion in order to
+         * make sure  we do not go off into somewhere cb1 didn't touch.  The
+         * root file should never be tagged DEFAULT here, so the reference check
+         * is still appropriate. */
+        if((esf->efc) && ((esf->efc->tag == H5F_EFC_TAG_CLOSE)
+                || ((esf->efc->tag == H5F_EFC_TAG_DEFAULT)
+                && (esf->nrefs == esf->efc->nrefs) && !(ent->nopen)))) {
+            /* tag should always be CLOSE is nrefs > 1 or DEFAULT if nrefs == 1
+             * here */
+            HDassert(((esf->nrefs > 1)
+                    && ((esf->efc->tag == H5F_EFC_TAG_CLOSE)))
+                    || ((esf->nrefs == 1)
+                    && (esf->efc->tag == H5F_EFC_TAG_DEFAULT)));
+
+            /* If tag is set to DONTCLOSE, we have already visited this file
+             * *or* it will be the start point of another iteration so just
+             * continue */
+            if(esf->efc->tag != H5F_EFC_TAG_DONTCLOSE) {
+                /* If tag is CLOSE, set to DONTCLOSE and add to the list of
+                 * uncloseable files. */
+                if(esf->efc->tag == H5F_EFC_TAG_CLOSE) {
+                    esf->efc->tag = H5F_EFC_TAG_DONTCLOSE;
+                    esf->efc->tmp_next = NULL;
+                    (*tail)->efc->tmp_next = esf;
+                    *tail = esf;
+                } /* end if */
+
+                /* Recurse into the entry */
+                H5F_efc_try_close_tag2(esf, tail);
+            } /* end if */
+        } /* end if */
+    } /* end for */
+
+    FUNC_LEAVE_NOAPI_VOID
+} /* end H5F_efc_try_close_tag2() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5F_efc_try_close
+ *
+ * Purpose:     Attempts to close the provided (shared) file by checking
+ *              to see if the releasing the EFC would cause its reference
+ *              count to drop to 0.  Necessary to handle the case where
+ *              chained EFCs form a cycle.  Note that this function does
+ *              not actually close the file (though it closes all children
+ *              as appropriate), as that is left up to the calling
+ *              function H5F_try_close().
+ *
+ *              Because H5F_try_close() has no way of telling if it is
+ *              called recursively from within this function, this
+ *              function serves as both the root of iteration and the
+ *              "callback" for the final pass (the one where the files are
+ *              actually closed).  The code for the callback case is at
+ *              the top of this function; luckily it only consists of a
+ *              (possible) call to H5F_efc_release().
+ *
+ *              The algorithm basically consists of 3 passes over the EFC
+ *              tree.  The first pass assumes that every reachable file is
+ *              closed, and keeps track of what the final reference count
+ *              would be for every reachable file.  The files are then
+ *              tagged as either closeable or uncloseable based on whether
+ *              this reference count drops to 0.
+ *
+ *              The second pass initiates a traversal from each file
+ *              marked as uncloseable in the first pass, and marks every
+ *              file reachable from the initial uncloseable file as
+ *              uncloseable.  This eliminates files that were marked as
+ *              closeable only because the first pass assumed that an
+ *              uncloseable file would be closed.
+ *
+ *              The final pass exploits the H5F_efc_release()->
+ *              H5F_efc_remove_ent()->H5F_try_close()->H5F_efc_try_close()
+ *              calling chain to recursively close the tree, but only the
+ *              files that are still marked as closeable.  All files
+ *              marked as closeable have their EFCs released, and will
+ *              eventually be closed when their last parent EFC is
+ *              released (the last part is guaranteed to be true by the
+ *              first 2 passes).
+ *
+ * Return:      Non-negative on success
+ *              Negative on failure
+ *
+ * Programmer:  Neil Fortner
+ *              Thursday, January 6, 2011
+ *
+ *-------------------------------------------------------------------------
+ */
+herr_t
+H5F_efc_try_close(H5F_t *f)
+{
+    H5F_file_t  *tail;                  /* Tail of linked list of found files.  Head will be f->shared. */
+    H5F_file_t  *uncloseable_head = NULL; /* Head of linked list of files found to be uncloseable by the first pass */
+    H5F_file_t  *uncloseable_tail = NULL; /* Tail of linked list of files found to be uncloseable by the first pass */
+    H5F_file_t  *sf;                    /* Temporary file pointer */
+    H5F_file_t  *next;                  /* Temporary file pointer */
+    herr_t      ret_value = SUCCEED;    /* Return value */
+
+    FUNC_ENTER_NOAPI_NOINIT(H5F_efc_try_close)
+
+    /* Sanity checks */
+    HDassert(f);
+    HDassert(f->shared);
+    HDassert(f->shared->efc);
+    HDassert(f->shared->nrefs > f->shared->efc->nrefs);
+    HDassert(f->shared->nrefs > 1);
+    HDassert(f->shared->efc->tag < 0);
+
+    if(f->shared->efc->tag == H5F_EFC_TAG_CLOSE) {
+        /* We must have reentered this function, and we should close this file.
+         * In actuality, we just release the EFC, the recursion should
+         * eventually reduce this file's reference count to 1 (though possibly
+         * not from this call to H5F_efc_release()). */
+        if(H5F_efc_release(f->shared->efc) < 0)
+            HGOTO_ERROR(H5E_FILE, H5E_CANTRELEASE, FAIL, "can't release external file cache")
+
+        /* If we marked the file as closeable, there must be no open files in
+         * its EFC.  This is because, in order to close an open child file, the
+         * client must keep a copy of the parent file open.  The algorithm
+         * detect that the parent file is open (directly or through an EFC) and
+         * refuse to close it.  Verify that all files were released from this
+         * EFC (i.e. none were open). */
+        HDassert(f->shared->efc->nfiles == 0);
+
+        HGOTO_DONE(SUCCEED)
+    } /* end if */
+
+    /* Conditions where we should not do anything and just return immediately */
+    /* If there are references that are not from an EFC or f, it will never
+     * be possible to close the file.  Just return.  Note that this holds true
+     * for the case that this file is being closed through H5F_efc_release()
+     * because that function (through H5F_efc_remove_ent()) decrements the EFC
+     * reference count before it calls H5F_try_close(). This may occur if this
+     * function is reentered. */
+    /* If the tag is H5F_EFC_TAG_DONTCLOSE, then we have definitely reentered
+     * this function, and this file has been marked as uncloseable, so we should
+     * not close/release it */
+    /* If nfiles is 0, then there is nothing to do.  Just return.  This may also
+     * occur on reentry (for example if this file was previously released). */
+    if((f->shared->nrefs != f->shared->efc->nrefs + 1)
+            || (f->shared->efc->tag == H5F_EFC_TAG_DONTCLOSE)
+            || (f->shared->efc->nfiles == 0))
+        /* We must have reentered this function, and we should not close this
+         * file.  Just return. */
+        HGOTO_DONE(SUCCEED)
+
+    /* If the file EFC were locked, that should always mean that there exists
+     * a reference to this file that is not in an EFC (it may have just been 
+     * removed from an EFC), and should have been caught by the above check */
+    /* If we get here then we must be beginning a new run.  Make sure that the
+     * temporary variables in f->shared->efc are at the default value */
+    HDassert(f->shared->efc->tag == H5F_EFC_TAG_DEFAULT);
+    HDassert(f->shared->efc->tmp_next == NULL);
+
+    /* Set up linked list for traversal into EFC tree.  f->shared is guaranteed
+     * to always be at the head. */
+    tail = f->shared;
+
+    /* Set up temporary reference count on root file */
+    f->shared->efc->tag = (int)f->shared->efc->nrefs;
+
+    /* First Pass: simulate closing all files reachable from this one, use "tag"
+     * field to keep track of final reference count for each file (including
+     * this one).  Keep list of files with starting reference count > 1 (head is
+     * f->shared). */
+    H5F_efc_try_close_tag1(f->shared, &tail);
+
+    /* Check if f->shared->efc->tag dropped to 0.  If it did not,
+     * we cannot close anything.  Just reset temporary values and return. */
+    if(f->shared->efc->tag > 0) {
+        sf = f->shared;
+        while(sf) {
+            next = sf->efc->tmp_next;
+            sf->efc->tag = H5F_EFC_TAG_DEFAULT;
+            sf->efc->tmp_next = NULL;
+            sf = next;
+        } /* end while */
+        HGOTO_DONE(SUCCEED)
+    } /* end if */
+
+    /* Run through the linked list , separating into two lists, one with tag ==
+     * 0 and one with tag > 0.  Mark them as either H5F_EFC_TAG_CLOSE or
+     * H5F_EFC_TAG_DONTCLOSE as appropriate. */
+    sf = f->shared;
+    tail = NULL;
+    while(sf) {
+        HDassert(sf->efc->tag >= 0);
+        next = sf->efc->tmp_next;
+        if(sf->efc->tag > 0) {
+            /* Remove from main list */
+            HDassert(tail);
+            tail->efc->tmp_next = sf->efc->tmp_next;
+            sf->efc->tmp_next = NULL;
+
+            /* Add to uncloseable list */
+            if(!uncloseable_head)
+                uncloseable_head = sf;
+            else
+                uncloseable_tail->efc->tmp_next = sf;
+            uncloseable_tail = sf;
+
+            /* Mark as uncloseable */
+            sf->efc->tag = H5F_EFC_TAG_DONTCLOSE;
+        } /* end if */
+        else {
+            sf->efc->tag = H5F_EFC_TAG_CLOSE;
+            tail = sf;
+        } /* end else */
+        sf = next;
+    } /* end while */
+
+    /* Second pass: Determine which of the reachable files found in pass 1
+     * cannot be closed by releasing the root file's EFC.  Run through the
+     * uncloseable list, for each item traverse the files reachable through the
+     * EFC, mark the file as uncloseable, and add it to the list of uncloseable
+     * files (for cleanup).  Use "tail" to store the original uncloseable tail
+     * so we know when to stop.  We do not need to keep track of the closeable
+     * list any more. */
+    sf = uncloseable_head;
+    if(sf) {
+        tail = uncloseable_tail;
+        HDassert(tail);
+        while(sf != tail->efc->tmp_next) {
+            H5F_efc_try_close_tag2(sf, &uncloseable_tail);
+            sf = sf->efc->tmp_next;
+        } /* end while */
+    } /* end if */
+
+    /* If the root file's tag is still H5F_EFC_TAG_CLOSE, release its EFC.  This
+     * should start the recursive release that should close all closeable files.
+     * Also, see the top of this function. */
+    if(f->shared->efc->tag == H5F_EFC_TAG_CLOSE) {
+        if(H5F_efc_release(f->shared->efc) < 0)
+            HGOTO_ERROR(H5E_FILE, H5E_CANTRELEASE, FAIL, "can't release external file cache")
+
+        /* Make sure the file's reference count is now 1 and will be closed by
+         * H5F_dest(). */
+        HDassert(f->shared->nrefs == 1);
+    } /* end if */
+
+    /* Clean up uncloseable files (reset tag and tmp_next).  All closeable files
+     * should have been closed, and therefore do not need to be cleaned up. */
+    if(uncloseable_head) {
+        sf = uncloseable_head;
+        while(sf) {
+            next = sf->efc->tmp_next;
+            HDassert(sf->efc->tag == H5F_EFC_TAG_DONTCLOSE);
+            sf->efc->tag = H5F_EFC_TAG_DEFAULT;
+            sf->efc->tmp_next = NULL;
+            sf = next;
+        } /* end while */
+    } /* end if */
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5F_efc_try_close() */
+
diff --git a/src/H5Fpkg.h b/src/H5Fpkg.h
index c98bda4..f63fa98 100644
--- a/src/H5Fpkg.h
+++ b/src/H5Fpkg.h
@@ -127,6 +127,10 @@
         + H5F_SUPERBLOCK_VARLEN_SIZE(v, f))
 
 
+/* Forward declaration external file cache struct used below (defined in
+ * H5Fefc.c) */
+typedef struct H5F_efc_t H5F_efc_t;
+
 /* Structure for metadata & "small [raw] data" block aggregation fields */
 struct H5F_blk_aggr_t {
     unsigned long       feature_flag;   /* Feature flag type */
@@ -200,6 +204,7 @@ typedef struct H5F_file_t {
     unsigned	nrefs;		/* Ref count for times file is opened	*/
     unsigned	flags;		/* Access Permissions for file          */
     H5F_mtab_t	mtab;		/* File mount table                     */
+    H5F_efc_t   *efc;           /* External file cache                  */
 
     /* Cached values from FCPL/superblock */
     uint8_t	sizeof_addr;	/* Size of addresses in file            */
@@ -328,6 +333,13 @@ H5_DLL herr_t H5F_sfile_add(H5F_file_t *shared);
 H5_DLL H5F_file_t * H5F_sfile_search(H5FD_t *lf);
 H5_DLL herr_t H5F_sfile_remove(H5F_file_t *shared);
 
+/* External file cache routines */
+H5_DLL H5F_efc_t *H5F_efc_create(unsigned max_nfiles);
+H5_DLL unsigned H5F_efc_max_nfiles(H5F_efc_t *efc);
+H5_DLL herr_t H5F_efc_release(H5F_efc_t *efc);
+H5_DLL herr_t H5F_efc_destroy(H5F_efc_t *efc);
+H5_DLL herr_t H5F_efc_try_close(H5F_t *f);
+
 /* Testing functions */
 #ifdef H5F_TESTING
 H5_DLL herr_t H5F_get_sohm_mesg_count_test(hid_t fid, unsigned type_id,
diff --git a/src/H5Fprivate.h b/src/H5Fprivate.h
index 10c2976..22d9acd 100644
--- a/src/H5Fprivate.h
+++ b/src/H5Fprivate.h
@@ -381,6 +381,7 @@ typedef struct H5F_blk_aggr_t H5F_blk_aggr_t;
 #define H5F_ACS_MULTI_TYPE_NAME                 "multi_type"    /* Data type in multi file driver */
 #define H5F_ACS_LATEST_FORMAT_NAME              "latest_format" /* 'Use latest format version' flag */
 #define H5F_ACS_WANT_POSIX_FD_NAME              "want_posix_fd" /* Internal: query the file descriptor from the core VFD, instead of the memory address */
+#define H5F_ACS_EFC_SIZE_NAME                   "efc_size"      /* Size of external file cache */
 
 /* ======================== File Mount properties ====================*/
 #define H5F_MNT_SYM_LOCAL_NAME 		"local"                 /* Whether absolute symlinks local to file. */
@@ -550,6 +551,11 @@ H5_DLL MPI_Comm H5F_mpi_get_comm(const H5F_t *f);
 H5_DLL int H5F_mpi_get_size(const H5F_t *f);
 #endif /* H5_HAVE_PARALLEL */
 
+/* External file cache routines */
+H5_DLL H5F_t *H5F_efc_open(H5F_t *parent, const char *name, unsigned flags,
+    hid_t fcpl_id, hid_t fapl_id, hid_t dxpl_id);
+H5_DLL herr_t H5F_efc_close(H5F_t *parent, H5F_t *file);
+
 /* Debugging functions */
 H5_DLL herr_t H5F_debug(H5F_t *f, FILE * stream, int indent, int fwidth);
 
diff --git a/src/H5Fpublic.h b/src/H5Fpublic.h
index 5ca64dc..d87dae6 100644
--- a/src/H5Fpublic.h
+++ b/src/H5Fpublic.h
@@ -198,6 +198,7 @@ H5_DLL ssize_t H5Fget_name(hid_t obj_id, char *name, size_t size);
 H5_DLL herr_t H5Fget_info2(hid_t obj_id, H5F_info2_t *finfo);
 H5_DLL ssize_t H5Fget_free_sections(hid_t file_id, H5F_mem_t type,
     size_t nsects, H5F_sect_info_t *sect_info/*out*/);
+H5_DLL herr_t H5Frelease_file_cache(hid_t file_id);
 
 /* Symbols defined for compatibility with previous versions of the HDF5 API.
  *
diff --git a/src/H5Lexternal.c b/src/H5Lexternal.c
index eb2ffe5..03f2e15 100644
--- a/src/H5Lexternal.c
+++ b/src/H5Lexternal.c
@@ -322,7 +322,7 @@ H5L_extern_traverse(const char UNUSED *link_name, hid_t cur_group,
     /* target file_name is an absolute pathname: see RM for detailed description */
     if(CHECK_ABSOLUTE(file_name) || CHECK_ABS_PATH(file_name)) {
         /* Try opening file */
-        if(NULL == (ext_file = H5F_open(file_name, intent, H5P_FILE_CREATE_DEFAULT, fapl_id, H5AC_dxpl_id))) {
+        if(NULL == (ext_file = H5F_efc_open(loc.oloc->file, file_name, intent, H5P_FILE_CREATE_DEFAULT, fapl_id, H5AC_dxpl_id))) {
             char *ptr = NULL;
 
             H5E_clear_stack(NULL);
@@ -334,7 +334,7 @@ H5L_extern_traverse(const char UNUSED *link_name, hid_t cur_group,
         } /* end if */
     } /* end if */
     else if(CHECK_ABS_DRIVE(file_name)) {
-        if(NULL == (ext_file = H5F_open(file_name, intent, H5P_FILE_CREATE_DEFAULT, fapl_id, H5AC_dxpl_id))) {
+        if(NULL == (ext_file = H5F_efc_open(loc.oloc->file, file_name, intent, H5P_FILE_CREATE_DEFAULT, fapl_id, H5AC_dxpl_id))) {
             H5E_clear_stack(NULL);
 	    /* strip "<drive-letter>:" */
 	    HDstrcpy(temp_file_name, &file_name[2]);
@@ -361,7 +361,7 @@ H5L_extern_traverse(const char UNUSED *link_name, hid_t cur_group,
                         HGOTO_ERROR(H5E_LINK, H5E_CANTGET, FAIL, "can't prepend prefix to filename")
 		    }
 
-                    ext_file = H5F_open(full_name, intent, H5P_FILE_CREATE_DEFAULT, fapl_id, H5AC_dxpl_id);
+                    ext_file = H5F_efc_open(loc.oloc->file, full_name, intent, H5P_FILE_CREATE_DEFAULT, fapl_id, H5AC_dxpl_id);
                     full_name = (char *)H5MM_xfree(full_name);
                     if(ext_file != NULL)
                         break;
@@ -379,7 +379,7 @@ H5L_extern_traverse(const char UNUSED *link_name, hid_t cur_group,
         if(my_prefix) {
             if(H5L_build_name(my_prefix, temp_file_name, &full_name/*out*/) < 0)
                 HGOTO_ERROR(H5E_LINK, H5E_CANTGET, FAIL, "can't prepend prefix to filename")
-            if(NULL == (ext_file = H5F_open(full_name, intent, H5P_FILE_CREATE_DEFAULT, fapl_id, H5AC_dxpl_id)))
+            if(NULL == (ext_file = H5F_efc_open(loc.oloc->file, full_name, intent, H5P_FILE_CREATE_DEFAULT, fapl_id, H5AC_dxpl_id)))
                 H5E_clear_stack(NULL);
             full_name = (char *)H5MM_xfree(full_name);
         } /* end if */
@@ -392,7 +392,7 @@ H5L_extern_traverse(const char UNUSED *link_name, hid_t cur_group,
         if(NULL != (extpath = H5F_EXTPATH(loc.oloc->file))) {
             if(H5L_build_name(extpath, temp_file_name, &full_name/*out*/) < 0)
                 HGOTO_ERROR(H5E_LINK, H5E_CANTGET, FAIL, "can't prepend prefix to filename")
-            if(NULL == (ext_file = H5F_open(full_name, intent, H5P_FILE_CREATE_DEFAULT, fapl_id, H5AC_dxpl_id)))
+            if(NULL == (ext_file = H5F_efc_open(loc.oloc->file, full_name, intent, H5P_FILE_CREATE_DEFAULT, fapl_id, H5AC_dxpl_id)))
                 H5E_clear_stack(NULL);
             full_name = (char *)H5MM_xfree(full_name);
         } /* end if */
@@ -400,7 +400,7 @@ H5L_extern_traverse(const char UNUSED *link_name, hid_t cur_group,
 
     /* try the relative file_name stored in temp_file_name */
     if(ext_file == NULL) {
-        if(NULL == (ext_file = H5F_open(temp_file_name, intent, H5P_FILE_CREATE_DEFAULT, fapl_id, H5AC_dxpl_id)))
+        if(NULL == (ext_file = H5F_efc_open(loc.oloc->file, temp_file_name, intent, H5P_FILE_CREATE_DEFAULT, fapl_id, H5AC_dxpl_id)))
             H5E_clear_stack(NULL);
     } /* end if */
 
@@ -427,32 +427,19 @@ H5L_extern_traverse(const char UNUSED *link_name, hid_t cur_group,
             HGOTO_ERROR(H5E_LINK, H5E_CANTGET, FAIL, "can't prepend prefix to filename")
 
         /* Try opening with the resolved name */
-        if(NULL == (ext_file = H5F_open(full_name, intent, H5P_FILE_CREATE_DEFAULT, fapl_id, H5AC_dxpl_id)))
+        if(NULL == (ext_file = H5F_efc_open(loc.oloc->file, full_name, intent, H5P_FILE_CREATE_DEFAULT, fapl_id, H5AC_dxpl_id)))
             HGOTO_ERROR(H5E_LINK, H5E_CANTOPENFILE, FAIL, "unable to open external file, external link file name = '%s', temp_file_name = '%s'", file_name, temp_file_name)
         full_name = (char *)H5MM_xfree(full_name);
     } /* end if */
 
 
-    /* Increment the number of open objects, to hold the file open */
-    H5F_incr_nopen_objs(ext_file);
-
     /* Retrieve the "group location" for the file's root group */
     if(H5G_loc_root(ext_file, &root_loc) < 0)
         HGOTO_ERROR(H5E_SYM, H5E_BADVALUE, FAIL, "unable to create location for file")
 
     /* Open the object referenced in the external file */
-    if((ext_obj = H5O_open_name(&root_loc, obj_name, lapl_id, FALSE)) < 0) {
-        H5F_decr_nopen_objs(ext_file);
+    if((ext_obj = H5O_open_name(&root_loc, obj_name, lapl_id, FALSE)) < 0)
         HGOTO_ERROR(H5E_SYM, H5E_CANTOPENOBJ, FAIL, "unable to open object")
-    } /* end if */
-
-    /* Decrement the number of open objects, to let the file close */
-    H5F_decr_nopen_objs(ext_file);
-
-    /* Close the external file */
-    if(H5F_try_close(ext_file) < 0)
-        HGOTO_ERROR(H5E_LINK, H5E_CANTCLOSEFILE, FAIL, "problem closing external file")
-    ext_file = NULL;
 
     /* Set return value */
     ret_value = ext_obj;
@@ -461,7 +448,7 @@ done:
     /* Release resources */
     if(fapl_id > 0 && H5I_dec_ref(fapl_id) < 0)
         HDONE_ERROR(H5E_ATOM, H5E_CANTRELEASE, FAIL, "unable to close atom for file access property list")
-    if(ext_file && H5F_try_close(ext_file) < 0)
+    if(ext_file && H5F_efc_close(loc.oloc->file, ext_file) < 0)
         HDONE_ERROR(H5E_LINK, H5E_CANTCLOSEFILE, FAIL, "problem closing external file")
     if(parent_group_name && parent_group_name != local_group_name)
         parent_group_name = (char *)H5MM_xfree(parent_group_name);
diff --git a/src/H5Pfapl.c b/src/H5Pfapl.c
index 10922da..7dea0b2 100644
--- a/src/H5Pfapl.c
+++ b/src/H5Pfapl.c
@@ -119,6 +119,9 @@
  */
 #define H5F_ACS_WANT_POSIX_FD_SIZE              sizeof(hbool_t)
 #define H5F_ACS_WANT_POSIX_FD_DEF               FALSE
+/* Definition for external file cache size */
+#define H5F_ACS_EFC_SIZE_SIZE                   sizeof(unsigned)
+#define H5F_ACS_EFC_SIZE_DEF                    0
 
 
 /******************/
@@ -211,6 +214,7 @@ H5P_facc_reg_prop(H5P_genclass_t *pclass)
     H5FD_mem_t mem_type = H5F_ACS_MULTI_TYPE_DEF;               /* Default file space type for multi VFD */
     hbool_t latest_format = H5F_ACS_LATEST_FORMAT_DEF;          /* Default setting for "use the latest version of the format" flag */
     hbool_t want_posix_fd = H5F_ACS_WANT_POSIX_FD_DEF;          /* Default setting for retrieving 'handle' from core VFD */
+    unsigned efc_size = H5F_ACS_EFC_SIZE_DEF;                   /* Default external file cache size */
     herr_t ret_value = SUCCEED;         /* Return value */
 
     FUNC_ENTER_NOAPI_NOINIT(H5P_facc_reg_prop)
@@ -292,6 +296,10 @@ H5P_facc_reg_prop(H5P_genclass_t *pclass)
     if(H5P_register_real(pclass, H5F_ACS_WANT_POSIX_FD_NAME, H5F_ACS_WANT_POSIX_FD_SIZE, &want_posix_fd, NULL, NULL, NULL, NULL, NULL, NULL, NULL) < 0)
          HGOTO_ERROR(H5E_PLIST, H5E_CANTINSERT, FAIL, "can't insert property into class")
 
+    /* Register the external file cache size */
+    if(H5P_register_real(pclass, H5F_ACS_EFC_SIZE_NAME, H5F_ACS_EFC_SIZE_SIZE, &efc_size, NULL, NULL, NULL, NULL, NULL, NULL, NULL) < 0)
+         HGOTO_ERROR(H5E_PLIST, H5E_CANTINSERT, FAIL, "can't insert property into class")
+
 done:
     FUNC_LEAVE_NOAPI(ret_value)
 } /* end H5P_facc_reg_prop() */
@@ -2016,3 +2024,80 @@ done:
     FUNC_LEAVE_API(ret_value)
 } /* end H5Pget_libver_bounds() */
 
+
+/*-------------------------------------------------------------------------
+ * Function:    H5Pset_elink_file_cache_size
+ *
+ * Purpose:     Sets the number of files opened through external links
+ *              from the file associated with this fapl to be held open
+ *              in that file's external file cache.  When the maximum
+ *              number of files is reached, the least recently used file
+ *              is closed (unless it is opened from somewhere else).
+ *
+ * Return:      Non-negative on success/Negative on failure
+ *
+ * Programmer:  Neil Fortner
+ *              Friday, December 17, 2010
+ *
+ *-------------------------------------------------------------------------
+ */
+herr_t
+H5Pset_elink_file_cache_size(hid_t plist_id, unsigned efc_size)
+{
+    H5P_genplist_t *plist;      /* Property list pointer */
+    herr_t ret_value = SUCCEED;   /* return value */
+
+    FUNC_ENTER_API(H5Pset_elink_file_cache_size, FAIL)
+    H5TRACE2("e", "iIu", plist_id, efc_size);
+
+    /* Get the plist structure */
+    if(NULL == (plist = H5P_object_verify(plist_id, H5P_FILE_ACCESS)))
+        HGOTO_ERROR(H5E_ATOM, H5E_BADATOM, FAIL, "can't find object for ID")
+
+    /* Set value */
+    if(H5P_set(plist, H5F_ACS_EFC_SIZE_NAME, &efc_size) < 0)
+        HGOTO_ERROR(H5E_PLIST, H5E_CANTSET, FAIL, "can't set elink file cache size")
+
+done:
+    FUNC_LEAVE_API(ret_value)
+} /* end H5Pset_elink_file_cache_size() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5Pget_elink_file_cache_size
+ *
+ * Purpose:     Gets the number of files opened through external links
+ *              from the file associated with this fapl to be held open
+ *              in that file's external file cache.  When the maximum
+ *              number of files is reached, the least recently used file
+ *              is closed (unless it is opened from somewhere else).
+ *
+ * Return:      Non-negative on success/Negative on failure
+ *
+ * Programmer:  Neil Fortner
+ *              Friday, December 17, 2010
+ *
+ *-------------------------------------------------------------------------
+ */
+herr_t
+H5Pget_elink_file_cache_size(hid_t plist_id, unsigned *efc_size)
+{
+    H5P_genplist_t *plist;      /* Property list pointer */
+    herr_t ret_value = SUCCEED;   /* return value */
+
+    FUNC_ENTER_API(H5Pget_elink_file_cache_size, FAIL)
+    H5TRACE2("e", "i*Iu", plist_id, efc_size);
+
+    /* Get the plist structure */
+    if(NULL == (plist = H5P_object_verify(plist_id, H5P_FILE_ACCESS)))
+        HGOTO_ERROR(H5E_ATOM, H5E_BADATOM, FAIL, "can't find object for ID")
+
+    /* Get value */
+    if(efc_size)
+        if(H5P_get(plist, H5F_ACS_EFC_SIZE_NAME, efc_size) < 0)
+            HGOTO_ERROR(H5E_PLIST, H5E_CANTGET, FAIL, "can't get elink file cache size")
+
+done:
+    FUNC_LEAVE_API(ret_value)
+} /* end H5Pget_elink_file_cache_size() */
+
diff --git a/src/H5Ppublic.h b/src/H5Ppublic.h
index 09eb796..6867b63 100644
--- a/src/H5Ppublic.h
+++ b/src/H5Ppublic.h
@@ -287,6 +287,8 @@ H5_DLL herr_t H5Pset_libver_bounds(hid_t plist_id, H5F_libver_t low,
     H5F_libver_t high);
 H5_DLL herr_t H5Pget_libver_bounds(hid_t plist_id, H5F_libver_t *low,
     H5F_libver_t *high);
+H5_DLL herr_t H5Pset_elink_file_cache_size(hid_t plist_id, unsigned efc_size);
+H5_DLL herr_t H5Pget_elink_file_cache_size(hid_t plist_id, unsigned *efc_size);
 
 /* Dataset creation property list (DCPL) routines */
 H5_DLL herr_t H5Pset_layout(hid_t plist_id, H5D_layout_t layout);
diff --git a/src/Makefile.am b/src/Makefile.am
index dc0edb3..ee0a3f5 100755
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -53,8 +53,8 @@ libhdf5_la_SOURCES= H5.c H5checksum.c H5dbg.c H5system.c H5timer.c H5trace.c \
         H5E.c H5Edeprec.c H5Eint.c \
         H5EA.c H5EAcache.c H5EAdbg.c H5EAdblkpage.c H5EAdblock.c H5EAhdr.c \
         H5EAiblock.c H5EAint.c H5EAsblock.c H5EAstat.c H5EAtest.c \
-        H5F.c H5Faccum.c H5Fdbg.c H5Fdeprec.c H5Ffake.c H5Fio.c H5Fmount.c \
-        H5Fmpi.c H5Fquery.c \
+        H5F.c H5Faccum.c H5Fdbg.c H5Fdeprec.c H5Fefc.c H5Ffake.c H5Fio.c \
+        H5Fmount.c H5Fmpi.c H5Fquery.c \
         H5Fsfile.c H5Fsuper.c H5Fsuper_cache.c H5Ftest.c \
         H5FA.c H5FAcache.c H5FAdbg.c H5FAdblock.c H5FAdblkpage.c H5FAhdr.c \
         H5FAstat.c H5FAtest.c \
diff --git a/src/Makefile.in b/src/Makefile.in
index 55e52ae..51003a0 100644
--- a/src/Makefile.in
+++ b/src/Makefile.in
@@ -106,19 +106,19 @@ am_libhdf5_la_OBJECTS = H5.lo H5checksum.lo H5dbg.lo H5system.lo \
 	H5Dtest.lo H5E.lo H5Edeprec.lo H5Eint.lo H5EA.lo H5EAcache.lo \
 	H5EAdbg.lo H5EAdblkpage.lo H5EAdblock.lo H5EAhdr.lo \
 	H5EAiblock.lo H5EAint.lo H5EAsblock.lo H5EAstat.lo H5EAtest.lo \
-	H5F.lo H5Faccum.lo H5Fdbg.lo H5Fdeprec.lo H5Ffake.lo H5Fio.lo \
-	H5Fmount.lo H5Fmpi.lo H5Fquery.lo H5Fsfile.lo H5Fsuper.lo \
-	H5Fsuper_cache.lo H5Ftest.lo H5FA.lo H5FAcache.lo H5FAdbg.lo \
-	H5FAdblock.lo H5FAdblkpage.lo H5FAhdr.lo H5FAstat.lo \
-	H5FAtest.lo H5FD.lo H5FDcore.lo H5FDdirect.lo H5FDfamily.lo \
-	H5FDint.lo H5FDlog.lo H5FDmpi.lo H5FDmpio.lo H5FDmpiposix.lo \
-	H5FDmulti.lo H5FDsec2.lo H5FDspace.lo H5FDstdio.lo H5FL.lo \
-	H5FO.lo H5FS.lo H5FScache.lo H5FSdbg.lo H5FSsection.lo \
-	H5FSstat.lo H5FStest.lo H5G.lo H5Gbtree2.lo H5Gcache.lo \
-	H5Gcompact.lo H5Gdense.lo H5Gdeprec.lo H5Gent.lo H5Gint.lo \
-	H5Glink.lo H5Gloc.lo H5Gname.lo H5Gnode.lo H5Gobj.lo H5Goh.lo \
-	H5Groot.lo H5Gstab.lo H5Gtest.lo H5Gtraverse.lo H5HF.lo \
-	H5HFbtree2.lo H5HFcache.lo H5HFdbg.lo H5HFdblock.lo \
+	H5F.lo H5Faccum.lo H5Fdbg.lo H5Fdeprec.lo H5Fefc.lo H5Ffake.lo \
+	H5Fio.lo H5Fmount.lo H5Fmpi.lo H5Fquery.lo H5Fsfile.lo \
+	H5Fsuper.lo H5Fsuper_cache.lo H5Ftest.lo H5FA.lo H5FAcache.lo \
+	H5FAdbg.lo H5FAdblock.lo H5FAdblkpage.lo H5FAhdr.lo \
+	H5FAstat.lo H5FAtest.lo H5FD.lo H5FDcore.lo H5FDdirect.lo \
+	H5FDfamily.lo H5FDint.lo H5FDlog.lo H5FDmpi.lo H5FDmpio.lo \
+	H5FDmpiposix.lo H5FDmulti.lo H5FDsec2.lo H5FDspace.lo \
+	H5FDstdio.lo H5FL.lo H5FO.lo H5FS.lo H5FScache.lo H5FSdbg.lo \
+	H5FSsection.lo H5FSstat.lo H5FStest.lo H5G.lo H5Gbtree2.lo \
+	H5Gcache.lo H5Gcompact.lo H5Gdense.lo H5Gdeprec.lo H5Gent.lo \
+	H5Gint.lo H5Glink.lo H5Gloc.lo H5Gname.lo H5Gnode.lo H5Gobj.lo \
+	H5Goh.lo H5Groot.lo H5Gstab.lo H5Gtest.lo H5Gtraverse.lo \
+	H5HF.lo H5HFbtree2.lo H5HFcache.lo H5HFdbg.lo H5HFdblock.lo \
 	H5HFdtable.lo H5HFhdr.lo H5HFhuge.lo H5HFiblock.lo H5HFiter.lo \
 	H5HFman.lo H5HFsection.lo H5HFspace.lo H5HFstat.lo H5HFtest.lo \
 	H5HFtiny.lo H5HG.lo H5HGcache.lo H5HGdbg.lo H5HL.lo \
@@ -486,8 +486,8 @@ libhdf5_la_SOURCES = H5.c H5checksum.c H5dbg.c H5system.c H5timer.c H5trace.c \
         H5E.c H5Edeprec.c H5Eint.c \
         H5EA.c H5EAcache.c H5EAdbg.c H5EAdblkpage.c H5EAdblock.c H5EAhdr.c \
         H5EAiblock.c H5EAint.c H5EAsblock.c H5EAstat.c H5EAtest.c \
-        H5F.c H5Faccum.c H5Fdbg.c H5Fdeprec.c H5Ffake.c H5Fio.c H5Fmount.c \
-        H5Fmpi.c H5Fquery.c \
+        H5F.c H5Faccum.c H5Fdbg.c H5Fdeprec.c H5Fefc.c H5Ffake.c H5Fio.c \
+        H5Fmount.c H5Fmpi.c H5Fquery.c \
         H5Fsfile.c H5Fsuper.c H5Fsuper_cache.c H5Ftest.c \
         H5FA.c H5FAcache.c H5FAdbg.c H5FAdblock.c H5FAdblkpage.c H5FAhdr.c \
         H5FAstat.c H5FAtest.c \
@@ -764,6 +764,7 @@ distclean-compile:
 @AMDEP_TRUE@@am__include@ @am__quote@./$(DEPDIR)/H5Faccum.Plo@am__quote@
 @AMDEP_TRUE@@am__include@ @am__quote@./$(DEPDIR)/H5Fdbg.Plo@am__quote@
 @AMDEP_TRUE@@am__include@ @am__quote@./$(DEPDIR)/H5Fdeprec.Plo@am__quote@
+@AMDEP_TRUE@@am__include@ @am__quote@./$(DEPDIR)/H5Fefc.Plo@am__quote@
 @AMDEP_TRUE@@am__include@ @am__quote@./$(DEPDIR)/H5Ffake.Plo@am__quote@
 @AMDEP_TRUE@@am__include@ @am__quote@./$(DEPDIR)/H5Fio.Plo@am__quote@
 @AMDEP_TRUE@@am__include@ @am__quote@./$(DEPDIR)/H5Fmount.Plo@am__quote@