subversion-1.9.7HEADsubversion-1.9.7master

1 files changed, 2324 insertions, 0 deletions

diff --git a/subversion/libsvn_fs_x/pack.c b/subversion/libsvn_fs_x/pack.c
new file mode 100644
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--- /dev/null
+++ b/subversion/libsvn_fs_x/pack.c
@@ -0,0 +1,2324 @@
+/* pack.c --- FSX shard packing functionality
+ *
+ * ====================================================================
+ *    Licensed to the Apache Software Foundation (ASF) under one
+ *    or more contributor license agreements.  See the NOTICE file
+ *    distributed with this work for additional information
+ *    regarding copyright ownership.  The ASF licenses this file
+ *    to you under the Apache License, Version 2.0 (the
+ *    "License"); you may not use this file except in compliance
+ *    with the License.  You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *    Unless required by applicable law or agreed to in writing,
+ *    software distributed under the License is distributed on an
+ *    "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ *    KIND, either express or implied.  See the License for the
+ *    specific language governing permissions and limitations
+ *    under the License.
+ * ====================================================================
+ */
+#include <assert.h>
+
+#include "svn_pools.h"
+#include "svn_dirent_uri.h"
+#include "svn_sorts.h"
+#include "private/svn_sorts_private.h"
+#include "private/svn_subr_private.h"
+#include "private/svn_string_private.h"
+#include "private/svn_temp_serializer.h"
+
+#include "fs_x.h"
+#include "pack.h"
+#include "util.h"
+#include "revprops.h"
+#include "transaction.h"
+#include "index.h"
+#include "low_level.h"
+#include "cached_data.h"
+#include "changes.h"
+#include "noderevs.h"
+#include "reps.h"
+
+#include "../libsvn_fs/fs-loader.h"
+
+#include "svn_private_config.h"
+#include "temp_serializer.h"
+
+/* Packing logic:
+ *
+ * We pack files on a pack file basis (e.g. 1000 revs) without changing
+ * existing pack files nor the revision files outside the range to pack.
+ *
+ * First, we will scan the revision file indexes to determine the number
+ * of items to "place" (i.e. determine their optimal position within the
+ * future pack file).  For each item, we will need a constant amount of
+ * memory to track it.  A MAX_MEM parameter sets a limit to the number of
+ * items we may place in one go.  That means, we may not be able to add
+ * all revisions at once.  Instead, we will run the placement for a subset
+ * of revisions at a time.  The very unlikely worst case will simply append
+ * all revision data with just a little reshuffling inside each revision.
+ *
+ * In a second step, we read all revisions in the selected range, build
+ * the item tracking information and copy the items themselves from the
+ * revision files to temporary files.  The latter serve as buckets for a
+ * very coarse bucket presort:  Separate change lists, file properties,
+ * directory properties and noderevs + representations from one another.
+ *
+ * The third step will determine an optimized placement for the items in
+ * each of the 4 buckets separately.  The first three will simply order
+ * their items by revision, starting with the newest once.  Placing rep
+ * and noderev items is a more elaborate process documented in the code.
+ *
+ * In short, we store items in the following order:
+ * - changed paths lists
+ * - node property
+ * - directory properties
+ * - directory representations corresponding noderevs, lexical path order
+ *   with special treatment of "trunk" and "branches"
+ * - same for file representations
+ *
+ * Step 4 copies the items from the temporary buckets into the final
+ * pack file and writes the temporary index files.
+ *
+ * Finally, after the last range of revisions, create the final indexes.
+ */
+
+/* Maximum amount of memory we allocate for placement information during
+ * the pack process.
+ */
+#define DEFAULT_MAX_MEM (64 * 1024 * 1024)
+
+/* Data structure describing a node change at PATH, REVISION.
+ * We will sort these instances by PATH and NODE_ID such that we can combine
+ * similar nodes in the same reps container and store containers in path
+ * major order.
+ */
+typedef struct path_order_t
+{
+  /* changed path */
+  svn_prefix_string__t *path;
+
+  /* node ID for this PATH in REVISION */
+  svn_fs_x__id_t node_id;
+
+  /* when this change happened */
+  svn_revnum_t revision;
+
+  /* this is a directory node */
+  svn_boolean_t is_dir;
+
+  /* length of the expanded representation content */
+  apr_int64_t expanded_size;
+
+  /* item ID of the noderev linked to the change. May be (0, 0). */
+  svn_fs_x__id_t noderev_id;
+
+  /* item ID of the representation containing the new data. May be (0, 0). */
+  svn_fs_x__id_t rep_id;
+} path_order_t;
+
+/* Represents a reference from item FROM to item TO.  FROM may be a noderev
+ * or rep_id while TO is (currently) always a representation.  We will sort
+ * them by TO which allows us to collect all dependent items.
+ */
+typedef struct reference_t
+{
+  svn_fs_x__id_t to;
+  svn_fs_x__id_t from;
+} reference_t;
+
+/* This structure keeps track of all the temporary data and status that
+ * needs to be kept around during the creation of one pack file.  After
+ * each revision range (in case we can't process all revs at once due to
+ * memory restrictions), parts of the data will get re-initialized.
+ */
+typedef struct pack_context_t
+{
+  /* file system that we operate on */
+  svn_fs_t *fs;
+
+  /* cancel function to invoke at regular intervals. May be NULL */
+  svn_cancel_func_t cancel_func;
+
+  /* baton to pass to CANCEL_FUNC */
+  void *cancel_baton;
+
+  /* first revision in the shard (and future pack file) */
+  svn_revnum_t shard_rev;
+
+  /* first revision in the range to process (>= SHARD_REV) */
+  svn_revnum_t start_rev;
+
+  /* first revision after the range to process (<= SHARD_END_REV) */
+  svn_revnum_t end_rev;
+
+  /* first revision after the current shard */
+  svn_revnum_t shard_end_rev;
+
+  /* log-to-phys proto index for the whole pack file */
+  apr_file_t *proto_l2p_index;
+
+  /* phys-to-log proto index for the whole pack file */
+  apr_file_t *proto_p2l_index;
+
+  /* full shard directory path (containing the unpacked revisions) */
+  const char *shard_dir;
+
+  /* full packed shard directory path (containing the pack file + indexes) */
+  const char *pack_file_dir;
+
+  /* full pack file path (including PACK_FILE_DIR) */
+  const char *pack_file_path;
+
+  /* current write position (i.e. file length) in the pack file */
+  apr_off_t pack_offset;
+
+  /* the pack file to ultimately write all data to */
+  apr_file_t *pack_file;
+
+  /* array of svn_fs_x__p2l_entry_t *, all referring to change lists.
+   * Will be filled in phase 2 and be cleared after each revision range. */
+  apr_array_header_t *changes;
+
+  /* temp file receiving all change list items (referenced by CHANGES).
+   * Will be filled in phase 2 and be cleared after each revision range. */
+  apr_file_t *changes_file;
+
+  /* array of svn_fs_x__p2l_entry_t *, all referring to file properties.
+   * Will be filled in phase 2 and be cleared after each revision range. */
+  apr_array_header_t *file_props;
+
+  /* temp file receiving all file prop items (referenced by FILE_PROPS).
+   * Will be filled in phase 2 and be cleared after each revision range.*/
+  apr_file_t *file_props_file;
+
+  /* array of svn_fs_x__p2l_entry_t *, all referring to directory properties.
+   * Will be filled in phase 2 and be cleared after each revision range. */
+  apr_array_header_t *dir_props;
+
+  /* temp file receiving all directory prop items (referenced by DIR_PROPS).
+   * Will be filled in phase 2 and be cleared after each revision range.*/
+  apr_file_t *dir_props_file;
+
+  /* container for all PATH members in PATH_ORDER. */
+  svn_prefix_tree__t *paths;
+
+  /* array of path_order_t *.  Will be filled in phase 2 and be cleared
+   * after each revision range.  Sorted by PATH, NODE_ID. */
+  apr_array_header_t *path_order;
+
+  /* array of reference_t* linking representations to their delta bases.
+   * Will be filled in phase 2 and be cleared after each revision range.
+   * It will be sorted by the FROM members (for rep->base rep lookup). */
+  apr_array_header_t *references;
+
+  /* array of svn_fs_x__p2l_entry_t*.  Will be filled in phase 2 and be
+   * cleared after each revision range.  During phase 3, we will set items
+   * to NULL that we already processed. */
+  apr_array_header_t *reps;
+
+  /* array of int, marking for each revision, the which offset their items
+   * begin in REPS.  Will be filled in phase 2 and be cleared after
+   * each revision range. */
+  apr_array_header_t *rev_offsets;
+
+  /* temp file receiving all items referenced by REPS.
+   * Will be filled in phase 2 and be cleared after each revision range.*/
+  apr_file_t *reps_file;
+
+  /* pool used for temporary data structures that will be cleaned up when
+   * the next range of revisions is being processed */
+  apr_pool_t *info_pool;
+} pack_context_t;
+
+/* Create and initialize a new pack context for packing shard SHARD_REV in
+ * SHARD_DIR into PACK_FILE_DIR within filesystem FS.  Allocate it in POOL
+ * and return the structure in *CONTEXT.
+ *
+ * Limit the number of items being copied per iteration to MAX_ITEMS.
+ * Set CANCEL_FUNC and CANCEL_BATON as well.
+ */
+static svn_error_t *
+initialize_pack_context(pack_context_t *context,
+                        svn_fs_t *fs,
+                        const char *pack_file_dir,
+                        const char *shard_dir,
+                        svn_revnum_t shard_rev,
+                        int max_items,
+                        svn_cancel_func_t cancel_func,
+                        void *cancel_baton,
+                        apr_pool_t *pool)
+{
+  svn_fs_x__data_t *ffd = fs->fsap_data;
+  const char *temp_dir;
+  int max_revs = MIN(ffd->max_files_per_dir, max_items);
+
+  SVN_ERR_ASSERT(shard_rev % ffd->max_files_per_dir == 0);
+
+  /* where we will place our various temp files */
+  SVN_ERR(svn_io_temp_dir(&temp_dir, pool));
+
+  /* store parameters */
+  context->fs = fs;
+  context->cancel_func = cancel_func;
+  context->cancel_baton = cancel_baton;
+
+  context->shard_rev = shard_rev;
+  context->start_rev = shard_rev;
+  context->end_rev = shard_rev;
+  context->shard_end_rev = shard_rev + ffd->max_files_per_dir;
+
+  /* Create the new directory and pack file. */
+  context->shard_dir = shard_dir;
+  context->pack_file_dir = pack_file_dir;
+  context->pack_file_path
+    = svn_dirent_join(pack_file_dir, PATH_PACKED, pool);
+  SVN_ERR(svn_io_file_open(&context->pack_file, context->pack_file_path,
+                           APR_WRITE | APR_BUFFERED | APR_BINARY | APR_EXCL
+                             | APR_CREATE, APR_OS_DEFAULT, pool));
+
+  /* Proto index files */
+  SVN_ERR(svn_fs_x__l2p_proto_index_open(
+             &context->proto_l2p_index,
+             svn_dirent_join(pack_file_dir,
+                             PATH_INDEX PATH_EXT_L2P_INDEX,
+                             pool),
+             pool));
+  SVN_ERR(svn_fs_x__p2l_proto_index_open(
+             &context->proto_p2l_index,
+             svn_dirent_join(pack_file_dir,
+                             PATH_INDEX PATH_EXT_P2L_INDEX,
+                             pool),
+             pool));
+
+  /* item buckets: one item info array and one temp file per bucket */
+  context->changes = apr_array_make(pool, max_items,
+                                    sizeof(svn_fs_x__p2l_entry_t *));
+  SVN_ERR(svn_io_open_unique_file3(&context->changes_file, NULL, temp_dir,
+                                   svn_io_file_del_on_close, pool, pool));
+  context->file_props = apr_array_make(pool, max_items,
+                                       sizeof(svn_fs_x__p2l_entry_t *));
+  SVN_ERR(svn_io_open_unique_file3(&context->file_props_file, NULL, temp_dir,
+                                   svn_io_file_del_on_close, pool, pool));
+  context->dir_props = apr_array_make(pool, max_items,
+                                      sizeof(svn_fs_x__p2l_entry_t *));
+  SVN_ERR(svn_io_open_unique_file3(&context->dir_props_file, NULL, temp_dir,
+                                   svn_io_file_del_on_close, pool, pool));
+
+  /* noderev and representation item bucket */
+  context->rev_offsets = apr_array_make(pool, max_revs, sizeof(int));
+  context->path_order = apr_array_make(pool, max_items,
+                                       sizeof(path_order_t *));
+  context->references = apr_array_make(pool, max_items,
+                                       sizeof(reference_t *));
+  context->reps = apr_array_make(pool, max_items,
+                                 sizeof(svn_fs_x__p2l_entry_t *));
+  SVN_ERR(svn_io_open_unique_file3(&context->reps_file, NULL, temp_dir,
+                                   svn_io_file_del_on_close, pool, pool));
+
+  /* the pool used for temp structures */
+  context->info_pool = svn_pool_create(pool);
+  context->paths = svn_prefix_tree__create(context->info_pool);
+
+  return SVN_NO_ERROR;
+}
+
+/* Clean up / free all revision range specific data and files in CONTEXT.
+ * Use SCRATCH_POOL for temporary allocations.
+ */
+static svn_error_t *
+reset_pack_context(pack_context_t *context,
+                   apr_pool_t *scratch_pool)
+{
+  apr_array_clear(context->changes);
+  SVN_ERR(svn_io_file_trunc(context->changes_file, 0, scratch_pool));
+  apr_array_clear(context->file_props);
+  SVN_ERR(svn_io_file_trunc(context->file_props_file, 0, scratch_pool));
+  apr_array_clear(context->dir_props);
+  SVN_ERR(svn_io_file_trunc(context->dir_props_file, 0, scratch_pool));
+
+  apr_array_clear(context->rev_offsets);
+  apr_array_clear(context->path_order);
+  apr_array_clear(context->references);
+  apr_array_clear(context->reps);
+  SVN_ERR(svn_io_file_trunc(context->reps_file, 0, scratch_pool));
+
+  svn_pool_clear(context->info_pool);
+
+  return SVN_NO_ERROR;
+}
+
+/* Call this after the last revision range.  It will finalize all index files
+ * for CONTEXT and close any open files.
+ * Use SCRATCH_POOL for temporary allocations.
+ */
+static svn_error_t *
+close_pack_context(pack_context_t *context,
+                   apr_pool_t *scratch_pool)
+{
+  const char *proto_l2p_index_path;
+  const char *proto_p2l_index_path;
+
+  /* need the file names for the actual index creation call further down */
+  SVN_ERR(svn_io_file_name_get(&proto_l2p_index_path,
+                               context->proto_l2p_index, scratch_pool));
+  SVN_ERR(svn_io_file_name_get(&proto_p2l_index_path,
+                               context->proto_p2l_index, scratch_pool));
+
+  /* finalize proto index files */
+  SVN_ERR(svn_io_file_close(context->proto_l2p_index, scratch_pool));
+  SVN_ERR(svn_io_file_close(context->proto_p2l_index, scratch_pool));
+
+  /* Append the actual index data to the pack file. */
+  SVN_ERR(svn_fs_x__add_index_data(context->fs, context->pack_file,
+                                    proto_l2p_index_path,
+                                    proto_p2l_index_path,
+                                    context->shard_rev,
+                                    scratch_pool));
+
+  /* remove proto index files */
+  SVN_ERR(svn_io_remove_file2(proto_l2p_index_path, FALSE, scratch_pool));
+  SVN_ERR(svn_io_remove_file2(proto_p2l_index_path, FALSE, scratch_pool));
+
+  SVN_ERR(svn_io_file_close(context->pack_file, scratch_pool));
+
+  return SVN_NO_ERROR;
+}
+
+/* Efficiently copy SIZE bytes from SOURCE to DEST.  Invoke the CANCEL_FUNC
+ * from CONTEXT at regular intervals.
+ * Use SCRATCH_POOL for temporary allocations.
+ */
+static svn_error_t *
+copy_file_data(pack_context_t *context,
+               apr_file_t *dest,
+               apr_file_t *source,
+               apr_off_t size,
+               apr_pool_t *scratch_pool)
+{
+  /* most non-representation items will be small.  Minimize the buffer
+   * and infrastructure overhead in that case. */
+  enum { STACK_BUFFER_SIZE = 1024 };
+
+  if (size < STACK_BUFFER_SIZE)
+    {
+      /* copy small data using a fixed-size buffer on stack */
+      char buffer[STACK_BUFFER_SIZE];
+      SVN_ERR(svn_io_file_read_full2(source, buffer, (apr_size_t)size,
+                                     NULL, NULL, scratch_pool));
+      SVN_ERR(svn_io_file_write_full(dest, buffer, (apr_size_t)size,
+                                     NULL, scratch_pool));
+    }
+  else
+    {
+      /* use streaming copies for larger data blocks.  That may require
+       * the allocation of larger buffers and we should make sure that
+       * this extra memory is released asap. */
+      svn_fs_x__data_t *ffd = context->fs->fsap_data;
+      apr_pool_t *copypool = svn_pool_create(scratch_pool);
+      char *buffer = apr_palloc(copypool, ffd->block_size);
+
+      while (size)
+        {
+          apr_size_t to_copy = (apr_size_t)(MIN(size, ffd->block_size));
+          if (context->cancel_func)
+            SVN_ERR(context->cancel_func(context->cancel_baton));
+
+          SVN_ERR(svn_io_file_read_full2(source, buffer, to_copy,
+                                         NULL, NULL, scratch_pool));
+          SVN_ERR(svn_io_file_write_full(dest, buffer, to_copy,
+                                         NULL, scratch_pool));
+
+          size -= to_copy;
+        }
+
+      svn_pool_destroy(copypool);
+    }
+
+  return SVN_NO_ERROR;
+}
+
+/* Writes SIZE bytes, all 0, to DEST.
+ * Use SCRATCH_POOL for temporary allocations.
+ */
+static svn_error_t *
+write_null_bytes(apr_file_t *dest,
+                 apr_off_t size,
+                 apr_pool_t *scratch_pool)
+{
+  /* Have a collection of high-quality, easy to access NUL bytes handy. */
+  enum { BUFFER_SIZE = 1024 };
+  static const char buffer[BUFFER_SIZE] = { 0 };
+
+  /* copy SIZE of them into the file's buffer */
+  while (size)
+    {
+      apr_size_t to_write = MIN(size, BUFFER_SIZE);
+      SVN_ERR(svn_io_file_write_full(dest, buffer, to_write, NULL,
+                                     scratch_pool));
+      size -= to_write;
+    }
+
+  return SVN_NO_ERROR;
+}
+
+/* Copy the "simple" item (changed paths list or property representation)
+ * from the current position in REV_FILE to TEMP_FILE using CONTEXT.  Add
+ * a copy of ENTRY to ENTRIES but with an updated offset value that points
+ * to the copy destination in TEMP_FILE.
+ * Use SCRATCH_POOL for temporary allocations.
+ */
+static svn_error_t *
+copy_item_to_temp(pack_context_t *context,
+                  apr_array_header_t *entries,
+                  apr_file_t *temp_file,
+                  svn_fs_x__revision_file_t *rev_file,
+                  svn_fs_x__p2l_entry_t *entry,
+                  apr_pool_t *scratch_pool)
+{
+  svn_fs_x__p2l_entry_t *new_entry
+    = svn_fs_x__p2l_entry_dup(entry, context->info_pool);
+
+  SVN_ERR(svn_fs_x__get_file_offset(&new_entry->offset, temp_file,
+                                    scratch_pool));
+  APR_ARRAY_PUSH(entries, svn_fs_x__p2l_entry_t *) = new_entry;
+
+  SVN_ERR(copy_file_data(context, temp_file, rev_file->file, entry->size,
+                         scratch_pool));
+
+  return SVN_NO_ERROR;
+}
+
+/* Return the offset within CONTEXT->REPS that corresponds to item
+ * ITEM_INDEX in  REVISION.
+ */
+static int
+get_item_array_index(pack_context_t *context,
+                     svn_revnum_t revision,
+                     apr_int64_t item_index)
+{
+  assert(revision >= context->start_rev);
+  return (int)item_index + APR_ARRAY_IDX(context->rev_offsets,
+                                         revision - context->start_rev,
+                                         int);
+}
+
+/* Write INFO to the correct position in CONTEXT->REPS.  The latter may
+ * need auto-expanding.  Overwriting an array element is not allowed.
+ */
+static void
+add_item_rep_mapping(pack_context_t *context,
+                     svn_fs_x__p2l_entry_t *entry)
+{
+  int idx;
+  assert(entry->item_count == 1);
+
+  /* index of INFO */
+  idx = get_item_array_index(context,
+                             entry->items[0].change_set,
+                             entry->items[0].number);
+
+  /* make sure the index exists in the array */
+  while (context->reps->nelts <= idx)
+    APR_ARRAY_PUSH(context->reps, void *) = NULL;
+
+  /* set the element.  If there is already an entry, there are probably
+   * two items claiming to be the same -> bail out */
+  assert(!APR_ARRAY_IDX(context->reps, idx, void *));
+  APR_ARRAY_IDX(context->reps, idx, void *) = entry;
+}
+
+/* Return the P2L entry from CONTEXT->REPS for the given ID.  If there is
+ * none (or not anymore), return NULL.  If RESET has been specified, set
+ * the array entry to NULL after returning the entry.
+ */
+static svn_fs_x__p2l_entry_t *
+get_item(pack_context_t *context,
+         const svn_fs_x__id_t *id,
+         svn_boolean_t reset)
+{
+  svn_fs_x__p2l_entry_t *result = NULL;
+  svn_revnum_t revision = svn_fs_x__get_revnum(id->change_set);
+  if (id->number && revision >= context->start_rev)
+    {
+      int idx = get_item_array_index(context, revision, id->number);
+      if (context->reps->nelts > idx)
+        {
+          result = APR_ARRAY_IDX(context->reps, idx, void *);
+          if (result && reset)
+            APR_ARRAY_IDX(context->reps, idx, void *) = NULL;
+        }
+    }
+
+  return result;
+}
+
+/* Copy representation item identified by ENTRY from the current position
+ * in REV_FILE into CONTEXT->REPS_FILE.  Add all tracking into needed by
+ * our placement algorithm to CONTEXT.
+ * Use SCRATCH_POOL for temporary allocations.
+ */
+static svn_error_t *
+copy_rep_to_temp(pack_context_t *context,
+                 svn_fs_x__revision_file_t *rev_file,
+                 svn_fs_x__p2l_entry_t *entry,
+                 apr_pool_t *scratch_pool)
+{
+  svn_fs_x__rep_header_t *rep_header;
+  apr_off_t source_offset = entry->offset;
+
+  /* create a copy of ENTRY, make it point to the copy destination and
+   * store it in CONTEXT */
+  entry = svn_fs_x__p2l_entry_dup(entry, context->info_pool);
+  SVN_ERR(svn_fs_x__get_file_offset(&entry->offset, context->reps_file,
+                                    scratch_pool));
+  add_item_rep_mapping(context, entry);
+
+  /* read & parse the representation header */
+  SVN_ERR(svn_fs_x__read_rep_header(&rep_header, rev_file->stream,
+                                    scratch_pool, scratch_pool));
+
+  /* if the representation is a delta against some other rep, link the two */
+  if (   rep_header->type == svn_fs_x__rep_delta
+      && rep_header->base_revision >= context->start_rev)
+    {
+      reference_t *reference = apr_pcalloc(context->info_pool,
+                                           sizeof(*reference));
+      reference->from = entry->items[0];
+      reference->to.change_set
+        = svn_fs_x__change_set_by_rev(rep_header->base_revision);
+      reference->to.number = rep_header->base_item_index;
+      APR_ARRAY_PUSH(context->references, reference_t *) = reference;
+    }
+
+  /* copy the whole rep (including header!) to our temp file */
+  SVN_ERR(svn_io_file_seek(rev_file->file, APR_SET, &source_offset,
+                           scratch_pool));
+  SVN_ERR(copy_file_data(context, context->reps_file, rev_file->file,
+                         entry->size, scratch_pool));
+
+  return SVN_NO_ERROR;
+}
+
+/* Directories first, dirs / files sorted by name in reverse lexical order.
+ * This maximizes the chance of two items being located close to one another
+ * in *all* pack files independent of their change order.  It also groups
+ * multi-project repos nicely according to their sub-projects.  The reverse
+ * order aspect gives "trunk" preference over "tags" and "branches", so
+ * trunk-related items are more likely to be contiguous.
+ */
+static int
+compare_dir_entries(const svn_sort__item_t *a,
+                    const svn_sort__item_t *b)
+{
+  const svn_fs_dirent_t *lhs = (const svn_fs_dirent_t *) a->value;
+  const svn_fs_dirent_t *rhs = (const svn_fs_dirent_t *) b->value;
+
+  if (lhs->kind != rhs->kind)
+    return lhs->kind == svn_node_dir ? -1 : 1;
+
+  return strcmp(lhs->name, rhs->name);
+}
+
+apr_array_header_t *
+svn_fs_x__order_dir_entries(svn_fs_t *fs,
+                            apr_hash_t *directory,
+                            apr_pool_t *result_pool,
+                            apr_pool_t *scratch_pool)
+{
+  apr_array_header_t *ordered
+    = svn_sort__hash(directory, compare_dir_entries, scratch_pool);
+
+  apr_array_header_t *result
+    = apr_array_make(result_pool, ordered->nelts, sizeof(svn_fs_dirent_t *));
+
+  int i;
+  for (i = 0; i < ordered->nelts; ++i)
+    APR_ARRAY_PUSH(result, svn_fs_dirent_t *)
+      = APR_ARRAY_IDX(ordered, i, svn_sort__item_t).value;
+
+  return result;
+}
+
+/* Return a duplicate of the the ORIGINAL path and with special sub-strins
+ * (e.g. "trunk") modified in such a way that have a lower lexicographic
+ * value than any other "normal" file name.
+ */
+static const char *
+tweak_path_for_ordering(const char *original,
+                        apr_pool_t *pool)
+{
+  /* We may add further special cases as needed. */
+  enum {SPECIAL_COUNT = 2};
+  static const char *special[SPECIAL_COUNT] = {"trunk", "branch"};
+  char *pos;
+  char *path = apr_pstrdup(pool, original);
+  int i;
+
+  /* Replace the first char of any "special" sub-string we find by
+   * a control char, i.e. '\1' .. '\31'.  In the rare event that this
+   * would clash with existing paths, no data will be lost but merely
+   * the node ordering will be sub-optimal.
+   */
+  for (i = 0; i < SPECIAL_COUNT; ++i)
+    for (pos = strstr(path, special[i]);
+         pos;
+         pos = strstr(pos + 1, special[i]))
+      {
+        *pos = (char)(i + '\1');
+      }
+
+   return path;
+}
+
+/* Copy node revision item identified by ENTRY from the current position
+ * in REV_FILE into CONTEXT->REPS_FILE.  Add all tracking into needed by
+ * our placement algorithm to CONTEXT.
+ * Use SCRATCH_POOL for temporary allocations.
+ */
+static svn_error_t *
+copy_node_to_temp(pack_context_t *context,
+                  svn_fs_x__revision_file_t *rev_file,
+                  svn_fs_x__p2l_entry_t *entry,
+                  apr_pool_t *scratch_pool)
+{
+  path_order_t *path_order = apr_pcalloc(context->info_pool,
+                                         sizeof(*path_order));
+  svn_fs_x__noderev_t *noderev;
+  const char *sort_path;
+  apr_off_t source_offset = entry->offset;
+
+  /* read & parse noderev */
+  SVN_ERR(svn_fs_x__read_noderev(&noderev, rev_file->stream, scratch_pool,
+                                 scratch_pool));
+
+  /* create a copy of ENTRY, make it point to the copy destination and
+   * store it in CONTEXT */
+  entry = svn_fs_x__p2l_entry_dup(entry, context->info_pool);
+  SVN_ERR(svn_fs_x__get_file_offset(&entry->offset, context->reps_file,
+                                     scratch_pool));
+  add_item_rep_mapping(context, entry);
+
+  /* copy the noderev to our temp file */
+  SVN_ERR(svn_io_file_seek(rev_file->file, APR_SET, &source_offset,
+                           scratch_pool));
+  SVN_ERR(copy_file_data(context, context->reps_file, rev_file->file,
+                         entry->size, scratch_pool));
+
+  /* if the node has a data representation, make that the node's "base".
+   * This will (often) cause the noderev to be placed right in front of
+   * its data representation. */
+
+  if (noderev->data_rep
+      &&    svn_fs_x__get_revnum(noderev->data_rep->id.change_set)
+         >= context->start_rev)
+    {
+      reference_t *reference = apr_pcalloc(context->info_pool,
+                                           sizeof(*reference));
+      reference->from = entry->items[0];
+      reference->to.change_set = noderev->data_rep->id.change_set;
+      reference->to.number = noderev->data_rep->id.number;
+      APR_ARRAY_PUSH(context->references, reference_t *) = reference;
+
+      path_order->rep_id = reference->to;
+      path_order->expanded_size = noderev->data_rep->expanded_size;
+    }
+
+  /* Sort path is the key used for ordering noderevs and associated reps.
+   * It will not be stored in the final pack file. */
+  sort_path = tweak_path_for_ordering(noderev->created_path, scratch_pool);
+  path_order->path = svn_prefix_string__create(context->paths, sort_path);
+  path_order->node_id = noderev->node_id;
+  path_order->revision = svn_fs_x__get_revnum(noderev->noderev_id.change_set);
+  path_order->is_dir = noderev->kind == svn_node_dir;
+  path_order->noderev_id = noderev->noderev_id;
+  APR_ARRAY_PUSH(context->path_order, path_order_t *) = path_order;
+
+  return SVN_NO_ERROR;
+}
+
+/* implements compare_fn_t. Place LHS before RHS, if the latter is older.
+ */
+static int
+compare_p2l_info(const svn_fs_x__p2l_entry_t * const * lhs,
+                 const svn_fs_x__p2l_entry_t * const * rhs)
+{
+  assert(*lhs != *rhs);
+  if ((*lhs)->item_count == 0)
+    return (*lhs)->item_count == 0 ? 0 : -1;
+  if ((*lhs)->item_count == 0)
+    return 1;
+
+  if ((*lhs)->items[0].change_set == (*rhs)->items[0].change_set)
+    return (*lhs)->items[0].number > (*rhs)->items[0].number ? -1 : 1;
+
+  return (*lhs)->items[0].change_set > (*rhs)->items[0].change_set ? -1 : 1;
+}
+
+/* Sort svn_fs_x__p2l_entry_t * array ENTRIES by age.  Place the latest
+ * items first.
+ */
+static void
+sort_items(apr_array_header_t *entries)
+{
+  svn_sort__array(entries,
+                  (int (*)(const void *, const void *))compare_p2l_info);
+}
+
+/* implements compare_fn_t.  Sort descending by PATH, NODE_ID and REVISION.
+ */
+static int
+compare_path_order(const path_order_t * const * lhs_p,
+                   const path_order_t * const * rhs_p)
+{
+  const path_order_t * lhs = *lhs_p;
+  const path_order_t * rhs = *rhs_p;
+
+  /* cluster all directories */
+  int diff = rhs->is_dir - lhs->is_dir;
+  if (diff)
+    return diff;
+
+  /* lexicographic order on path and node (i.e. latest first) */
+  diff = svn_prefix_string__compare(lhs->path, rhs->path);
+  if (diff)
+    return diff;
+
+  /* reverse order on node (i.e. latest first) */
+  diff = svn_fs_x__id_compare(&rhs->node_id, &lhs->node_id);
+  if (diff)
+    return diff;
+
+  /* reverse order on revision (i.e. latest first) */
+  if (lhs->revision != rhs->revision)
+    return lhs->revision < rhs->revision ? 1 : -1;
+
+  return 0;
+}
+
+/* implements compare_fn_t.  Sort ascending by TO, FROM.
+ */
+static int
+compare_references(const reference_t * const * lhs_p,
+                   const reference_t * const * rhs_p)
+{
+  const reference_t * lhs = *lhs_p;
+  const reference_t * rhs = *rhs_p;
+
+  int diff = svn_fs_x__id_compare(&lhs->to, &rhs->to);
+  return diff ? diff : svn_fs_x__id_compare(&lhs->from, &rhs->from);
+}
+
+/* Order the data collected in CONTEXT such that we can place them in the
+ * desired order.
+ */
+static void
+sort_reps(pack_context_t *context)
+{
+  svn_sort__array(context->path_order,
+                  (int (*)(const void *, const void *))compare_path_order);
+  svn_sort__array(context->references,
+                  (int (*)(const void *, const void *))compare_references);
+}
+
+/* Return the remaining unused bytes in the current block in CONTEXT's
+ * pack file.
+ */
+static apr_ssize_t
+get_block_left(pack_context_t *context)
+{
+  svn_fs_x__data_t *ffd = context->fs->fsap_data;
+  return ffd->block_size - (context->pack_offset % ffd->block_size);
+}
+
+/* To prevent items from overlapping a block boundary, we will usually
+ * put them into the next block and top up the old one with NUL bytes.
+ * Pad CONTEXT's pack file to the end of the current block, if that padding
+ * is short enough.  Use SCRATCH_POOL for temporary allocations.
+ */
+static svn_error_t *
+auto_pad_block(pack_context_t *context,
+               apr_pool_t *scratch_pool)
+{
+  svn_fs_x__data_t *ffd = context->fs->fsap_data;
+
+  /* This is the maximum number of bytes "wasted" that way per block.
+   * Larger items will cross the block boundaries. */
+  const apr_off_t max_padding = MAX(ffd->block_size / 50, 512);
+
+  /* Is wasted space small enough to align the current item to the next
+   * block? */
+  apr_off_t padding = get_block_left(context);
+
+  if (padding < max_padding)
+    {
+      /* Yes. To up with NUL bytes and don't forget to create
+       * an P2L index entry marking this section as unused. */
+      svn_fs_x__p2l_entry_t null_entry;
+
+      null_entry.offset = context->pack_offset;
+      null_entry.size = padding;
+      null_entry.type = SVN_FS_X__ITEM_TYPE_UNUSED;
+      null_entry.fnv1_checksum = 0;
+      null_entry.item_count = 0;
+      null_entry.items = NULL;
+
+      SVN_ERR(write_null_bytes(context->pack_file, padding, scratch_pool));
+      SVN_ERR(svn_fs_x__p2l_proto_index_add_entry
+                  (context->proto_p2l_index, &null_entry, scratch_pool));
+      context->pack_offset += padding;
+    }
+
+  return SVN_NO_ERROR;
+}
+
+/* Return the index of the first entry in CONTEXT->REFERENCES that
+ * references ITEM->ITEMS[0] if such entries exist.  All matching items
+ * will be consecutive.
+ */
+static int
+find_first_reference(pack_context_t *context,
+                     svn_fs_x__p2l_entry_t *item)
+{
+  int lower = 0;
+  int upper = context->references->nelts - 1;
+
+  while (lower <= upper)
+    {
+      int current = lower + (upper - lower) / 2;
+      reference_t *reference
+        = APR_ARRAY_IDX(context->references, current, reference_t *);
+
+      if (svn_fs_x__id_compare(&reference->to, item->items) < 0)
+        lower = current + 1;
+      else
+        upper = current - 1;
+    }
+
+  return lower;
+}
+
+/* Check whether entry number IDX in CONTEXT->REFERENCES references ITEM.
+ */
+static svn_boolean_t
+is_reference_match(pack_context_t *context,
+                   int idx,
+                   svn_fs_x__p2l_entry_t *item)
+{
+  reference_t *reference;
+  if (context->references->nelts <= idx)
+    return FALSE;
+
+  reference = APR_ARRAY_IDX(context->references, idx, reference_t *);
+  return svn_fs_x__id_eq(&reference->to, item->items);
+}
+
+/* Starting at IDX in CONTEXT->PATH_ORDER, select all representations and
+ * noderevs that should be placed into the same container, respectively.
+ * Append the path_order_t * elements encountered in SELECTED, the
+ * svn_fs_x__p2l_entry_t * of the representations that should be placed
+ * into the same reps container will be appended to REP_PARTS and the
+ * svn_fs_x__p2l_entry_t * of the noderevs referencing those reps will
+ * be appended to NODE_PARTS.
+ *
+ * Remove all returned items from the CONTEXT->REPS container and prevent
+ * them from being placed a second time later on.  That also means that the
+ * caller has to place all items returned.
+ */
+static svn_error_t *
+select_reps(pack_context_t *context,
+            int idx,
+            apr_array_header_t *selected,
+            apr_array_header_t *node_parts,
+            apr_array_header_t *rep_parts)
+{
+  apr_array_header_t *path_order = context->path_order;
+  path_order_t *start_path = APR_ARRAY_IDX(path_order, idx, path_order_t *);
+
+  svn_fs_x__p2l_entry_t *node_part;
+  svn_fs_x__p2l_entry_t *rep_part;
+  svn_fs_x__p2l_entry_t *depending;
+  int i, k;
+
+  /* collect all path_order records as well as rep and noderev items
+   * that occupy the same path with the same node. */
+  for (; idx < path_order->nelts; ++idx)
+    {
+      path_order_t *current_path
+        = APR_ARRAY_IDX(path_order, idx, path_order_t *);
+
+      if (!svn_fs_x__id_eq(&start_path->node_id, &current_path->node_id))
+        break;
+
+      APR_ARRAY_IDX(path_order, idx, path_order_t *) = NULL;
+      node_part = get_item(context, &current_path->noderev_id, TRUE);
+      rep_part = get_item(context, &current_path->rep_id, TRUE);
+
+      if (node_part && rep_part)
+        APR_ARRAY_PUSH(selected, path_order_t *) = current_path;
+
+      if (node_part)
+        APR_ARRAY_PUSH(node_parts, svn_fs_x__p2l_entry_t *) = node_part;
+      if (rep_part)
+        APR_ARRAY_PUSH(rep_parts, svn_fs_x__p2l_entry_t *) = rep_part;
+    }
+
+  /* collect depending reps and noderevs that reference any of the collected
+   * reps */
+  for (i = 0; i < rep_parts->nelts; ++i)
+    {
+      rep_part = APR_ARRAY_IDX(rep_parts, i, svn_fs_x__p2l_entry_t*);
+      for (k = find_first_reference(context, rep_part);
+           is_reference_match(context, k, rep_part);
+           ++k)
+        {
+          reference_t *reference
+            = APR_ARRAY_IDX(context->references, k, reference_t *);
+
+          depending = get_item(context, &reference->from, TRUE);
+          if (!depending)
+            continue;
+
+          if (depending->type == SVN_FS_X__ITEM_TYPE_NODEREV)
+            APR_ARRAY_PUSH(node_parts, svn_fs_x__p2l_entry_t *) = depending;
+          else
+            APR_ARRAY_PUSH(rep_parts, svn_fs_x__p2l_entry_t *) = depending;
+        }
+    }
+
+  return SVN_NO_ERROR;
+}
+
+/* Return TRUE, if all path_order_t * in SELECTED reference contents that is
+ * not longer than LIMIT.
+ */
+static svn_boolean_t
+reps_fit_into_containers(apr_array_header_t *selected,
+                         apr_uint64_t limit)
+{
+  int i;
+  for (i = 0; i < selected->nelts; ++i)
+    if (APR_ARRAY_IDX(selected, i, path_order_t *)->expanded_size > limit)
+      return FALSE;
+
+  return TRUE;
+}
+
+/* Write the *CONTAINER containing the noderevs described by the
+ * svn_fs_x__p2l_entry_t * in ITEMS to the pack file on CONTEXT.
+ * Append a P2L entry for the container to CONTAINER->REPS.
+ * Afterwards, clear ITEMS and re-allocate *CONTAINER in CONTAINER_POOL
+ * so the caller may fill them again.
+ * Use SCRATCH_POOL for temporary allocations.
+ */
+static svn_error_t *
+write_nodes_container(pack_context_t *context,
+                      svn_fs_x__noderevs_t **container,
+                      apr_array_header_t *items,
+                      apr_pool_t *container_pool,
+                      apr_pool_t *scratch_pool)
+{
+  int i;
+  apr_off_t offset = 0;
+  svn_fs_x__p2l_entry_t *container_entry;
+  svn_stream_t *pack_stream;
+
+  if (items->nelts == 0)
+    return SVN_NO_ERROR;
+
+  /* serialize container */
+  container_entry = apr_palloc(context->info_pool, sizeof(*container_entry));
+  pack_stream = svn_checksum__wrap_write_stream_fnv1a_32x4
+                                (&container_entry->fnv1_checksum,
+                                 svn_stream_from_aprfile2(context->pack_file,
+                                                          TRUE, scratch_pool),
+                                 scratch_pool);
+  SVN_ERR(svn_fs_x__write_noderevs_container(pack_stream, *container,
+                                             scratch_pool));
+  SVN_ERR(svn_stream_close(pack_stream));
+  SVN_ERR(svn_io_file_seek(context->pack_file, APR_CUR, &offset,
+                           scratch_pool));
+
+  /* replace first noderev item in ENTRIES with the container
+     and set all others to NULL */
+  container_entry->offset = context->pack_offset;
+  container_entry->size = offset - container_entry->offset;
+  container_entry->type = SVN_FS_X__ITEM_TYPE_NODEREVS_CONT;
+  container_entry->item_count = items->nelts;
+  container_entry->items = apr_palloc(context->info_pool,
+      sizeof(svn_fs_x__id_t) * container_entry->item_count);
+
+  for (i = 0; i < items->nelts; ++i)
+    container_entry->items[i]
+      = APR_ARRAY_IDX(items, i, svn_fs_x__p2l_entry_t *)->items[0];
+
+  context->pack_offset = offset;
+  APR_ARRAY_PUSH(context->reps, svn_fs_x__p2l_entry_t *)
+    = container_entry;
+
+  /* Write P2L index for copied items, i.e. the 1 container */
+  SVN_ERR(svn_fs_x__p2l_proto_index_add_entry
+            (context->proto_p2l_index, container_entry, scratch_pool));
+
+  svn_pool_clear(container_pool);
+  *container = svn_fs_x__noderevs_create(16, container_pool);
+  apr_array_clear(items);
+
+  return SVN_NO_ERROR;
+}
+
+/* Read the noderevs given by the svn_fs_x__p2l_entry_t * in NODE_PARTS
+ * from TEMP_FILE and add them to *CONTAINER and NODES_IN_CONTAINER.
+ * Whenever the container grows bigger than the current block in CONTEXT,
+ * write the data to disk and continue in the next block.
+ *
+ * Use CONTAINER_POOL to re-allocate the *CONTAINER as necessary and
+ * SCRATCH_POOL to temporary allocations.
+ */
+static svn_error_t *
+store_nodes(pack_context_t *context,
+            apr_file_t *temp_file,
+            apr_array_header_t *node_parts,
+            svn_fs_x__noderevs_t **container,
+            apr_array_header_t *nodes_in_container,
+            apr_pool_t *container_pool,
+            apr_pool_t *scratch_pool)
+{
+  int i;
+
+  apr_pool_t *iterpool = svn_pool_create(scratch_pool);
+  svn_stream_t *stream
+    = svn_stream_from_aprfile2(temp_file, TRUE, scratch_pool);
+
+  /* number of bytes in the current block not being spent on fixed-size
+     items (i.e. those not put into the container). */
+  apr_size_t capacity_left = get_block_left(context);
+
+  /* Estimated noderev container size */
+  apr_size_t last_container_size = 0, container_size = 0;
+
+  /* Estimate extra capacity we will gain from container compression. */
+  apr_size_t pack_savings = 0;
+  for (i = 0; i < node_parts->nelts; ++i)
+    {
+      svn_fs_x__noderev_t *noderev;
+      svn_fs_x__p2l_entry_t *entry
+        = APR_ARRAY_IDX(node_parts, i, svn_fs_x__p2l_entry_t *);
+
+      /* if we reached the limit, check whether we saved some space
+         through the container. */
+      if (capacity_left + pack_savings < container_size + entry->size)
+        container_size = svn_fs_x__noderevs_estimate_size(*container);
+
+      /* If necessary and the container is large enough, try harder
+         by actually serializing the container and determine current
+         savings due to compression. */
+      if (   capacity_left + pack_savings < container_size + entry->size
+          && container_size > last_container_size + 2000)
+        {
+          svn_stringbuf_t *serialized
+            = svn_stringbuf_create_ensure(container_size, iterpool);
+          svn_stream_t *temp_stream
+            = svn_stream_from_stringbuf(serialized, iterpool);
+
+          SVN_ERR(svn_fs_x__write_noderevs_container(temp_stream, *container,
+                                                     iterpool));
+          SVN_ERR(svn_stream_close(temp_stream));
+
+          last_container_size = container_size;
+          pack_savings = container_size - serialized->len;
+        }
+
+      /* still doesn't fit? -> block is full. Flush */
+      if (   capacity_left + pack_savings < container_size + entry->size
+          && nodes_in_container->nelts < 2)
+        {
+          SVN_ERR(auto_pad_block(context, iterpool));
+          capacity_left = get_block_left(context);
+        }
+
+      /* still doesn't fit? -> block is full. Flush */
+      if (capacity_left + pack_savings < container_size + entry->size)
+        {
+          SVN_ERR(write_nodes_container(context, container,
+                                        nodes_in_container, container_pool,
+                                        iterpool));
+
+          capacity_left = get_block_left(context);
+          pack_savings = 0;
+          container_size = 0;
+        }
+
+      /* item will fit into the block. */
+      SVN_ERR(svn_io_file_seek(temp_file, APR_SET, &entry->offset, iterpool));
+      SVN_ERR(svn_fs_x__read_noderev(&noderev, stream, iterpool, iterpool));
+      svn_fs_x__noderevs_add(*container, noderev);
+
+      container_size += entry->size;
+      APR_ARRAY_PUSH(nodes_in_container, svn_fs_x__p2l_entry_t *) = entry;
+
+      svn_pool_clear(iterpool);
+    }
+
+  svn_pool_destroy(iterpool);
+
+  return SVN_NO_ERROR;
+}
+
+
+/* Finalize CONTAINER and write it to CONTEXT's pack file.
+ * Append an P2L entry containing the given SUB_ITEMS to NEW_ENTRIES.
+ * Use SCRATCH_POOL for temporary allocations.
+ */
+static svn_error_t *
+write_reps_container(pack_context_t *context,
+                     svn_fs_x__reps_builder_t *container,
+                     apr_array_header_t *sub_items,
+                     apr_array_header_t *new_entries,
+                     apr_pool_t *scratch_pool)
+{
+  apr_off_t offset = 0;
+  svn_fs_x__p2l_entry_t container_entry;
+
+  svn_stream_t *pack_stream
+    = svn_checksum__wrap_write_stream_fnv1a_32x4
+                                (&container_entry.fnv1_checksum,
+                                 svn_stream_from_aprfile2(context->pack_file,
+                                                          TRUE, scratch_pool),
+                                 scratch_pool);
+
+  SVN_ERR(svn_fs_x__write_reps_container(pack_stream, container,
+                                         scratch_pool));
+  SVN_ERR(svn_stream_close(pack_stream));
+  SVN_ERR(svn_io_file_seek(context->pack_file, APR_CUR, &offset,
+                           scratch_pool));
+
+  container_entry.offset = context->pack_offset;
+  container_entry.size = offset - container_entry.offset;
+  container_entry.type = SVN_FS_X__ITEM_TYPE_REPS_CONT;
+  container_entry.item_count = sub_items->nelts;
+  container_entry.items = (svn_fs_x__id_t *)sub_items->elts;
+
+  context->pack_offset = offset;
+  APR_ARRAY_PUSH(new_entries, svn_fs_x__p2l_entry_t *)
+    = svn_fs_x__p2l_entry_dup(&container_entry, context->info_pool);
+
+  SVN_ERR(svn_fs_x__p2l_proto_index_add_entry
+            (context->proto_p2l_index, &container_entry, scratch_pool));
+
+  return SVN_NO_ERROR;
+}
+
+/* Read the (property) representations identified by svn_fs_x__p2l_entry_t
+ * elements in ENTRIES from TEMP_FILE, aggregate them and write them into
+ * CONTEXT->PACK_FILE.  Use SCRATCH_POOL for temporary allocations.
+ */
+static svn_error_t *
+write_reps_containers(pack_context_t *context,
+                      apr_array_header_t *entries,
+                      apr_file_t *temp_file,
+                      apr_array_header_t *new_entries,
+                      apr_pool_t *scratch_pool)
+{
+  apr_pool_t *iterpool = svn_pool_create(scratch_pool);
+  apr_pool_t *container_pool = svn_pool_create(scratch_pool);
+  int i;
+
+  apr_ssize_t block_left = get_block_left(context);
+
+  svn_fs_x__reps_builder_t *container
+    = svn_fs_x__reps_builder_create(context->fs, container_pool);
+  apr_array_header_t *sub_items
+    = apr_array_make(scratch_pool, 64, sizeof(svn_fs_x__id_t));
+  svn_fs_x__revision_file_t *file;
+
+  SVN_ERR(svn_fs_x__wrap_temp_rev_file(&file, context->fs, temp_file,
+                                       scratch_pool));
+
+  /* copy all items in strict order */
+  for (i = entries->nelts-1; i >= 0; --i)
+    {
+      svn_fs_x__representation_t representation = { 0 };
+      svn_stringbuf_t *contents;
+      svn_stream_t *stream;
+      apr_size_t list_index;
+      svn_fs_x__p2l_entry_t *entry
+        = APR_ARRAY_IDX(entries, i, svn_fs_x__p2l_entry_t *);
+
+      if ((block_left < entry->size) && sub_items->nelts)
+        {
+          block_left = get_block_left(context)
+                     - svn_fs_x__reps_estimate_size(container);
+        }
+
+      if ((block_left < entry->size) && sub_items->nelts)
+        {
+          SVN_ERR(write_reps_container(context, container, sub_items,
+                                       new_entries, iterpool));
+
+          apr_array_clear(sub_items);
+          svn_pool_clear(container_pool);
+          container = svn_fs_x__reps_builder_create(context->fs,
+                                                    container_pool);
+          block_left = get_block_left(context);
+        }
+
+      /* still enough space in current block? */
+      if (block_left < entry->size)
+        {
+          SVN_ERR(auto_pad_block(context, iterpool));
+          block_left = get_block_left(context);
+        }
+
+      assert(entry->item_count == 1);
+      representation.id = entry->items[0];
+
+      /* select the change list in the source file, parse it and add it to
+       * the container */
+      SVN_ERR(svn_io_file_seek(temp_file, APR_SET, &entry->offset,
+                               iterpool));
+      SVN_ERR(svn_fs_x__get_representation_length(&representation.size,
+                                             &representation.expanded_size,
+                                             context->fs, file,
+                                             entry, iterpool));
+      SVN_ERR(svn_fs_x__get_contents(&stream, context->fs, &representation,
+                                     FALSE, iterpool));
+      contents = svn_stringbuf_create_ensure(representation.expanded_size,
+                                             iterpool);
+      contents->len = representation.expanded_size;
+
+      /* The representation is immutable.  Read it normally. */
+      SVN_ERR(svn_stream_read_full(stream, contents->data, &contents->len));
+      SVN_ERR(svn_stream_close(stream));
+
+      SVN_ERR(svn_fs_x__reps_add(&list_index, container,
+                                 svn_stringbuf__morph_into_string(contents)));
+      SVN_ERR_ASSERT(list_index == sub_items->nelts);
+      block_left -= entry->size;
+
+      APR_ARRAY_PUSH(sub_items, svn_fs_x__id_t) = entry->items[0];
+
+      svn_pool_clear(iterpool);
+    }
+
+  if (sub_items->nelts)
+    SVN_ERR(write_reps_container(context, container, sub_items,
+                                 new_entries, iterpool));
+
+  svn_pool_destroy(iterpool);
+  svn_pool_destroy(container_pool);
+
+  return SVN_NO_ERROR;
+}
+
+/* Return TRUE if the estimated size of the NODES_IN_CONTAINER plus the
+ * representations given as svn_fs_x__p2l_entry_t * in ENTRIES may exceed
+ * the space left in the current block.
+ */
+static svn_boolean_t
+should_flush_nodes_container(pack_context_t *context,
+                             svn_fs_x__noderevs_t *nodes_container,
+                             apr_array_header_t *entries)
+{
+  apr_ssize_t block_left = get_block_left(context);
+  apr_ssize_t rep_sum = 0;
+  apr_ssize_t container_size
+    = svn_fs_x__noderevs_estimate_size(nodes_container);
+
+  int i;
+  for (i = 0; i < entries->nelts; ++i)
+    {
+      svn_fs_x__p2l_entry_t *entry
+        = APR_ARRAY_IDX(entries, i, svn_fs_x__p2l_entry_t *);
+      rep_sum += entry->size;
+    }
+
+  return block_left < rep_sum + container_size;
+}
+
+/* Read the contents of the first COUNT non-NULL, non-empty items in ITEMS
+ * from TEMP_FILE and write them to CONTEXT->PACK_FILE.
+ * Use SCRATCH_POOL for temporary allocations.
+ */
+static svn_error_t *
+store_items(pack_context_t *context,
+            apr_file_t *temp_file,
+            apr_array_header_t *items,
+            int count,
+            apr_pool_t *scratch_pool)
+{
+  int i;
+  apr_pool_t *iterpool = svn_pool_create(scratch_pool);
+
+  /* copy all items in strict order */
+  for (i = 0; i < count; ++i)
+    {
+      svn_fs_x__p2l_entry_t *entry
+        = APR_ARRAY_IDX(items, i, svn_fs_x__p2l_entry_t *);
+      if (!entry
+          || entry->type == SVN_FS_X__ITEM_TYPE_UNUSED
+          || entry->item_count == 0)
+        continue;
+
+      /* select the item in the source file and copy it into the target
+       * pack file */
+      SVN_ERR(svn_io_file_seek(temp_file, APR_SET, &entry->offset,
+                               iterpool));
+      SVN_ERR(copy_file_data(context, context->pack_file, temp_file,
+                             entry->size, iterpool));
+
+      /* write index entry and update current position */
+      entry->offset = context->pack_offset;
+      context->pack_offset += entry->size;
+
+      SVN_ERR(svn_fs_x__p2l_proto_index_add_entry
+                  (context->proto_p2l_index, entry, iterpool));
+
+      APR_ARRAY_PUSH(context->reps, svn_fs_x__p2l_entry_t *) = entry;
+      svn_pool_clear(iterpool);
+    }
+
+  svn_pool_destroy(iterpool);
+
+  return SVN_NO_ERROR;
+}
+
+/* Copy (append) the items identified by svn_fs_x__p2l_entry_t * elements
+ * in ENTRIES strictly in order from TEMP_FILE into CONTEXT->PACK_FILE.
+ * Use SCRATCH_POOL for temporary allocations.
+ */
+static svn_error_t *
+copy_reps_from_temp(pack_context_t *context,
+                    apr_file_t *temp_file,
+                    apr_pool_t *scratch_pool)
+{
+  svn_fs_x__data_t *ffd = context->fs->fsap_data;
+
+  apr_pool_t *iterpool = svn_pool_create(scratch_pool);
+  apr_pool_t *container_pool = svn_pool_create(scratch_pool);
+  apr_array_header_t *path_order = context->path_order;
+  apr_array_header_t *reps = context->reps;
+  apr_array_header_t *selected = apr_array_make(scratch_pool, 16,
+                                                path_order->elt_size);
+  apr_array_header_t *node_parts = apr_array_make(scratch_pool, 16,
+                                                  reps->elt_size);
+  apr_array_header_t *rep_parts = apr_array_make(scratch_pool, 16,
+                                                 reps->elt_size);
+  apr_array_header_t *nodes_in_container = apr_array_make(scratch_pool, 16,
+                                                          reps->elt_size);
+  int i, k;
+  int initial_reps_count = reps->nelts;
+
+  /* 1 container for all noderevs in the current block.  We will try to
+   * not write it to disk until the current block fills up, i.e. aim for
+   * a single noderevs container per block. */
+  svn_fs_x__noderevs_t *nodes_container
+    = svn_fs_x__noderevs_create(16, container_pool);
+
+  /* copy items in path order. Create block-sized containers. */
+  for (i = 0; i < path_order->nelts; ++i)
+    {
+      if (APR_ARRAY_IDX(path_order, i, path_order_t *) == NULL)
+        continue;
+
+      /* Collect reps to combine and all noderevs referencing them */
+      SVN_ERR(select_reps(context, i, selected, node_parts, rep_parts));
+
+      /* store the noderevs container in front of the reps */
+      SVN_ERR(store_nodes(context, temp_file, node_parts, &nodes_container,
+                          nodes_in_container, container_pool, iterpool));
+
+      /* actually flush the noderevs to disk if the reps container is likely
+       * to fill the block, i.e. no further noderevs will be added to the
+       * nodes container. */
+      if (should_flush_nodes_container(context, nodes_container, node_parts))
+        SVN_ERR(write_nodes_container(context, &nodes_container,
+                                      nodes_in_container, container_pool,
+                                      iterpool));
+
+      /* if all reps are short enough put them into one container.
+       * Otherwise, just store all containers here. */
+      if (reps_fit_into_containers(selected, 2 * ffd->block_size))
+        SVN_ERR(write_reps_containers(context, rep_parts, temp_file,
+                                      context->reps, iterpool));
+      else
+        SVN_ERR(store_items(context, temp_file, rep_parts, rep_parts->nelts,
+                            iterpool));
+
+      /* processed all items */
+      apr_array_clear(selected);
+      apr_array_clear(node_parts);
+      apr_array_clear(rep_parts);
+
+      svn_pool_clear(iterpool);
+    }
+
+  /* flush noderevs container to disk */
+  if (nodes_in_container->nelts)
+    SVN_ERR(write_nodes_container(context, &nodes_container,
+                                  nodes_in_container, container_pool,
+                                  iterpool));
+
+  /* copy all items in strict order */
+  SVN_ERR(store_items(context, temp_file, reps, initial_reps_count,
+                      scratch_pool));
+
+  /* vaccum ENTRIES array: eliminate NULL entries */
+  for (i = 0, k = 0; i < reps->nelts; ++i)
+    {
+      svn_fs_x__p2l_entry_t *entry
+        = APR_ARRAY_IDX(reps, i, svn_fs_x__p2l_entry_t *);
+      if (entry)
+        {
+          APR_ARRAY_IDX(reps, k, svn_fs_x__p2l_entry_t *) = entry;
+          ++k;
+        }
+    }
+  reps->nelts = k;
+
+  svn_pool_destroy(iterpool);
+  svn_pool_destroy(container_pool);
+
+  return SVN_NO_ERROR;
+}
+
+/* Finalize CONTAINER and write it to CONTEXT's pack file.
+ * Append an P2L entry containing the given SUB_ITEMS to NEW_ENTRIES.
+ * Use SCRATCH_POOL for temporary allocations.
+ */
+static svn_error_t *
+write_changes_container(pack_context_t *context,
+                        svn_fs_x__changes_t *container,
+                        apr_array_header_t *sub_items,
+                        apr_array_header_t *new_entries,
+                        apr_pool_t *scratch_pool)
+{
+  apr_off_t offset = 0;
+  svn_fs_x__p2l_entry_t container_entry;
+
+  svn_stream_t *pack_stream
+    = svn_checksum__wrap_write_stream_fnv1a_32x4
+                                (&container_entry.fnv1_checksum,
+                                 svn_stream_from_aprfile2(context->pack_file,
+                                                          TRUE, scratch_pool),
+                                 scratch_pool);
+
+  SVN_ERR(svn_fs_x__write_changes_container(pack_stream,
+                                             container,
+                                             scratch_pool));
+  SVN_ERR(svn_stream_close(pack_stream));
+  SVN_ERR(svn_io_file_seek(context->pack_file, APR_CUR, &offset,
+                           scratch_pool));
+
+  container_entry.offset = context->pack_offset;
+  container_entry.size = offset - container_entry.offset;
+  container_entry.type = SVN_FS_X__ITEM_TYPE_CHANGES_CONT;
+  container_entry.item_count = sub_items->nelts;
+  container_entry.items = (svn_fs_x__id_t *)sub_items->elts;
+
+  context->pack_offset = offset;
+  APR_ARRAY_PUSH(new_entries, svn_fs_x__p2l_entry_t *)
+    = svn_fs_x__p2l_entry_dup(&container_entry, context->info_pool);
+
+  SVN_ERR(svn_fs_x__p2l_proto_index_add_entry
+            (context->proto_p2l_index, &container_entry, scratch_pool));
+
+  return SVN_NO_ERROR;
+}
+
+/* Read the change lists identified by svn_fs_x__p2l_entry_t * elements
+ * in ENTRIES strictly in from TEMP_FILE, aggregate them and write them
+ * into CONTEXT->PACK_FILE.  Use SCRATCH_POOL for temporary allocations.
+ */
+static svn_error_t *
+write_changes_containers(pack_context_t *context,
+                         apr_array_header_t *entries,
+                         apr_file_t *temp_file,
+                         apr_pool_t *scratch_pool)
+{
+  apr_pool_t *iterpool = svn_pool_create(scratch_pool);
+  apr_pool_t *container_pool = svn_pool_create(scratch_pool);
+  int i;
+
+  apr_ssize_t block_left = get_block_left(context);
+  apr_ssize_t estimated_addition = 0;
+
+  svn_fs_x__changes_t *container
+    = svn_fs_x__changes_create(1000, container_pool);
+  apr_array_header_t *sub_items
+    = apr_array_make(scratch_pool, 64, sizeof(svn_fs_x__id_t));
+  apr_array_header_t *new_entries
+    = apr_array_make(context->info_pool, 16, entries->elt_size);
+  svn_stream_t *temp_stream
+    = svn_stream_from_aprfile2(temp_file, TRUE, scratch_pool);
+
+  /* copy all items in strict order */
+  for (i = entries->nelts-1; i >= 0; --i)
+    {
+      apr_array_header_t *changes;
+      apr_size_t list_index;
+      svn_fs_x__p2l_entry_t *entry
+        = APR_ARRAY_IDX(entries, i, svn_fs_x__p2l_entry_t *);
+
+      /* zip compression alone will significantly reduce the size of large
+       * change lists. So, we will probably need even less than this estimate.
+       */
+      apr_ssize_t estimated_size = (entry->size / 5) + 250;
+
+      /* If necessary and enough data has been added to the container since
+       * the last test, try harder by actually serializing the container and
+       * determine current savings due to compression. */
+      if (block_left < estimated_size && estimated_addition > 2000)
+        {
+          svn_stringbuf_t *serialized
+            = svn_stringbuf_create_ensure(get_block_left(context), iterpool);
+          svn_stream_t *memory_stream
+            = svn_stream_from_stringbuf(serialized, iterpool);
+
+          SVN_ERR(svn_fs_x__write_changes_container(memory_stream,
+                                                     container, iterpool));
+          SVN_ERR(svn_stream_close(temp_stream));
+
+          block_left = get_block_left(context) - serialized->len;
+          estimated_addition = 0;
+        }
+
+      if ((block_left < estimated_size) && sub_items->nelts)
+        {
+          SVN_ERR(write_changes_container(context, container, sub_items,
+                                          new_entries, iterpool));
+
+          apr_array_clear(sub_items);
+          svn_pool_clear(container_pool);
+          container = svn_fs_x__changes_create(1000, container_pool);
+          block_left = get_block_left(context);
+          estimated_addition = 0;
+        }
+
+      /* still enough space in current block? */
+      if (block_left < estimated_size)
+        {
+          SVN_ERR(auto_pad_block(context, iterpool));
+          block_left = get_block_left(context);
+        }
+
+      /* select the change list in the source file, parse it and add it to
+       * the container */
+      SVN_ERR(svn_io_file_seek(temp_file, APR_SET, &entry->offset,
+                               iterpool));
+      SVN_ERR(svn_fs_x__read_changes(&changes, temp_stream, scratch_pool,
+                                     iterpool));
+      SVN_ERR(svn_fs_x__changes_append_list(&list_index, container, changes));
+      SVN_ERR_ASSERT(list_index == sub_items->nelts);
+      block_left -= estimated_size;
+      estimated_addition += estimated_size;
+
+      APR_ARRAY_PUSH(sub_items, svn_fs_x__id_t) = entry->items[0];
+
+      svn_pool_clear(iterpool);
+    }
+
+  if (sub_items->nelts)
+    SVN_ERR(write_changes_container(context, container, sub_items,
+                                    new_entries, iterpool));
+
+  *entries = *new_entries;
+  svn_pool_destroy(iterpool);
+  svn_pool_destroy(container_pool);
+
+  return SVN_NO_ERROR;
+}
+
+/* Read the (property) representations identified by svn_fs_x__p2l_entry_t
+ * elements in ENTRIES from TEMP_FILE, aggregate them and write them into
+ * CONTEXT->PACK_FILE.  Use SCRATCH_POOL for temporary allocations.
+ */
+static svn_error_t *
+write_property_containers(pack_context_t *context,
+                          apr_array_header_t *entries,
+                          apr_file_t *temp_file,
+                          apr_pool_t *scratch_pool)
+{
+  apr_array_header_t *new_entries
+    = apr_array_make(context->info_pool, 16, entries->elt_size);
+
+  SVN_ERR(write_reps_containers(context, entries, temp_file, new_entries,
+                                scratch_pool));
+
+  *entries = *new_entries;
+
+  return SVN_NO_ERROR;
+}
+
+/* Append all entries of svn_fs_x__p2l_entry_t * array TO_APPEND to
+ * svn_fs_x__p2l_entry_t * array DEST.
+ */
+static void
+append_entries(apr_array_header_t *dest,
+               apr_array_header_t *to_append)
+{
+  int i;
+  for (i = 0; i < to_append->nelts; ++i)
+    APR_ARRAY_PUSH(dest, svn_fs_x__p2l_entry_t *)
+      = APR_ARRAY_IDX(to_append, i, svn_fs_x__p2l_entry_t *);
+}
+
+/* Write the log-to-phys proto index file for CONTEXT and use POOL for
+ * temporary allocations.  All items in all buckets must have been placed
+ * by now.
+ */
+static svn_error_t *
+write_l2p_index(pack_context_t *context,
+                apr_pool_t *pool)
+{
+  apr_pool_t *scratch_pool = svn_pool_create(pool);
+  const char *temp_name;
+  const char *proto_index;
+  apr_off_t offset = 0;
+
+  /* lump all items into one bucket.  As target, use the bucket that
+   * probably has the most entries already. */
+  append_entries(context->reps, context->changes);
+  append_entries(context->reps, context->file_props);
+  append_entries(context->reps, context->dir_props);
+
+  /* Let the index code do the expensive L2P -> P2L transformation. */
+  SVN_ERR(svn_fs_x__l2p_index_from_p2l_entries(&temp_name,
+                                               context->fs,
+                                               context->reps,
+                                               pool, scratch_pool));
+
+  /* Append newly written segment to exisiting proto index file. */
+  SVN_ERR(svn_io_file_name_get(&proto_index, context->proto_l2p_index,
+                               scratch_pool));
+
+  SVN_ERR(svn_io_file_flush(context->proto_l2p_index, scratch_pool));
+  SVN_ERR(svn_io_append_file(temp_name, proto_index, scratch_pool));
+  SVN_ERR(svn_io_remove_file2(temp_name, FALSE, scratch_pool));
+  SVN_ERR(svn_io_file_seek(context->proto_l2p_index, APR_END, &offset,
+                           scratch_pool));
+
+  /* Done. */
+  svn_pool_destroy(scratch_pool);
+
+  return SVN_NO_ERROR;
+}
+
+/* Pack the current revision range of CONTEXT, i.e. this covers phases 2
+ * to 4.  Use SCRATCH_POOL for temporary allocations.
+ */
+static svn_error_t *
+pack_range(pack_context_t *context,
+           apr_pool_t *scratch_pool)
+{
+  svn_fs_x__data_t *ffd = context->fs->fsap_data;
+  apr_pool_t *revpool = svn_pool_create(scratch_pool);
+  apr_pool_t *iterpool = svn_pool_create(scratch_pool);
+
+  /* Phase 2: Copy items into various buckets and build tracking info */
+  svn_revnum_t revision;
+  for (revision = context->start_rev; revision < context->end_rev; ++revision)
+    {
+      apr_off_t offset = 0;
+      svn_fs_x__revision_file_t *rev_file;
+
+      /* Get the rev file dimensions (mainly index locations). */
+      SVN_ERR(svn_fs_x__open_pack_or_rev_file(&rev_file, context->fs,
+                                              revision, revpool, iterpool));
+      SVN_ERR(svn_fs_x__auto_read_footer(rev_file));
+
+      /* store the indirect array index */
+      APR_ARRAY_PUSH(context->rev_offsets, int) = context->reps->nelts;
+
+      /* read the phys-to-log index file until we covered the whole rev file.
+       * That index contains enough info to build both target indexes from it. */
+      while (offset < rev_file->l2p_offset)
+        {
+          /* read one cluster */
+          int i;
+          apr_array_header_t *entries;
+          svn_pool_clear(iterpool);
+
+          SVN_ERR(svn_fs_x__p2l_index_lookup(&entries, context->fs,
+                                             rev_file, revision, offset,
+                                             ffd->p2l_page_size, iterpool,
+                                             iterpool));
+
+          for (i = 0; i < entries->nelts; ++i)
+            {
+              svn_fs_x__p2l_entry_t *entry
+                = &APR_ARRAY_IDX(entries, i, svn_fs_x__p2l_entry_t);
+
+              /* skip first entry if that was duplicated due crossing a
+                 cluster boundary */
+              if (offset > entry->offset)
+                continue;
+
+              /* process entry while inside the rev file */
+              offset = entry->offset;
+              if (offset < rev_file->l2p_offset)
+                {
+                  SVN_ERR(svn_io_file_seek(rev_file->file, APR_SET, &offset,
+                                           iterpool));
+
+                  if (entry->type == SVN_FS_X__ITEM_TYPE_CHANGES)
+                    SVN_ERR(copy_item_to_temp(context,
+                                              context->changes,
+                                              context->changes_file,
+                                              rev_file, entry, iterpool));
+                  else if (entry->type == SVN_FS_X__ITEM_TYPE_FILE_PROPS)
+                    SVN_ERR(copy_item_to_temp(context,
+                                              context->file_props,
+                                              context->file_props_file,
+                                              rev_file, entry, iterpool));
+                  else if (entry->type == SVN_FS_X__ITEM_TYPE_DIR_PROPS)
+                    SVN_ERR(copy_item_to_temp(context,
+                                              context->dir_props,
+                                              context->dir_props_file,
+                                              rev_file, entry, iterpool));
+                  else if (   entry->type == SVN_FS_X__ITEM_TYPE_FILE_REP
+                           || entry->type == SVN_FS_X__ITEM_TYPE_DIR_REP)
+                    SVN_ERR(copy_rep_to_temp(context, rev_file, entry,
+                                             iterpool));
+                  else if (entry->type == SVN_FS_X__ITEM_TYPE_NODEREV)
+                    SVN_ERR(copy_node_to_temp(context, rev_file, entry,
+                                              iterpool));
+                  else
+                    SVN_ERR_ASSERT(entry->type == SVN_FS_X__ITEM_TYPE_UNUSED);
+
+                  offset += entry->size;
+                }
+            }
+
+          if (context->cancel_func)
+            SVN_ERR(context->cancel_func(context->cancel_baton));
+        }
+
+      svn_pool_clear(revpool);
+    }
+
+  svn_pool_destroy(iterpool);
+
+  /* phase 3: placement.
+   * Use "newest first" placement for simple items. */
+  sort_items(context->changes);
+  sort_items(context->file_props);
+  sort_items(context->dir_props);
+
+  /* follow dependencies recursively for noderevs and data representations */
+  sort_reps(context);
+
+  /* phase 4: copy bucket data to pack file.  Write P2L index. */
+  SVN_ERR(write_changes_containers(context, context->changes,
+                                   context->changes_file, revpool));
+  svn_pool_clear(revpool);
+  SVN_ERR(write_property_containers(context, context->file_props,
+                                    context->file_props_file, revpool));
+  svn_pool_clear(revpool);
+  SVN_ERR(write_property_containers(context, context->dir_props,
+                                    context->dir_props_file, revpool));
+  svn_pool_clear(revpool);
+  SVN_ERR(copy_reps_from_temp(context, context->reps_file, revpool));
+  svn_pool_clear(revpool);
+
+  /* write L2P index as well (now that we know all target offsets) */
+  SVN_ERR(write_l2p_index(context, revpool));
+
+  svn_pool_destroy(revpool);
+
+  return SVN_NO_ERROR;
+}
+
+/* Append CONTEXT->START_REV to the context's pack file with no re-ordering.
+ * This function will only be used for very large revisions (>>100k changes).
+ * Use SCRATCH_POOL for temporary allocations.
+ */
+static svn_error_t *
+append_revision(pack_context_t *context,
+                apr_pool_t *scratch_pool)
+{
+  svn_fs_x__data_t *ffd = context->fs->fsap_data;
+  apr_off_t offset = 0;
+  apr_pool_t *iterpool = svn_pool_create(scratch_pool);
+  svn_fs_x__revision_file_t *rev_file;
+  apr_finfo_t finfo;
+
+  /* Get the size of the file. */
+  const char *path = svn_dirent_join(context->shard_dir,
+                                     apr_psprintf(iterpool, "%ld",
+                                                  context->start_rev),
+                                     scratch_pool);
+  SVN_ERR(svn_io_stat(&finfo, path, APR_FINFO_SIZE, scratch_pool));
+
+  /* Copy all the bits from the rev file to the end of the pack file. */
+  SVN_ERR(svn_fs_x__open_pack_or_rev_file(&rev_file, context->fs,
+                                          context->start_rev, scratch_pool,
+                                          iterpool));
+  SVN_ERR(copy_file_data(context, context->pack_file, rev_file->file,
+                         finfo.size, iterpool));
+
+  /* mark the start of a new revision */
+  SVN_ERR(svn_fs_x__l2p_proto_index_add_revision(context->proto_l2p_index,
+                                                 scratch_pool));
+
+  /* read the phys-to-log index file until we covered the whole rev file.
+   * That index contains enough info to build both target indexes from it. */
+  while (offset < finfo.size)
+    {
+      /* read one cluster */
+      int i;
+      apr_array_header_t *entries;
+      SVN_ERR(svn_fs_x__p2l_index_lookup(&entries, context->fs, rev_file,
+                                         context->start_rev, offset,
+                                         ffd->p2l_page_size, iterpool,
+                                         iterpool));
+
+      for (i = 0; i < entries->nelts; ++i)
+        {
+          svn_fs_x__p2l_entry_t *entry
+            = &APR_ARRAY_IDX(entries, i, svn_fs_x__p2l_entry_t);
+
+          /* skip first entry if that was duplicated due crossing a
+             cluster boundary */
+          if (offset > entry->offset)
+            continue;
+
+          /* process entry while inside the rev file */
+          offset = entry->offset;
+          if (offset < finfo.size)
+            {
+              /* there should be true containers */
+              SVN_ERR_ASSERT(entry->item_count == 1);
+
+              entry->offset += context->pack_offset;
+              offset += entry->size;
+              SVN_ERR(svn_fs_x__l2p_proto_index_add_entry
+                        (context->proto_l2p_index, entry->offset, 0,
+                         entry->items[0].number, iterpool));
+              SVN_ERR(svn_fs_x__p2l_proto_index_add_entry
+                        (context->proto_p2l_index, entry, iterpool));
+            }
+        }
+
+      svn_pool_clear(iterpool);
+    }
+
+  svn_pool_destroy(iterpool);
+  context->pack_offset += finfo.size;
+
+  return SVN_NO_ERROR;
+}
+
+/* Format 7 packing logic.
+ *
+ * Pack the revision shard starting at SHARD_REV in filesystem FS from
+ * SHARD_DIR into the PACK_FILE_DIR, using SCRATCH_POOL for temporary
+ * allocations.  Limit the extra memory consumption to MAX_MEM bytes.
+ * CANCEL_FUNC and CANCEL_BATON are what you think they are.
+ */
+static svn_error_t *
+pack_log_addressed(svn_fs_t *fs,
+                   const char *pack_file_dir,
+                   const char *shard_dir,
+                   svn_revnum_t shard_rev,
+                   apr_size_t max_mem,
+                   svn_cancel_func_t cancel_func,
+                   void *cancel_baton,
+                   apr_pool_t *scratch_pool)
+{
+  enum
+    {
+      /* estimated amount of memory used to represent one item in memory
+       * during rev file packing */
+      PER_ITEM_MEM = APR_ALIGN_DEFAULT(sizeof(path_order_t))
+                   + APR_ALIGN_DEFAULT(2 *sizeof(void*))
+                   + APR_ALIGN_DEFAULT(sizeof(reference_t))
+                   + APR_ALIGN_DEFAULT(sizeof(svn_fs_x__p2l_entry_t))
+                   + 6 * sizeof(void*)
+    };
+
+  int max_items = max_mem / PER_ITEM_MEM > INT_MAX
+                ? INT_MAX
+                : (int)(max_mem / PER_ITEM_MEM);
+  apr_array_header_t *max_ids;
+  pack_context_t context = { 0 };
+  int i;
+  apr_size_t item_count = 0;
+  apr_pool_t *iterpool = svn_pool_create(scratch_pool);
+
+  /* set up a pack context */
+  SVN_ERR(initialize_pack_context(&context, fs, pack_file_dir, shard_dir,
+                                  shard_rev, max_items, cancel_func,
+                                  cancel_baton, scratch_pool));
+
+  /* phase 1: determine the size of the revisions to pack */
+  SVN_ERR(svn_fs_x__l2p_get_max_ids(&max_ids, fs, shard_rev,
+                                    context.shard_end_rev - shard_rev,
+                                    scratch_pool, scratch_pool));
+
+  /* pack revisions in ranges that don't exceed MAX_MEM */
+  for (i = 0; i < max_ids->nelts; ++i)
+    if (APR_ARRAY_IDX(max_ids, i, apr_uint64_t) + item_count <= max_items)
+      {
+        context.end_rev++;
+      }
+    else
+      {
+        /* some unpacked revisions before this one? */
+        if (context.start_rev < context.end_rev)
+          {
+            /* pack them intelligently (might be just 1 rev but still ...) */
+            SVN_ERR(pack_range(&context, iterpool));
+            SVN_ERR(reset_pack_context(&context, iterpool));
+            item_count = 0;
+          }
+
+        /* next revision range is to start with the current revision */
+        context.start_rev = i + context.shard_rev;
+        context.end_rev = context.start_rev + 1;
+
+        /* if this is a very large revision, we must place it as is */
+        if (APR_ARRAY_IDX(max_ids, i, apr_uint64_t) > max_items)
+          {
+            SVN_ERR(append_revision(&context, iterpool));
+            context.start_rev++;
+          }
+        else
+          item_count += (apr_size_t)APR_ARRAY_IDX(max_ids, i, apr_uint64_t);
+
+        svn_pool_clear(iterpool);
+      }
+
+  /* non-empty revision range at the end? */
+  if (context.start_rev < context.end_rev)
+    SVN_ERR(pack_range(&context, iterpool));
+
+  /* last phase: finalize indexes and clean up */
+  SVN_ERR(reset_pack_context(&context, iterpool));
+  SVN_ERR(close_pack_context(&context, iterpool));
+  svn_pool_destroy(iterpool);
+
+  return SVN_NO_ERROR;
+}
+
+/* Given REV in FS, set *REV_OFFSET to REV's offset in the packed file.
+   Use SCRATCH_POOL for temporary allocations. */
+svn_error_t *
+svn_fs_x__get_packed_offset(apr_off_t *rev_offset,
+                            svn_fs_t *fs,
+                            svn_revnum_t rev,
+                            apr_pool_t *scratch_pool)
+{
+  svn_fs_x__data_t *ffd = fs->fsap_data;
+  svn_stream_t *manifest_stream;
+  svn_boolean_t is_cached;
+  svn_revnum_t shard;
+  apr_int64_t shard_pos;
+  apr_array_header_t *manifest;
+  apr_pool_t *iterpool;
+
+  shard = rev / ffd->max_files_per_dir;
+
+  /* position of the shard within the manifest */
+  shard_pos = rev % ffd->max_files_per_dir;
+
+  /* fetch exactly that element into *rev_offset, if the manifest is found
+     in the cache */
+  SVN_ERR(svn_cache__get_partial((void **) rev_offset, &is_cached,
+                                 ffd->packed_offset_cache, &shard,
+                                 svn_fs_x__get_sharded_offset, &shard_pos,
+                                 scratch_pool));
+
+  if (is_cached)
+      return SVN_NO_ERROR;
+
+  /* Open the manifest file. */
+  SVN_ERR(svn_stream_open_readonly(&manifest_stream,
+                    svn_fs_x__path_rev_packed(fs, rev, PATH_MANIFEST,
+                                              scratch_pool),
+                    scratch_pool, scratch_pool));
+
+  /* While we're here, let's just read the entire manifest file into an array,
+     so we can cache the entire thing. */
+  iterpool = svn_pool_create(scratch_pool);
+  manifest = apr_array_make(scratch_pool, ffd->max_files_per_dir,
+                            sizeof(apr_off_t));
+  while (1)
+    {
+      svn_boolean_t eof;
+      apr_int64_t val;
+
+      svn_pool_clear(iterpool);
+      SVN_ERR(svn_fs_x__read_number_from_stream(&val, &eof, manifest_stream,
+                                                iterpool));
+      if (eof)
+        break;
+
+      APR_ARRAY_PUSH(manifest, apr_off_t) = (apr_off_t)val;
+    }
+  svn_pool_destroy(iterpool);
+
+  *rev_offset = APR_ARRAY_IDX(manifest, rev % ffd->max_files_per_dir,
+                              apr_off_t);
+
+  /* Close up shop and cache the array. */
+  SVN_ERR(svn_stream_close(manifest_stream));
+  return svn_cache__set(ffd->packed_offset_cache, &shard, manifest,
+                        scratch_pool);
+}
+
+/* In filesystem FS, pack the revision SHARD containing exactly
+ * MAX_FILES_PER_DIR revisions from SHARD_PATH into the PACK_FILE_DIR,
+ * using SCRATCH_POOL for temporary allocations.  Try to limit the amount of
+ * temporary memory needed to MAX_MEM bytes.  CANCEL_FUNC and CANCEL_BATON
+ * are what you think they are.
+ *
+ * If for some reason we detect a partial packing already performed, we
+ * remove the pack file and start again.
+ *
+ * The actual packing will be done in a format-specific sub-function.
+ */
+static svn_error_t *
+pack_rev_shard(svn_fs_t *fs,
+               const char *pack_file_dir,
+               const char *shard_path,
+               apr_int64_t shard,
+               int max_files_per_dir,
+               apr_size_t max_mem,
+               svn_cancel_func_t cancel_func,
+               void *cancel_baton,
+               apr_pool_t *scratch_pool)
+{
+  const char *pack_file_path;
+  svn_revnum_t shard_rev = (svn_revnum_t) (shard * max_files_per_dir);
+
+  /* Some useful paths. */
+  pack_file_path = svn_dirent_join(pack_file_dir, PATH_PACKED, scratch_pool);
+
+  /* Remove any existing pack file for this shard, since it is incomplete. */
+  SVN_ERR(svn_io_remove_dir2(pack_file_dir, TRUE, cancel_func, cancel_baton,
+                             scratch_pool));
+
+  /* Create the new directory and pack file. */
+  SVN_ERR(svn_io_dir_make(pack_file_dir, APR_OS_DEFAULT, scratch_pool));
+
+  /* Index information files */
+  SVN_ERR(pack_log_addressed(fs, pack_file_dir, shard_path, shard_rev,
+                              max_mem, cancel_func, cancel_baton,
+                             scratch_pool));
+
+  SVN_ERR(svn_io_copy_perms(shard_path, pack_file_dir, scratch_pool));
+  SVN_ERR(svn_io_set_file_read_only(pack_file_path, FALSE, scratch_pool));
+
+  return SVN_NO_ERROR;
+}
+
+/* In the file system at FS_PATH, pack the SHARD in REVS_DIR and
+ * REVPROPS_DIR containing exactly MAX_FILES_PER_DIR revisions, using
+ * SCRATCH_POOL temporary for allocations.  REVPROPS_DIR will be NULL if
+ * revprop packing is not supported.  COMPRESSION_LEVEL and MAX_PACK_SIZE
+ * will be ignored in that case.
+ *
+ * CANCEL_FUNC and CANCEL_BATON are what you think they are; similarly
+ * NOTIFY_FUNC and NOTIFY_BATON.
+ *
+ * If for some reason we detect a partial packing already performed, we
+ * remove the pack file and start again.
+ */
+static svn_error_t *
+pack_shard(const char *revs_dir,
+           const char *revsprops_dir,
+           svn_fs_t *fs,
+           apr_int64_t shard,
+           int max_files_per_dir,
+           apr_off_t max_pack_size,
+           int compression_level,
+           svn_fs_pack_notify_t notify_func,
+           void *notify_baton,
+           svn_cancel_func_t cancel_func,
+           void *cancel_baton,
+           apr_pool_t *scratch_pool)
+{
+  svn_fs_x__data_t *ffd = fs->fsap_data;
+  const char *rev_shard_path, *rev_pack_file_dir;
+  const char *revprops_shard_path, *revprops_pack_file_dir;
+
+  /* Notify caller we're starting to pack this shard. */
+  if (notify_func)
+    SVN_ERR(notify_func(notify_baton, shard, svn_fs_pack_notify_start,
+                        scratch_pool));
+
+  /* Some useful paths. */
+  rev_pack_file_dir = svn_dirent_join(revs_dir,
+                  apr_psprintf(scratch_pool,
+                               "%" APR_INT64_T_FMT PATH_EXT_PACKED_SHARD,
+                               shard),
+                  scratch_pool);
+  rev_shard_path = svn_dirent_join(revs_dir,
+                      apr_psprintf(scratch_pool, "%" APR_INT64_T_FMT, shard),
+                      scratch_pool);
+
+  /* pack the revision content */
+  SVN_ERR(pack_rev_shard(fs, rev_pack_file_dir, rev_shard_path,
+                         shard, max_files_per_dir, DEFAULT_MAX_MEM,
+                         cancel_func, cancel_baton, scratch_pool));
+
+  /* if enabled, pack the revprops in an equivalent way */
+  if (revsprops_dir)
+    {
+      revprops_pack_file_dir = svn_dirent_join(revsprops_dir,
+                   apr_psprintf(scratch_pool,
+                                "%" APR_INT64_T_FMT PATH_EXT_PACKED_SHARD,
+                                shard),
+                   scratch_pool);
+      revprops_shard_path = svn_dirent_join(revsprops_dir,
+                   apr_psprintf(scratch_pool, "%" APR_INT64_T_FMT, shard),
+                   scratch_pool);
+
+      SVN_ERR(svn_fs_x__pack_revprops_shard(revprops_pack_file_dir,
+                                            revprops_shard_path,
+                                            shard, max_files_per_dir,
+                                            (int)(0.9 * max_pack_size),
+                                            compression_level,
+                                            cancel_func, cancel_baton,
+                                            scratch_pool));
+    }
+
+  /* Update the min-unpacked-rev file to reflect our newly packed shard. */
+  SVN_ERR(svn_fs_x__write_min_unpacked_rev(fs,
+                          (svn_revnum_t)((shard + 1) * max_files_per_dir),
+                          scratch_pool));
+  ffd->min_unpacked_rev = (svn_revnum_t)((shard + 1) * max_files_per_dir);
+
+  /* Finally, remove the existing shard directories.
+   * For revprops, clean up older obsolete shards as well as they might
+   * have been left over from an interrupted FS upgrade. */
+  SVN_ERR(svn_io_remove_dir2(rev_shard_path, TRUE,
+                             cancel_func, cancel_baton, scratch_pool));
+  if (revsprops_dir)
+    {
+      svn_node_kind_t kind = svn_node_dir;
+      apr_int64_t to_cleanup = shard;
+      do
+        {
+          SVN_ERR(svn_fs_x__delete_revprops_shard(revprops_shard_path,
+                                                  to_cleanup,
+                                                  max_files_per_dir,
+                                                  cancel_func, cancel_baton,
+                                                  scratch_pool));
+
+          /* If the previous shard exists, clean it up as well.
+             Don't try to clean up shard 0 as it we can't tell quickly
+             whether it actually needs cleaning up. */
+          revprops_shard_path = svn_dirent_join(revsprops_dir,
+                                                apr_psprintf(scratch_pool,
+                                                          "%" APR_INT64_T_FMT,
+                                                          --to_cleanup),
+                                                scratch_pool);
+          SVN_ERR(svn_io_check_path(revprops_shard_path, &kind, scratch_pool));
+        }
+      while (kind == svn_node_dir && to_cleanup > 0);
+    }
+
+  /* Notify caller we're starting to pack this shard. */
+  if (notify_func)
+    SVN_ERR(notify_func(notify_baton, shard, svn_fs_pack_notify_end,
+                        scratch_pool));
+
+  return SVN_NO_ERROR;
+}
+
+typedef struct pack_baton_t
+{
+  svn_fs_t *fs;
+  svn_fs_pack_notify_t notify_func;
+  void *notify_baton;
+  svn_cancel_func_t cancel_func;
+  void *cancel_baton;
+} pack_baton_t;
+
+
+/* The work-horse for svn_fs_x__pack, called with the FS write lock.
+   This implements the svn_fs_x__with_write_lock() 'body' callback
+   type.  BATON is a 'pack_baton_t *'.
+
+   WARNING: if you add a call to this function, please note:
+     The code currently assumes that any piece of code running with
+     the write-lock set can rely on the ffd->min_unpacked_rev and
+     ffd->min_unpacked_revprop caches to be up-to-date (and, by
+     extension, on not having to use a retry when calling
+     svn_fs_x__path_rev_absolute() and friends).  If you add a call
+     to this function, consider whether you have to call
+     update_min_unpacked_rev().
+     See this thread: http://thread.gmane.org/1291206765.3782.3309.camel@edith
+ */
+static svn_error_t *
+pack_body(void *baton,
+          apr_pool_t *scratch_pool)
+{
+  pack_baton_t *pb = baton;
+  svn_fs_x__data_t *ffd = pb->fs->fsap_data;
+  apr_int64_t completed_shards;
+  apr_int64_t i;
+  svn_revnum_t youngest;
+  apr_pool_t *iterpool;
+  const char *rev_data_path;
+  const char *revprops_data_path = NULL;
+
+  /* If we aren't using sharding, we can't do any packing, so quit. */
+  SVN_ERR(svn_fs_x__read_min_unpacked_rev(&ffd->min_unpacked_rev, pb->fs,
+                                          scratch_pool));
+
+  SVN_ERR(svn_fs_x__youngest_rev(&youngest, pb->fs, scratch_pool));
+  completed_shards = (youngest + 1) / ffd->max_files_per_dir;
+
+  /* See if we've already completed all possible shards thus far. */
+  if (ffd->min_unpacked_rev == (completed_shards * ffd->max_files_per_dir))
+    return SVN_NO_ERROR;
+
+  rev_data_path = svn_dirent_join(pb->fs->path, PATH_REVS_DIR, scratch_pool);
+  revprops_data_path = svn_dirent_join(pb->fs->path, PATH_REVPROPS_DIR,
+                                        scratch_pool);
+
+  iterpool = svn_pool_create(scratch_pool);
+  for (i = ffd->min_unpacked_rev / ffd->max_files_per_dir;
+       i < completed_shards;
+       i++)
+    {
+      svn_pool_clear(iterpool);
+
+      if (pb->cancel_func)
+        SVN_ERR(pb->cancel_func(pb->cancel_baton));
+
+      SVN_ERR(pack_shard(rev_data_path, revprops_data_path,
+                         pb->fs, i, ffd->max_files_per_dir,
+                         ffd->revprop_pack_size,
+                         ffd->compress_packed_revprops
+                           ? SVN__COMPRESSION_ZLIB_DEFAULT
+                           : SVN__COMPRESSION_NONE,
+                         pb->notify_func, pb->notify_baton,
+                         pb->cancel_func, pb->cancel_baton, iterpool));
+    }
+
+  svn_pool_destroy(iterpool);
+  return SVN_NO_ERROR;
+}
+
+svn_error_t *
+svn_fs_x__pack(svn_fs_t *fs,
+               svn_fs_pack_notify_t notify_func,
+               void *notify_baton,
+               svn_cancel_func_t cancel_func,
+               void *cancel_baton,
+               apr_pool_t *scratch_pool)
+{
+  pack_baton_t pb = { 0 };
+  pb.fs = fs;
+  pb.notify_func = notify_func;
+  pb.notify_baton = notify_baton;
+  pb.cancel_func = cancel_func;
+  pb.cancel_baton = cancel_baton;
+  return svn_fs_x__with_pack_lock(fs, pack_body, &pb, scratch_pool);
+}