Merge branch 'ds/multi-pack-index'

When there are too many packfiles in a repository (which is not
recommended), looking up an object in these would require
consulting many pack .idx files; a new mechanism to have a single
file that consolidates all of these .idx files is introduced.

* ds/multi-pack-index: (32 commits)
  pack-objects: consider packs in multi-pack-index
  midx: test a few commands that use get_all_packs
  treewide: use get_all_packs
  packfile: add all_packs list
  midx: fix bug that skips midx with alternates
  midx: stop reporting garbage
  midx: mark bad packed objects
  multi-pack-index: store local property
  multi-pack-index: provide more helpful usage info
  midx: clear midx on repack
  packfile: skip loading index if in multi-pack-index
  midx: prevent duplicate packfile loads
  midx: use midx in approximate_object_count
  midx: use existing midx when writing new one
  midx: use midx in abbreviation calculations
  midx: read objects from multi-pack-index
  config: create core.multiPackIndex setting
  midx: write object offsets
  midx: write object id fanout chunk
  midx: write object ids in a chunk
  ...

31 files changed, 1859 insertions, 66 deletions

diff --git a/.gitignore b/.gitignore
index ffceea7d59..9d1363a1eb 100644
--- a/.gitignore
+++ b/.gitignore
@@ -99,8 +99,9 @@
 /git-mergetool--lib
 /git-mktag
 /git-mktree
-/git-name-rev
+/git-multi-pack-index
 /git-mv
+/git-name-rev
 /git-notes
 /git-p4
 /git-pack-redundant
diff --git a/Documentation/config.txt b/Documentation/config.txt
index 69a27eb688..6ecd70df0a 100644
--- a/Documentation/config.txt
+++ b/Documentation/config.txt
@@ -937,6 +937,11 @@ core.useReplaceRefs::
 	option was given on the command line. See linkgit:git[1] and
 	linkgit:git-replace[1] for more information.
 
+core.multiPackIndex::
+	Use the multi-pack-index file to track multiple packfiles using a
+	single index. See link:technical/multi-pack-index.html[the
+	multi-pack-index design document].
+
 core.sparseCheckout::
 	Enable "sparse checkout" feature. See section "Sparse checkout" in
 	linkgit:git-read-tree[1] for more information.
diff --git a/Documentation/git-multi-pack-index.txt b/Documentation/git-multi-pack-index.txt
new file mode 100644
index 0000000000..1f97e79912
--- /dev/null
+++ b/Documentation/git-multi-pack-index.txt
@@ -0,0 +1,56 @@
+git-multi-pack-index(1)
+=======================
+
+NAME
+----
+git-multi-pack-index - Write and verify multi-pack-indexes
+
+
+SYNOPSIS
+--------
+[verse]
+'git multi-pack-index' [--object-dir=<dir>] <verb>
+
+DESCRIPTION
+-----------
+Write or verify a multi-pack-index (MIDX) file.
+
+OPTIONS
+-------
+
+--object-dir=<dir>::
+	Use given directory for the location of Git objects. We check
+	`<dir>/packs/multi-pack-index` for the current MIDX file, and
+	`<dir>/packs` for the pack-files to index.
+
+write::
+	When given as the verb, write a new MIDX file to
+	`<dir>/packs/multi-pack-index`.
+
+
+EXAMPLES
+--------
+
+* Write a MIDX file for the packfiles in the current .git folder.
++
+-----------------------------------------------
+$ git multi-pack-index write
+-----------------------------------------------
+
+* Write a MIDX file for the packfiles in an alternate object store.
++
+-----------------------------------------------
+$ git multi-pack-index --object-dir <alt> write
+-----------------------------------------------
+
+
+SEE ALSO
+--------
+See link:technical/multi-pack-index.html[The Multi-Pack-Index Design
+Document] and link:technical/pack-format.html[The Multi-Pack-Index
+Format] for more information on the multi-pack-index feature.
+
+
+GIT
+---
+Part of the linkgit:git[1] suite
diff --git a/Documentation/technical/multi-pack-index.txt b/Documentation/technical/multi-pack-index.txt
new file mode 100644
index 0000000000..d7e57639f7
--- /dev/null
+++ b/Documentation/technical/multi-pack-index.txt
@@ -0,0 +1,109 @@
+Multi-Pack-Index (MIDX) Design Notes
+====================================
+
+The Git object directory contains a 'pack' directory containing
+packfiles (with suffix ".pack") and pack-indexes (with suffix
+".idx"). The pack-indexes provide a way to lookup objects and
+navigate to their offset within the pack, but these must come
+in pairs with the packfiles. This pairing depends on the file
+names, as the pack-index differs only in suffix with its pack-
+file. While the pack-indexes provide fast lookup per packfile,
+this performance degrades as the number of packfiles increases,
+because abbreviations need to inspect every packfile and we are
+more likely to have a miss on our most-recently-used packfile.
+For some large repositories, repacking into a single packfile
+is not feasible due to storage space or excessive repack times.
+
+The multi-pack-index (MIDX for short) stores a list of objects
+and their offsets into multiple packfiles. It contains:
+
+- A list of packfile names.
+- A sorted list of object IDs.
+- A list of metadata for the ith object ID including:
+  - A value j referring to the jth packfile.
+  - An offset within the jth packfile for the object.
+- If large offsets are required, we use another list of large
+  offsets similar to version 2 pack-indexes.
+
+Thus, we can provide O(log N) lookup time for any number
+of packfiles.
+
+Design Details
+--------------
+
+- The MIDX is stored in a file named 'multi-pack-index' in the
+  .git/objects/pack directory. This could be stored in the pack
+  directory of an alternate. It refers only to packfiles in that
+  same directory.
+
+- The pack.multiIndex config setting must be on to consume MIDX files.
+
+- The file format includes parameters for the object ID hash
+  function, so a future change of hash algorithm does not require
+  a change in format.
+
+- The MIDX keeps only one record per object ID. If an object appears
+  in multiple packfiles, then the MIDX selects the copy in the most-
+  recently modified packfile.
+
+- If there exist packfiles in the pack directory not registered in
+  the MIDX, then those packfiles are loaded into the `packed_git`
+  list and `packed_git_mru` cache.
+
+- The pack-indexes (.idx files) remain in the pack directory so we
+  can delete the MIDX file, set core.midx to false, or downgrade
+  without any loss of information.
+
+- The MIDX file format uses a chunk-based approach (similar to the
+  commit-graph file) that allows optional data to be added.
+
+Future Work
+-----------
+
+- Add a 'verify' subcommand to the 'git midx' builtin to verify the
+  contents of the multi-pack-index file match the offsets listed in
+  the corresponding pack-indexes.
+
+- The multi-pack-index allows many packfiles, especially in a context
+  where repacking is expensive (such as a very large repo), or
+  unexpected maintenance time is unacceptable (such as a high-demand
+  build machine). However, the multi-pack-index needs to be rewritten
+  in full every time. We can extend the format to be incremental, so
+  writes are fast. By storing a small "tip" multi-pack-index that
+  points to large "base" MIDX files, we can keep writes fast while
+  still reducing the number of binary searches required for object
+  lookups.
+
+- The reachability bitmap is currently paired directly with a single
+  packfile, using the pack-order as the object order to hopefully
+  compress the bitmaps well using run-length encoding. This could be
+  extended to pair a reachability bitmap with a multi-pack-index. If
+  the multi-pack-index is extended to store a "stable object order"
+  (a function Order(hash) = integer that is constant for a given hash,
+  even as the multi-pack-index is updated) then a reachability bitmap
+  could point to a multi-pack-index and be updated independently.
+
+- Packfiles can be marked as "special" using empty files that share
+  the initial name but replace ".pack" with ".keep" or ".promisor".
+  We can add an optional chunk of data to the multi-pack-index that
+  records flags of information about the packfiles. This allows new
+  states, such as 'repacked' or 'redeltified', that can help with
+  pack maintenance in a multi-pack environment. It may also be
+  helpful to organize packfiles by object type (commit, tree, blob,
+  etc.) and use this metadata to help that maintenance.
+
+- The partial clone feature records special "promisor" packs that
+  may point to objects that are not stored locally, but available
+  on request to a server. The multi-pack-index does not currently
+  track these promisor packs.
+
+Related Links
+-------------
+[0] https://bugs.chromium.org/p/git/issues/detail?id=6
+    Chromium work item for: Multi-Pack Index (MIDX)
+
+[1] https://public-inbox.org/git/20180107181459.222909-1-dstolee@microsoft.com/
+    An earlier RFC for the multi-pack-index feature
+
+[2] https://public-inbox.org/git/alpine.DEB.2.20.1803091557510.23109@alexmv-linux/
+    Git Merge 2018 Contributor's summit notes (includes discussion of MIDX)
diff --git a/Documentation/technical/pack-format.txt b/Documentation/technical/pack-format.txt
index 70a99fd142..cab5bdd2ff 100644
--- a/Documentation/technical/pack-format.txt
+++ b/Documentation/technical/pack-format.txt
@@ -252,3 +252,80 @@ Pack file entry: <+
     corresponding packfile.
 
     20-byte SHA-1-checksum of all of the above.
+
+== multi-pack-index (MIDX) files have the following format:
+
+The multi-pack-index files refer to multiple pack-files and loose objects.
+
+In order to allow extensions that add extra data to the MIDX, we organize
+the body into "chunks" and provide a lookup table at the beginning of the
+body. The header includes certain length values, such as the number of packs,
+the number of base MIDX files, hash lengths and types.
+
+All 4-byte numbers are in network order.
+
+HEADER:
+
+	4-byte signature:
+	    The signature is: {'M', 'I', 'D', 'X'}
+
+	1-byte version number:
+	    Git only writes or recognizes version 1.
+
+	1-byte Object Id Version
+	    Git only writes or recognizes version 1 (SHA1).
+
+	1-byte number of "chunks"
+
+	1-byte number of base multi-pack-index files:
+	    This value is currently always zero.
+
+	4-byte number of pack files
+
+CHUNK LOOKUP:
+
+	(C + 1) * 12 bytes providing the chunk offsets:
+	    First 4 bytes describe chunk id. Value 0 is a terminating label.
+	    Other 8 bytes provide offset in current file for chunk to start.
+	    (Chunks are provided in file-order, so you can infer the length
+	    using the next chunk position if necessary.)
+
+	The remaining data in the body is described one chunk at a time, and
+	these chunks may be given in any order. Chunks are required unless
+	otherwise specified.
+
+CHUNK DATA:
+
+	Packfile Names (ID: {'P', 'N', 'A', 'M'})
+	    Stores the packfile names as concatenated, null-terminated strings.
+	    Packfiles must be listed in lexicographic order for fast lookups by
+	    name. This is the only chunk not guaranteed to be a multiple of four
+	    bytes in length, so should be the last chunk for alignment reasons.
+
+	OID Fanout (ID: {'O', 'I', 'D', 'F'})
+	    The ith entry, F[i], stores the number of OIDs with first
+	    byte at most i. Thus F[255] stores the total
+	    number of objects.
+
+	OID Lookup (ID: {'O', 'I', 'D', 'L'})
+	    The OIDs for all objects in the MIDX are stored in lexicographic
+	    order in this chunk.
+
+	Object Offsets (ID: {'O', 'O', 'F', 'F'})
+	    Stores two 4-byte values for every object.
+	    1: The pack-int-id for the pack storing this object.
+	    2: The offset within the pack.
+		If all offsets are less than 2^31, then the large offset chunk
+		will not exist and offsets are stored as in IDX v1.
+		If there is at least one offset value larger than 2^32-1, then
+		the large offset chunk must exist. If the large offset chunk
+		exists and the 31st bit is on, then removing that bit reveals
+		the row in the large offsets containing the 8-byte offset of
+		this object.
+
+	[Optional] Object Large Offsets (ID: {'L', 'O', 'F', 'F'})
+	    8-byte offsets into large packfiles.
+
+TRAILER:
+
+	20-byte SHA1-checksum of the above contents.
diff --git a/Makefile b/Makefile
index 5a969f5830..bd83683a87 100644
--- a/Makefile
+++ b/Makefile
@@ -723,6 +723,7 @@ TEST_BUILTINS_OBJS += test-online-cpus.o
 TEST_BUILTINS_OBJS += test-path-utils.o
 TEST_BUILTINS_OBJS += test-prio-queue.o
 TEST_BUILTINS_OBJS += test-read-cache.o
+TEST_BUILTINS_OBJS += test-read-midx.o
 TEST_BUILTINS_OBJS += test-ref-store.o
 TEST_BUILTINS_OBJS += test-regex.o
 TEST_BUILTINS_OBJS += test-repository.o
@@ -900,6 +901,7 @@ LIB_OBJS += merge.o
 LIB_OBJS += merge-blobs.o
 LIB_OBJS += merge-recursive.o
 LIB_OBJS += mergesort.o
+LIB_OBJS += midx.o
 LIB_OBJS += name-hash.o
 LIB_OBJS += negotiator/default.o
 LIB_OBJS += negotiator/skipping.o
@@ -1060,6 +1062,7 @@ BUILTIN_OBJS += builtin/merge-recursive.o
 BUILTIN_OBJS += builtin/merge-tree.o
 BUILTIN_OBJS += builtin/mktag.o
 BUILTIN_OBJS += builtin/mktree.o
+BUILTIN_OBJS += builtin/multi-pack-index.o
 BUILTIN_OBJS += builtin/mv.o
 BUILTIN_OBJS += builtin/name-rev.o
 BUILTIN_OBJS += builtin/notes.o
diff --git a/builtin.h b/builtin.h
index 99206df4bd..962f0489ab 100644
--- a/builtin.h
+++ b/builtin.h
@@ -191,6 +191,7 @@ extern int cmd_merge_recursive(int argc, const char **argv, const char *prefix);
 extern int cmd_merge_tree(int argc, const char **argv, const char *prefix);
 extern int cmd_mktag(int argc, const char **argv, const char *prefix);
 extern int cmd_mktree(int argc, const char **argv, const char *prefix);
+extern int cmd_multi_pack_index(int argc, const char **argv, const char *prefix);
 extern int cmd_mv(int argc, const char **argv, const char *prefix);
 extern int cmd_name_rev(int argc, const char **argv, const char *prefix);
 extern int cmd_notes(int argc, const char **argv, const char *prefix);
diff --git a/builtin/count-objects.c b/builtin/count-objects.c
index d51e2ce1ec..a7cad052c6 100644
--- a/builtin/count-objects.c
+++ b/builtin/count-objects.c
@@ -123,7 +123,7 @@ int cmd_count_objects(int argc, const char **argv, const char *prefix)
 		struct strbuf pack_buf = STRBUF_INIT;
 		struct strbuf garbage_buf = STRBUF_INIT;
 
-		for (p = get_packed_git(the_repository); p; p = p->next) {
+		for (p = get_all_packs(the_repository); p; p = p->next) {
 			if (!p->pack_local)
 				continue;
 			if (open_pack_index(p))
diff --git a/builtin/fsck.c b/builtin/fsck.c
index 250f5af118..63c8578cc1 100644
--- a/builtin/fsck.c
+++ b/builtin/fsck.c
@@ -740,7 +740,7 @@ int cmd_fsck(int argc, const char **argv, const char *prefix)
 			struct progress *progress = NULL;
 
 			if (show_progress) {
-				for (p = get_packed_git(the_repository); p;
+				for (p = get_all_packs(the_repository); p;
 				     p = p->next) {
 					if (open_pack_index(p))
 						continue;
@@ -749,7 +749,7 @@ int cmd_fsck(int argc, const char **argv, const char *prefix)
 
 				progress = start_progress(_("Checking objects"), total);
 			}
-			for (p = get_packed_git(the_repository); p;
+			for (p = get_all_packs(the_repository); p;
 			     p = p->next) {
 				/* verify gives error messages itself */
 				if (verify_pack(p, fsck_obj_buffer,
diff --git a/builtin/gc.c b/builtin/gc.c
index 57069442b0..2b592260e9 100644
--- a/builtin/gc.c
+++ b/builtin/gc.c
@@ -183,7 +183,7 @@ static struct packed_git *find_base_packs(struct string_list *packs,
 {
 	struct packed_git *p, *base = NULL;
 
-	for (p = get_packed_git(the_repository); p; p = p->next) {
+	for (p = get_all_packs(the_repository); p; p = p->next) {
 		if (!p->pack_local)
 			continue;
 		if (limit) {
@@ -208,7 +208,7 @@ static int too_many_packs(void)
 	if (gc_auto_pack_limit <= 0)
 		return 0;
 
-	for (cnt = 0, p = get_packed_git(the_repository); p; p = p->next) {
+	for (cnt = 0, p = get_all_packs(the_repository); p; p = p->next) {
 		if (!p->pack_local)
 			continue;
 		if (p->pack_keep)
diff --git a/builtin/multi-pack-index.c b/builtin/multi-pack-index.c
new file mode 100644
index 0000000000..2633efd95d
--- /dev/null
+++ b/builtin/multi-pack-index.c
@@ -0,0 +1,47 @@
+#include "builtin.h"
+#include "cache.h"
+#include "config.h"
+#include "parse-options.h"
+#include "midx.h"
+
+static char const * const builtin_multi_pack_index_usage[] = {
+	N_("git multi-pack-index [--object-dir=<dir>] write"),
+	NULL
+};
+
+static struct opts_multi_pack_index {
+	const char *object_dir;
+} opts;
+
+int cmd_multi_pack_index(int argc, const char **argv,
+			 const char *prefix)
+{
+	static struct option builtin_multi_pack_index_options[] = {
+		OPT_FILENAME(0, "object-dir", &opts.object_dir,
+		  N_("object directory containing set of packfile and pack-index pairs")),
+		OPT_END(),
+	};
+
+	git_config(git_default_config, NULL);
+
+	argc = parse_options(argc, argv, prefix,
+			     builtin_multi_pack_index_options,
+			     builtin_multi_pack_index_usage, 0);
+
+	if (!opts.object_dir)
+		opts.object_dir = get_object_directory();
+
+	if (argc == 0)
+		usage_with_options(builtin_multi_pack_index_usage,
+				   builtin_multi_pack_index_options);
+
+	if (argc > 1) {
+		die(_("too many arguments"));
+		return 1;
+	}
+
+	if (!strcmp(argv[0], "write"))
+		return write_midx_file(opts.object_dir);
+
+	die(_("unrecognized verb: %s"), argv[0]);
+}
diff --git a/builtin/pack-objects.c b/builtin/pack-objects.c
index d1144a8f7e..caa4cd0211 100644
--- a/builtin/pack-objects.c
+++ b/builtin/pack-objects.c
@@ -31,6 +31,7 @@
 #include "packfile.h"
 #include "object-store.h"
 #include "dir.h"
+#include "midx.h"
 
 #define IN_PACK(obj) oe_in_pack(&to_pack, obj)
 #define SIZE(obj) oe_size(&to_pack, obj)
@@ -1040,6 +1041,7 @@ static int want_object_in_pack(const struct object_id *oid,
 {
 	int want;
 	struct list_head *pos;
+	struct multi_pack_index *m;
 
 	if (!exclude && local && has_loose_object_nonlocal(oid))
 		return 0;
@@ -1054,6 +1056,32 @@ static int want_object_in_pack(const struct object_id *oid,
 		if (want != -1)
 			return want;
 	}
+
+	for (m = get_multi_pack_index(the_repository); m; m = m->next) {
+		struct pack_entry e;
+		if (fill_midx_entry(oid, &e, m)) {
+			struct packed_git *p = e.p;
+			off_t offset;
+
+			if (p == *found_pack)
+				offset = *found_offset;
+			else
+				offset = find_pack_entry_one(oid->hash, p);
+
+			if (offset) {
+				if (!*found_pack) {
+					if (!is_pack_valid(p))
+						continue;
+					*found_offset = offset;
+					*found_pack = p;
+				}
+				want = want_found_object(exclude, p);
+				if (want != -1)
+					return want;
+			}
+		}
+	}
+
 	list_for_each(pos, get_packed_git_mru(the_repository)) {
 		struct packed_git *p = list_entry(pos, struct packed_git, mru);
 		off_t offset;
@@ -2806,7 +2834,7 @@ static void add_objects_in_unpacked_packs(struct rev_info *revs)
 
 	memset(&in_pack, 0, sizeof(in_pack));
 
-	for (p = get_packed_git(the_repository); p; p = p->next) {
+	for (p = get_all_packs(the_repository); p; p = p->next) {
 		struct object_id oid;
 		struct object *o;
 
@@ -2870,7 +2898,7 @@ static int has_sha1_pack_kept_or_nonlocal(const struct object_id *oid)
 	struct packed_git *p;
 
 	p = (last_found != (void *)1) ? last_found :
-					get_packed_git(the_repository);
+					get_all_packs(the_repository);
 
 	while (p) {
 		if ((!p->pack_local || p->pack_keep ||
@@ -2880,7 +2908,7 @@ static int has_sha1_pack_kept_or_nonlocal(const struct object_id *oid)
 			return 1;
 		}
 		if (p == last_found)
-			p = get_packed_git(the_repository);
+			p = get_all_packs(the_repository);
 		else
 			p = p->next;
 		if (p == last_found)
@@ -2916,7 +2944,7 @@ static void loosen_unused_packed_objects(struct rev_info *revs)
 	uint32_t i;
 	struct object_id oid;
 
-	for (p = get_packed_git(the_repository); p; p = p->next) {
+	for (p = get_all_packs(the_repository); p; p = p->next) {
 		if (!p->pack_local || p->pack_keep || p->pack_keep_in_core)
 			continue;
 
@@ -3063,7 +3091,7 @@ static void add_extra_kept_packs(const struct string_list *names)
 	if (!names->nr)
 		return;
 
-	for (p = get_packed_git(the_repository); p; p = p->next) {
+	for (p = get_all_packs(the_repository); p; p = p->next) {
 		const char *name = basename(p->pack_name);
 		int i;
 
@@ -3336,7 +3364,7 @@ int cmd_pack_objects(int argc, const char **argv, const char *prefix)
 	add_extra_kept_packs(&keep_pack_list);
 	if (ignore_packed_keep_on_disk) {
 		struct packed_git *p;
-		for (p = get_packed_git(the_repository); p; p = p->next)
+		for (p = get_all_packs(the_repository); p; p = p->next)
 			if (p->pack_local && p->pack_keep)
 				break;
 		if (!p) /* no keep-able packs found */
@@ -3349,7 +3377,7 @@ int cmd_pack_objects(int argc, const char **argv, const char *prefix)
 		 * it also covers non-local objects
 		 */
 		struct packed_git *p;
-		for (p = get_packed_git(the_repository); p; p = p->next) {
+		for (p = get_all_packs(the_repository); p; p = p->next) {
 			if (!p->pack_local) {
 				have_non_local_packs = 1;
 				break;
diff --git a/builtin/pack-redundant.c b/builtin/pack-redundant.c
index 0494dceff7..cf9a9aabd4 100644
--- a/builtin/pack-redundant.c
+++ b/builtin/pack-redundant.c
@@ -577,7 +577,7 @@ static struct pack_list * add_pack(struct packed_git *p)
 
 static struct pack_list * add_pack_file(const char *filename)
 {
-	struct packed_git *p = get_packed_git(the_repository);
+	struct packed_git *p = get_all_packs(the_repository);
 
 	if (strlen(filename) < 40)
 		die("Bad pack filename: %s", filename);
@@ -592,7 +592,7 @@ static struct pack_list * add_pack_file(const char *filename)
 
 static void load_all(void)
 {
-	struct packed_git *p = get_packed_git(the_repository);
+	struct packed_git *p = get_all_packs(the_repository);
 
 	while (p) {
 		add_pack(p);
diff --git a/builtin/repack.c b/builtin/repack.c
index d5886039cc..42be88e86c 100644
--- a/builtin/repack.c
+++ b/builtin/repack.c
@@ -8,6 +8,7 @@
 #include "strbuf.h"
 #include "string-list.h"
 #include "argv-array.h"
+#include "midx.h"
 #include "packfile.h"
 #include "object-store.h"
 
@@ -280,6 +281,7 @@ int cmd_repack(int argc, const char **argv, const char *prefix)
 	int keep_unreachable = 0;
 	struct string_list keep_pack_list = STRING_LIST_INIT_NODUP;
 	int no_update_server_info = 0;
+	int midx_cleared = 0;
 	struct pack_objects_args po_args = {NULL};
 
 	struct option builtin_repack_options[] = {
@@ -418,6 +420,13 @@ int cmd_repack(int argc, const char **argv, const char *prefix)
 	for_each_string_list_item(item, &names) {
 		for (ext = 0; ext < ARRAY_SIZE(exts); ext++) {
 			char *fname, *fname_old;
+
+			if (!midx_cleared) {
+				/* if we move a packfile, it will invalidated the midx */
+				clear_midx_file(get_object_directory());
+				midx_cleared = 1;
+			}
+
 			fname = mkpathdup("%s/pack-%s%s", packdir,
 						item->string, exts[ext].name);
 			if (!file_exists(fname)) {
diff --git a/command-list.txt b/command-list.txt
index a9dda3b8af..c36ea3c182 100644
--- a/command-list.txt
+++ b/command-list.txt
@@ -123,6 +123,7 @@ git-merge-index                         plumbingmanipulators
 git-merge-one-file                      purehelpers
 git-mergetool                           ancillarymanipulators           complete
 git-merge-tree                          ancillaryinterrogators
+git-multi-pack-index                    plumbingmanipulators
 git-mktag                               plumbingmanipulators
 git-mktree                              plumbingmanipulators
 git-mv                                  mainporcelain           worktree
diff --git a/fast-import.c b/fast-import.c
index 89bb0c9db3..f8c3acd3b5 100644
--- a/fast-import.c
+++ b/fast-import.c
@@ -1068,7 +1068,7 @@ static int store_object(
 		duplicate_count_by_type[type]++;
 		return 1;
 	} else if (find_sha1_pack(oid.hash,
-				  get_packed_git(the_repository))) {
+				  get_all_packs(the_repository))) {
 		e->type = type;
 		e->pack_id = MAX_PACK_ID;
 		e->idx.offset = 1; /* just not zero! */
@@ -1266,7 +1266,7 @@ static void stream_blob(uintmax_t len, struct object_id *oidout, uintmax_t mark)
 		truncate_pack(&checkpoint);
 
 	} else if (find_sha1_pack(oid.hash,
-				  get_packed_git(the_repository))) {
+				  get_all_packs(the_repository))) {
 		e->type = OBJ_BLOB;
 		e->pack_id = MAX_PACK_ID;
 		e->idx.offset = 1; /* just not zero! */
diff --git a/git.c b/git.c
index c27c38738b..a6f4b44af5 100644
--- a/git.c
+++ b/git.c
@@ -508,6 +508,7 @@ static struct cmd_struct commands[] = {
 	{ "merge-tree", cmd_merge_tree, RUN_SETUP | NO_PARSEOPT },
 	{ "mktag", cmd_mktag, RUN_SETUP | NO_PARSEOPT },
 	{ "mktree", cmd_mktree, RUN_SETUP },
+	{ "multi-pack-index", cmd_multi_pack_index, RUN_SETUP_GENTLY },
 	{ "mv", cmd_mv, RUN_SETUP | NEED_WORK_TREE },
 	{ "name-rev", cmd_name_rev, RUN_SETUP },
 	{ "notes", cmd_notes, RUN_SETUP },
diff --git a/http-backend.c b/http-backend.c
index 458642ef72..9e894f197f 100644
--- a/http-backend.c
+++ b/http-backend.c
@@ -595,13 +595,13 @@ static void get_info_packs(struct strbuf *hdr, char *arg)
 	size_t cnt = 0;
 
 	select_getanyfile(hdr);
-	for (p = get_packed_git(the_repository); p; p = p->next) {
+	for (p = get_all_packs(the_repository); p; p = p->next) {
 		if (p->pack_local)
 			cnt++;
 	}
 
 	strbuf_grow(&buf, cnt * 53 + 2);
-	for (p = get_packed_git(the_repository); p; p = p->next) {
+	for (p = get_all_packs(the_repository); p; p = p->next) {
 		if (p->pack_local)
 			strbuf_addf(&buf, "P %s\n", p->pack_name + objdirlen + 6);
 	}
diff --git a/midx.c b/midx.c
new file mode 100644
index 0000000000..f3e8dbc108
--- /dev/null
+++ b/midx.c
@@ -0,0 +1,930 @@
+#include "cache.h"
+#include "config.h"
+#include "csum-file.h"
+#include "dir.h"
+#include "lockfile.h"
+#include "packfile.h"
+#include "object-store.h"
+#include "sha1-lookup.h"
+#include "midx.h"
+
+#define MIDX_SIGNATURE 0x4d494458 /* "MIDX" */
+#define MIDX_VERSION 1
+#define MIDX_BYTE_FILE_VERSION 4
+#define MIDX_BYTE_HASH_VERSION 5
+#define MIDX_BYTE_NUM_CHUNKS 6
+#define MIDX_BYTE_NUM_PACKS 8
+#define MIDX_HASH_VERSION 1
+#define MIDX_HEADER_SIZE 12
+#define MIDX_HASH_LEN 20
+#define MIDX_MIN_SIZE (MIDX_HEADER_SIZE + MIDX_HASH_LEN)
+
+#define MIDX_MAX_CHUNKS 5
+#define MIDX_CHUNK_ALIGNMENT 4
+#define MIDX_CHUNKID_PACKNAMES 0x504e414d /* "PNAM" */
+#define MIDX_CHUNKID_OIDFANOUT 0x4f494446 /* "OIDF" */
+#define MIDX_CHUNKID_OIDLOOKUP 0x4f49444c /* "OIDL" */
+#define MIDX_CHUNKID_OBJECTOFFSETS 0x4f4f4646 /* "OOFF" */
+#define MIDX_CHUNKID_LARGEOFFSETS 0x4c4f4646 /* "LOFF" */
+#define MIDX_CHUNKLOOKUP_WIDTH (sizeof(uint32_t) + sizeof(uint64_t))
+#define MIDX_CHUNK_FANOUT_SIZE (sizeof(uint32_t) * 256)
+#define MIDX_CHUNK_OFFSET_WIDTH (2 * sizeof(uint32_t))
+#define MIDX_CHUNK_LARGE_OFFSET_WIDTH (sizeof(uint64_t))
+#define MIDX_LARGE_OFFSET_NEEDED 0x80000000
+
+static char *get_midx_filename(const char *object_dir)
+{
+	return xstrfmt("%s/pack/multi-pack-index", object_dir);
+}
+
+struct multi_pack_index *load_multi_pack_index(const char *object_dir, int local)
+{
+	struct multi_pack_index *m = NULL;
+	int fd;
+	struct stat st;
+	size_t midx_size;
+	void *midx_map = NULL;
+	uint32_t hash_version;
+	char *midx_name = get_midx_filename(object_dir);
+	uint32_t i;
+	const char *cur_pack_name;
+
+	fd = git_open(midx_name);
+
+	if (fd < 0)
+		goto cleanup_fail;
+	if (fstat(fd, &st)) {
+		error_errno(_("failed to read %s"), midx_name);
+		goto cleanup_fail;
+	}
+
+	midx_size = xsize_t(st.st_size);
+
+	if (midx_size < MIDX_MIN_SIZE) {
+		error(_("multi-pack-index file %s is too small"), midx_name);
+		goto cleanup_fail;
+	}
+
+	FREE_AND_NULL(midx_name);
+
+	midx_map = xmmap(NULL, midx_size, PROT_READ, MAP_PRIVATE, fd, 0);
+
+	FLEX_ALLOC_MEM(m, object_dir, object_dir, strlen(object_dir));
+	m->fd = fd;
+	m->data = midx_map;
+	m->data_len = midx_size;
+	m->local = local;
+
+	m->signature = get_be32(m->data);
+	if (m->signature != MIDX_SIGNATURE) {
+		error(_("multi-pack-index signature 0x%08x does not match signature 0x%08x"),
+		      m->signature, MIDX_SIGNATURE);
+		goto cleanup_fail;
+	}
+
+	m->version = m->data[MIDX_BYTE_FILE_VERSION];
+	if (m->version != MIDX_VERSION) {
+		error(_("multi-pack-index version %d not recognized"),
+		      m->version);
+		goto cleanup_fail;
+	}
+
+	hash_version = m->data[MIDX_BYTE_HASH_VERSION];
+	if (hash_version != MIDX_HASH_VERSION) {
+		error(_("hash version %u does not match"), hash_version);
+		goto cleanup_fail;
+	}
+	m->hash_len = MIDX_HASH_LEN;
+
+	m->num_chunks = m->data[MIDX_BYTE_NUM_CHUNKS];
+
+	m->num_packs = get_be32(m->data + MIDX_BYTE_NUM_PACKS);
+
+	for (i = 0; i < m->num_chunks; i++) {
+		uint32_t chunk_id = get_be32(m->data + MIDX_HEADER_SIZE +
+					     MIDX_CHUNKLOOKUP_WIDTH * i);
+		uint64_t chunk_offset = get_be64(m->data + MIDX_HEADER_SIZE + 4 +
+						 MIDX_CHUNKLOOKUP_WIDTH * i);
+
+		switch (chunk_id) {
+			case MIDX_CHUNKID_PACKNAMES:
+				m->chunk_pack_names = m->data + chunk_offset;
+				break;
+
+			case MIDX_CHUNKID_OIDFANOUT:
+				m->chunk_oid_fanout = (uint32_t *)(m->data + chunk_offset);
+				break;
+
+			case MIDX_CHUNKID_OIDLOOKUP:
+				m->chunk_oid_lookup = m->data + chunk_offset;
+				break;
+
+			case MIDX_CHUNKID_OBJECTOFFSETS:
+				m->chunk_object_offsets = m->data + chunk_offset;
+				break;
+
+			case MIDX_CHUNKID_LARGEOFFSETS:
+				m->chunk_large_offsets = m->data + chunk_offset;
+				break;
+
+			case 0:
+				die(_("terminating multi-pack-index chunk id appears earlier than expected"));
+				break;
+
+			default:
+				/*
+				 * Do nothing on unrecognized chunks, allowing future
+				 * extensions to add optional chunks.
+				 */
+				break;
+		}
+	}
+
+	if (!m->chunk_pack_names)
+		die(_("multi-pack-index missing required pack-name chunk"));
+	if (!m->chunk_oid_fanout)
+		die(_("multi-pack-index missing required OID fanout chunk"));
+	if (!m->chunk_oid_lookup)
+		die(_("multi-pack-index missing required OID lookup chunk"));
+	if (!m->chunk_object_offsets)
+		die(_("multi-pack-index missing required object offsets chunk"));
+
+	m->num_objects = ntohl(m->chunk_oid_fanout[255]);
+
+	m->pack_names = xcalloc(m->num_packs, sizeof(*m->pack_names));
+	m->packs = xcalloc(m->num_packs, sizeof(*m->packs));
+
+	cur_pack_name = (const char *)m->chunk_pack_names;
+	for (i = 0; i < m->num_packs; i++) {
+		m->pack_names[i] = cur_pack_name;
+
+		cur_pack_name += strlen(cur_pack_name) + 1;
+
+		if (i && strcmp(m->pack_names[i], m->pack_names[i - 1]) <= 0) {
+			error(_("multi-pack-index pack names out of order: '%s' before '%s'"),
+			      m->pack_names[i - 1],
+			      m->pack_names[i]);
+			goto cleanup_fail;
+		}
+	}
+
+	return m;
+
+cleanup_fail:
+	free(m);
+	free(midx_name);
+	if (midx_map)
+		munmap(midx_map, midx_size);
+	if (0 <= fd)
+		close(fd);
+	return NULL;
+}
+
+static void close_midx(struct multi_pack_index *m)
+{
+	uint32_t i;
+	munmap((unsigned char *)m->data, m->data_len);
+	close(m->fd);
+	m->fd = -1;
+
+	for (i = 0; i < m->num_packs; i++) {
+		if (m->packs[i]) {
+			close_pack(m->packs[i]);
+			free(m->packs);
+		}
+	}
+	FREE_AND_NULL(m->packs);
+	FREE_AND_NULL(m->pack_names);
+}
+
+int prepare_midx_pack(struct multi_pack_index *m, uint32_t pack_int_id)
+{
+	struct strbuf pack_name = STRBUF_INIT;
+
+	if (pack_int_id >= m->num_packs)
+		BUG("bad pack-int-id");
+
+	if (m->packs[pack_int_id])
+		return 0;
+
+	strbuf_addf(&pack_name, "%s/pack/%s", m->object_dir,
+		    m->pack_names[pack_int_id]);
+
+	m->packs[pack_int_id] = add_packed_git(pack_name.buf, pack_name.len, m->local);
+	strbuf_release(&pack_name);
+	return !m->packs[pack_int_id];
+}
+
+int bsearch_midx(const struct object_id *oid, struct multi_pack_index *m, uint32_t *result)
+{
+	return bsearch_hash(oid->hash, m->chunk_oid_fanout, m->chunk_oid_lookup,
+			    MIDX_HASH_LEN, result);
+}
+
+struct object_id *nth_midxed_object_oid(struct object_id *oid,
+					struct multi_pack_index *m,
+					uint32_t n)
+{
+	if (n >= m->num_objects)
+		return NULL;
+
+	hashcpy(oid->hash, m->chunk_oid_lookup + m->hash_len * n);
+	return oid;
+}
+
+static off_t nth_midxed_offset(struct multi_pack_index *m, uint32_t pos)
+{
+	const unsigned char *offset_data;
+	uint32_t offset32;
+
+	offset_data = m->chunk_object_offsets + pos * MIDX_CHUNK_OFFSET_WIDTH;
+	offset32 = get_be32(offset_data + sizeof(uint32_t));
+
+	if (m->chunk_large_offsets && offset32 & MIDX_LARGE_OFFSET_NEEDED) {
+		if (sizeof(offset32) < sizeof(uint64_t))
+			die(_("multi-pack-index stores a 64-bit offset, but off_t is too small"));
+
+		offset32 ^= MIDX_LARGE_OFFSET_NEEDED;
+		return get_be64(m->chunk_large_offsets + sizeof(uint64_t) * offset32);
+	}
+
+	return offset32;
+}
+
+static uint32_t nth_midxed_pack_int_id(struct multi_pack_index *m, uint32_t pos)
+{
+	return get_be32(m->chunk_object_offsets + pos * MIDX_CHUNK_OFFSET_WIDTH);
+}
+
+static int nth_midxed_pack_entry(struct multi_pack_index *m, struct pack_entry *e, uint32_t pos)
+{
+	uint32_t pack_int_id;
+	struct packed_git *p;
+
+	if (pos >= m->num_objects)
+		return 0;
+
+	pack_int_id = nth_midxed_pack_int_id(m, pos);
+
+	if (prepare_midx_pack(m, pack_int_id))
+		die(_("error preparing packfile from multi-pack-index"));
+	p = m->packs[pack_int_id];
+
+	/*
+	* We are about to tell the caller where they can locate the
+	* requested object.  We better make sure the packfile is
+	* still here and can be accessed before supplying that
+	* answer, as it may have been deleted since the MIDX was
+	* loaded!
+	*/
+	if (!is_pack_valid(p))
+		return 0;
+
+	if (p->num_bad_objects) {
+		uint32_t i;
+		struct object_id oid;
+		nth_midxed_object_oid(&oid, m, pos);
+		for (i = 0; i < p->num_bad_objects; i++)
+			if (!hashcmp(oid.hash,
+				     p->bad_object_sha1 + the_hash_algo->rawsz * i))
+				return 0;
+	}
+
+	e->offset = nth_midxed_offset(m, pos);
+	e->p = p;
+
+	return 1;
+}
+
+int fill_midx_entry(const struct object_id *oid, struct pack_entry *e, struct multi_pack_index *m)
+{
+	uint32_t pos;
+
+	if (!bsearch_midx(oid, m, &pos))
+		return 0;
+
+	return nth_midxed_pack_entry(m, e, pos);
+}
+
+int midx_contains_pack(struct multi_pack_index *m, const char *idx_name)
+{
+	uint32_t first = 0, last = m->num_packs;
+
+	while (first < last) {
+		uint32_t mid = first + (last - first) / 2;
+		const char *current;
+		int cmp;
+
+		current = m->pack_names[mid];
+		cmp = strcmp(idx_name, current);
+		if (!cmp)
+			return 1;
+		if (cmp > 0) {
+			first = mid + 1;
+			continue;
+		}
+		last = mid;
+	}
+
+	return 0;
+}
+
+int prepare_multi_pack_index_one(struct repository *r, const char *object_dir, int local)
+{
+	struct multi_pack_index *m;
+	struct multi_pack_index *m_search;
+	int config_value;
+
+	if (repo_config_get_bool(r, "core.multipackindex", &config_value) ||
+	    !config_value)
+		return 0;
+
+	for (m_search = r->objects->multi_pack_index; m_search; m_search = m_search->next)
+		if (!strcmp(object_dir, m_search->object_dir))
+			return 1;
+
+	m = load_multi_pack_index(object_dir, local);
+
+	if (m) {
+		m->next = r->objects->multi_pack_index;
+		r->objects->multi_pack_index = m;
+		return 1;
+	}
+
+	return 0;
+}
+
+static size_t write_midx_header(struct hashfile *f,
+				unsigned char num_chunks,
+				uint32_t num_packs)
+{
+	unsigned char byte_values[4];
+
+	hashwrite_be32(f, MIDX_SIGNATURE);
+	byte_values[0] = MIDX_VERSION;
+	byte_values[1] = MIDX_HASH_VERSION;
+	byte_values[2] = num_chunks;
+	byte_values[3] = 0; /* unused */
+	hashwrite(f, byte_values, sizeof(byte_values));
+	hashwrite_be32(f, num_packs);
+
+	return MIDX_HEADER_SIZE;
+}
+
+struct pack_list {
+	struct packed_git **list;
+	char **names;
+	uint32_t nr;
+	uint32_t alloc_list;
+	uint32_t alloc_names;
+	size_t pack_name_concat_len;
+	struct multi_pack_index *m;
+};
+
+static void add_pack_to_midx(const char *full_path, size_t full_path_len,
+			     const char *file_name, void *data)
+{
+	struct pack_list *packs = (struct pack_list *)data;
+
+	if (ends_with(file_name, ".idx")) {
+		if (packs->m && midx_contains_pack(packs->m, file_name))
+			return;
+
+		ALLOC_GROW(packs->list, packs->nr + 1, packs->alloc_list);
+		ALLOC_GROW(packs->names, packs->nr + 1, packs->alloc_names);
+
+		packs->list[packs->nr] = add_packed_git(full_path,
+							full_path_len,
+							0);
+
+		if (!packs->list[packs->nr]) {
+			warning(_("failed to add packfile '%s'"),
+				full_path);
+			return;
+		}
+
+		if (open_pack_index(packs->list[packs->nr])) {
+			warning(_("failed to open pack-index '%s'"),
+				full_path);
+			close_pack(packs->list[packs->nr]);
+			FREE_AND_NULL(packs->list[packs->nr]);
+			return;
+		}
+
+		packs->names[packs->nr] = xstrdup(file_name);
+		packs->pack_name_concat_len += strlen(file_name) + 1;
+		packs->nr++;
+	}
+}
+
+struct pack_pair {
+	uint32_t pack_int_id;
+	char *pack_name;
+};
+
+static int pack_pair_compare(const void *_a, const void *_b)
+{
+	struct pack_pair *a = (struct pack_pair *)_a;
+	struct pack_pair *b = (struct pack_pair *)_b;
+	return strcmp(a->pack_name, b->pack_name);
+}
+
+static void sort_packs_by_name(char **pack_names, uint32_t nr_packs, uint32_t *perm)
+{
+	uint32_t i;
+	struct pack_pair *pairs;
+
+	ALLOC_ARRAY(pairs, nr_packs);
+
+	for (i = 0; i < nr_packs; i++) {
+		pairs[i].pack_int_id = i;
+		pairs[i].pack_name = pack_names[i];
+	}
+
+	QSORT(pairs, nr_packs, pack_pair_compare);
+
+	for (i = 0; i < nr_packs; i++) {
+		pack_names[i] = pairs[i].pack_name;
+		perm[pairs[i].pack_int_id] = i;
+	}
+
+	free(pairs);
+}
+
+struct pack_midx_entry {
+	struct object_id oid;
+	uint32_t pack_int_id;
+	time_t pack_mtime;
+	uint64_t offset;
+};
+
+static int midx_oid_compare(const void *_a, const void *_b)
+{
+	const struct pack_midx_entry *a = (const struct pack_midx_entry *)_a;
+	const struct pack_midx_entry *b = (const struct pack_midx_entry *)_b;
+	int cmp = oidcmp(&a->oid, &b->oid);
+
+	if (cmp)
+		return cmp;
+
+	if (a->pack_mtime > b->pack_mtime)
+		return -1;
+	else if (a->pack_mtime < b->pack_mtime)
+		return 1;
+
+	return a->pack_int_id - b->pack_int_id;
+}
+
+static int nth_midxed_pack_midx_entry(struct multi_pack_index *m,
+				      uint32_t *pack_perm,
+				      struct pack_midx_entry *e,
+				      uint32_t pos)
+{
+	if (pos >= m->num_objects)
+		return 1;
+
+	nth_midxed_object_oid(&e->oid, m, pos);
+	e->pack_int_id = pack_perm[nth_midxed_pack_int_id(m, pos)];
+	e->offset = nth_midxed_offset(m, pos);
+
+	/* consider objects in midx to be from "old" packs */
+	e->pack_mtime = 0;
+	return 0;
+}
+
+static void fill_pack_entry(uint32_t pack_int_id,
+			    struct packed_git *p,
+			    uint32_t cur_object,
+			    struct pack_midx_entry *entry)
+{
+	if (!nth_packed_object_oid(&entry->oid, p, cur_object))
+		die(_("failed to locate object %d in packfile"), cur_object);
+
+	entry->pack_int_id = pack_int_id;
+	entry->pack_mtime = p->mtime;
+
+	entry->offset = nth_packed_object_offset(p, cur_object);
+}
+
+/*
+ * It is possible to artificially get into a state where there are many
+ * duplicate copies of objects. That can create high memory pressure if
+ * we are to create a list of all objects before de-duplication. To reduce
+ * this memory pressure without a significant performance drop, automatically
+ * group objects by the first byte of their object id. Use the IDX fanout
+ * tables to group the data, copy to a local array, then sort.
+ *
+ * Copy only the de-duplicated entries (selected by most-recent modified time
+ * of a packfile containing the object).
+ */
+static struct pack_midx_entry *get_sorted_entries(struct multi_pack_index *m,
+						  struct packed_git **p,
+						  uint32_t *perm,
+						  uint32_t nr_packs,
+						  uint32_t *nr_objects)
+{
+	uint32_t cur_fanout, cur_pack, cur_object;
+	uint32_t alloc_fanout, alloc_objects, total_objects = 0;
+	struct pack_midx_entry *entries_by_fanout = NULL;
+	struct pack_midx_entry *deduplicated_entries = NULL;
+	uint32_t start_pack = m ? m->num_packs : 0;
+
+	for (cur_pack = start_pack; cur_pack < nr_packs; cur_pack++)
+		total_objects += p[cur_pack]->num_objects;
+
+	/*
+	 * As we de-duplicate by fanout value, we expect the fanout
+	 * slices to be evenly distributed, with some noise. Hence,
+	 * allocate slightly more than one 256th.
+	 */
+	alloc_objects = alloc_fanout = total_objects > 3200 ? total_objects / 200 : 16;
+
+	ALLOC_ARRAY(entries_by_fanout, alloc_fanout);
+	ALLOC_ARRAY(deduplicated_entries, alloc_objects);
+	*nr_objects = 0;
+
+	for (cur_fanout = 0; cur_fanout < 256; cur_fanout++) {
+		uint32_t nr_fanout = 0;
+
+		if (m) {
+			uint32_t start = 0, end;
+
+			if (cur_fanout)
+				start = ntohl(m->chunk_oid_fanout[cur_fanout - 1]);
+			end = ntohl(m->chunk_oid_fanout[cur_fanout]);
+
+			for (cur_object = start; cur_object < end; cur_object++) {
+				ALLOC_GROW(entries_by_fanout, nr_fanout + 1, alloc_fanout);
+				nth_midxed_pack_midx_entry(m, perm,
+							   &entries_by_fanout[nr_fanout],
+							   cur_object);
+				nr_fanout++;
+			}
+		}
+
+		for (cur_pack = start_pack; cur_pack < nr_packs; cur_pack++) {
+			uint32_t start = 0, end;
+
+			if (cur_fanout)
+				start = get_pack_fanout(p[cur_pack], cur_fanout - 1);
+			end = get_pack_fanout(p[cur_pack], cur_fanout);
+
+			for (cur_object = start; cur_object < end; cur_object++) {
+				ALLOC_GROW(entries_by_fanout, nr_fanout + 1, alloc_fanout);
+				fill_pack_entry(perm[cur_pack], p[cur_pack], cur_object, &entries_by_fanout[nr_fanout]);
+				nr_fanout++;
+			}
+		}
+
+		QSORT(entries_by_fanout, nr_fanout, midx_oid_compare);
+
+		/*
+		 * The batch is now sorted by OID and then mtime (descending).
+		 * Take only the first duplicate.
+		 */
+		for (cur_object = 0; cur_object < nr_fanout; cur_object++) {
+			if (cur_object && !oidcmp(&entries_by_fanout[cur_object - 1].oid,
+						  &entries_by_fanout[cur_object].oid))
+				continue;
+
+			ALLOC_GROW(deduplicated_entries, *nr_objects + 1, alloc_objects);
+			memcpy(&deduplicated_entries[*nr_objects],
+			       &entries_by_fanout[cur_object],
+			       sizeof(struct pack_midx_entry));
+			(*nr_objects)++;
+		}
+	}
+
+	free(entries_by_fanout);
+	return deduplicated_entries;
+}
+
+static size_t write_midx_pack_names(struct hashfile *f,
+				    char **pack_names,
+				    uint32_t num_packs)
+{
+	uint32_t i;
+	unsigned char padding[MIDX_CHUNK_ALIGNMENT];
+	size_t written = 0;
+
+	for (i = 0; i < num_packs; i++) {
+		size_t writelen = strlen(pack_names[i]) + 1;
+
+		if (i && strcmp(pack_names[i], pack_names[i - 1]) <= 0)
+			BUG("incorrect pack-file order: %s before %s",
+			    pack_names[i - 1],
+			    pack_names[i]);
+
+		hashwrite(f, pack_names[i], writelen);
+		written += writelen;
+	}
+
+	/* add padding to be aligned */
+	i = MIDX_CHUNK_ALIGNMENT - (written % MIDX_CHUNK_ALIGNMENT);
+	if (i < MIDX_CHUNK_ALIGNMENT) {
+		memset(padding, 0, sizeof(padding));
+		hashwrite(f, padding, i);
+		written += i;
+	}
+
+	return written;
+}
+
+static size_t write_midx_oid_fanout(struct hashfile *f,
+				    struct pack_midx_entry *objects,
+				    uint32_t nr_objects)
+{
+	struct pack_midx_entry *list = objects;
+	struct pack_midx_entry *last = objects + nr_objects;
+	uint32_t count = 0;
+	uint32_t i;
+
+	/*
+	* Write the first-level table (the list is sorted,
+	* but we use a 256-entry lookup to be able to avoid
+	* having to do eight extra binary search iterations).
+	*/
+	for (i = 0; i < 256; i++) {
+		struct pack_midx_entry *next = list;
+
+		while (next < last && next->oid.hash[0] == i) {
+			count++;
+			next++;
+		}
+
+		hashwrite_be32(f, count);
+		list = next;
+	}
+
+	return MIDX_CHUNK_FANOUT_SIZE;
+}
+
+static size_t write_midx_oid_lookup(struct hashfile *f, unsigned char hash_len,
+				    struct pack_midx_entry *objects,
+				    uint32_t nr_objects)
+{
+	struct pack_midx_entry *list = objects;
+	uint32_t i;
+	size_t written = 0;
+
+	for (i = 0; i < nr_objects; i++) {
+		struct pack_midx_entry *obj = list++;
+
+		if (i < nr_objects - 1) {
+			struct pack_midx_entry *next = list;
+			if (oidcmp(&obj->oid, &next->oid) >= 0)
+				BUG("OIDs not in order: %s >= %s",
+				    oid_to_hex(&obj->oid),
+				    oid_to_hex(&next->oid));
+		}
+
+		hashwrite(f, obj->oid.hash, (int)hash_len);
+		written += hash_len;
+	}
+
+	return written;
+}
+
+static size_t write_midx_object_offsets(struct hashfile *f, int large_offset_needed,
+					struct pack_midx_entry *objects, uint32_t nr_objects)
+{
+	struct pack_midx_entry *list = objects;
+	uint32_t i, nr_large_offset = 0;
+	size_t written = 0;
+
+	for (i = 0; i < nr_objects; i++) {
+		struct pack_midx_entry *obj = list++;
+
+		hashwrite_be32(f, obj->pack_int_id);
+
+		if (large_offset_needed && obj->offset >> 31)
+			hashwrite_be32(f, MIDX_LARGE_OFFSET_NEEDED | nr_large_offset++);
+		else if (!large_offset_needed && obj->offset >> 32)
+			BUG("object %s requires a large offset (%"PRIx64") but the MIDX is not writing large offsets!",
+			    oid_to_hex(&obj->oid),
+			    obj->offset);
+		else
+			hashwrite_be32(f, (uint32_t)obj->offset);
+
+		written += MIDX_CHUNK_OFFSET_WIDTH;
+	}
+
+	return written;
+}
+
+static size_t write_midx_large_offsets(struct hashfile *f, uint32_t nr_large_offset,
+				       struct pack_midx_entry *objects, uint32_t nr_objects)
+{
+	struct pack_midx_entry *list = objects;
+	size_t written = 0;
+
+	while (nr_large_offset) {
+		struct pack_midx_entry *obj = list++;
+		uint64_t offset = obj->offset;
+
+		if (!(offset >> 31))
+			continue;
+
+		hashwrite_be32(f, offset >> 32);
+		hashwrite_be32(f, offset & 0xffffffffUL);
+		written += 2 * sizeof(uint32_t);
+
+		nr_large_offset--;
+	}
+
+	return written;
+}
+
+int write_midx_file(const char *object_dir)
+{
+	unsigned char cur_chunk, num_chunks = 0;
+	char *midx_name;
+	uint32_t i;
+	struct hashfile *f = NULL;
+	struct lock_file lk;
+	struct pack_list packs;
+	uint32_t *pack_perm = NULL;
+	uint64_t written = 0;
+	uint32_t chunk_ids[MIDX_MAX_CHUNKS + 1];
+	uint64_t chunk_offsets[MIDX_MAX_CHUNKS + 1];
+	uint32_t nr_entries, num_large_offsets = 0;
+	struct pack_midx_entry *entries = NULL;
+	int large_offsets_needed = 0;
+
+	midx_name = get_midx_filename(object_dir);
+	if (safe_create_leading_directories(midx_name)) {
+		UNLEAK(midx_name);
+		die_errno(_("unable to create leading directories of %s"),
+			  midx_name);
+	}
+
+	packs.m = load_multi_pack_index(object_dir, 1);
+
+	packs.nr = 0;
+	packs.alloc_list = packs.m ? packs.m->num_packs : 16;
+	packs.alloc_names = packs.alloc_list;
+	packs.list = NULL;
+	packs.names = NULL;
+	packs.pack_name_concat_len = 0;
+	ALLOC_ARRAY(packs.list, packs.alloc_list);
+	ALLOC_ARRAY(packs.names, packs.alloc_names);
+
+	if (packs.m) {
+		for (i = 0; i < packs.m->num_packs; i++) {
+			ALLOC_GROW(packs.list, packs.nr + 1, packs.alloc_list);
+			ALLOC_GROW(packs.names, packs.nr + 1, packs.alloc_names);
+
+			packs.list[packs.nr] = NULL;
+			packs.names[packs.nr] = xstrdup(packs.m->pack_names[i]);
+			packs.pack_name_concat_len += strlen(packs.names[packs.nr]) + 1;
+			packs.nr++;
+		}
+	}
+
+	for_each_file_in_pack_dir(object_dir, add_pack_to_midx, &packs);
+
+	if (packs.m && packs.nr == packs.m->num_packs)
+		goto cleanup;
+
+	if (packs.pack_name_concat_len % MIDX_CHUNK_ALIGNMENT)
+		packs.pack_name_concat_len += MIDX_CHUNK_ALIGNMENT -
+					      (packs.pack_name_concat_len % MIDX_CHUNK_ALIGNMENT);
+
+	ALLOC_ARRAY(pack_perm, packs.nr);
+	sort_packs_by_name(packs.names, packs.nr, pack_perm);
+
+	entries = get_sorted_entries(packs.m, packs.list, pack_perm, packs.nr, &nr_entries);
+
+	for (i = 0; i < nr_entries; i++) {
+		if (entries[i].offset > 0x7fffffff)
+			num_large_offsets++;
+		if (entries[i].offset > 0xffffffff)
+			large_offsets_needed = 1;
+	}
+
+	hold_lock_file_for_update(&lk, midx_name, LOCK_DIE_ON_ERROR);
+	f = hashfd(lk.tempfile->fd, lk.tempfile->filename.buf);
+	FREE_AND_NULL(midx_name);
+
+	if (packs.m)
+		close_midx(packs.m);
+
+	cur_chunk = 0;
+	num_chunks = large_offsets_needed ? 5 : 4;
+
+	written = write_midx_header(f, num_chunks, packs.nr);
+
+	chunk_ids[cur_chunk] = MIDX_CHUNKID_PACKNAMES;
+	chunk_offsets[cur_chunk] = written + (num_chunks + 1) * MIDX_CHUNKLOOKUP_WIDTH;
+
+	cur_chunk++;
+	chunk_ids[cur_chunk] = MIDX_CHUNKID_OIDFANOUT;
+	chunk_offsets[cur_chunk] = chunk_offsets[cur_chunk - 1] + packs.pack_name_concat_len;
+
+	cur_chunk++;
+	chunk_ids[cur_chunk] = MIDX_CHUNKID_OIDLOOKUP;
+	chunk_offsets[cur_chunk] = chunk_offsets[cur_chunk - 1] + MIDX_CHUNK_FANOUT_SIZE;
+
+	cur_chunk++;
+	chunk_ids[cur_chunk] = MIDX_CHUNKID_OBJECTOFFSETS;
+	chunk_offsets[cur_chunk] = chunk_offsets[cur_chunk - 1] + nr_entries * MIDX_HASH_LEN;
+
+	cur_chunk++;
+	chunk_offsets[cur_chunk] = chunk_offsets[cur_chunk - 1] + nr_entries * MIDX_CHUNK_OFFSET_WIDTH;
+	if (large_offsets_needed) {
+		chunk_ids[cur_chunk] = MIDX_CHUNKID_LARGEOFFSETS;
+
+		cur_chunk++;
+		chunk_offsets[cur_chunk] = chunk_offsets[cur_chunk - 1] +
+					   num_large_offsets * MIDX_CHUNK_LARGE_OFFSET_WIDTH;
+	}
+
+	chunk_ids[cur_chunk] = 0;
+
+	for (i = 0; i <= num_chunks; i++) {
+		if (i && chunk_offsets[i] < chunk_offsets[i - 1])
+			BUG("incorrect chunk offsets: %"PRIu64" before %"PRIu64,
+			    chunk_offsets[i - 1],
+			    chunk_offsets[i]);
+
+		if (chunk_offsets[i] % MIDX_CHUNK_ALIGNMENT)
+			BUG("chunk offset %"PRIu64" is not properly aligned",
+			    chunk_offsets[i]);
+
+		hashwrite_be32(f, chunk_ids[i]);
+		hashwrite_be32(f, chunk_offsets[i] >> 32);
+		hashwrite_be32(f, chunk_offsets[i]);
+
+		written += MIDX_CHUNKLOOKUP_WIDTH;
+	}
+
+	for (i = 0; i < num_chunks; i++) {
+		if (written != chunk_offsets[i])
+			BUG("incorrect chunk offset (%"PRIu64" != %"PRIu64") for chunk id %"PRIx32,
+			    chunk_offsets[i],
+			    written,
+			    chunk_ids[i]);
+
+		switch (chunk_ids[i]) {
+			case MIDX_CHUNKID_PACKNAMES:
+				written += write_midx_pack_names(f, packs.names, packs.nr);
+				break;
+
+			case MIDX_CHUNKID_OIDFANOUT:
+				written += write_midx_oid_fanout(f, entries, nr_entries);
+				break;
+
+			case MIDX_CHUNKID_OIDLOOKUP:
+				written += write_midx_oid_lookup(f, MIDX_HASH_LEN, entries, nr_entries);
+				break;
+
+			case MIDX_CHUNKID_OBJECTOFFSETS:
+				written += write_midx_object_offsets(f, large_offsets_needed, entries, nr_entries);
+				break;
+
+			case MIDX_CHUNKID_LARGEOFFSETS:
+				written += write_midx_large_offsets(f, num_large_offsets, entries, nr_entries);
+				break;
+
+			default:
+				BUG("trying to write unknown chunk id %"PRIx32,
+				    chunk_ids[i]);
+		}
+	}
+
+	if (written != chunk_offsets[num_chunks])
+		BUG("incorrect final offset %"PRIu64" != %"PRIu64,
+		    written,
+		    chunk_offsets[num_chunks]);
+
+	finalize_hashfile(f, NULL, CSUM_FSYNC | CSUM_HASH_IN_STREAM);
+	commit_lock_file(&lk);
+
+cleanup:
+	for (i = 0; i < packs.nr; i++) {
+		if (packs.list[i]) {
+			close_pack(packs.list[i]);
+			free(packs.list[i]);
+		}
+		free(packs.names[i]);
+	}
+
+	free(packs.list);
+	free(packs.names);
+	free(entries);
+	free(pack_perm);
+	free(midx_name);
+	return 0;
+}
+
+void clear_midx_file(const char *object_dir)
+{
+	char *midx = get_midx_filename(object_dir);
+
+	if (remove_path(midx)) {
+		UNLEAK(midx);
+		die(_("failed to clear multi-pack-index at %s"), midx);
+	}
+
+	free(midx);
+}
diff --git a/midx.h b/midx.h
new file mode 100644
index 0000000000..a210f1af2a
--- /dev/null
+++ b/midx.h
@@ -0,0 +1,47 @@
+#ifndef __MIDX_H__
+#define __MIDX_H__
+
+#include "repository.h"
+
+struct multi_pack_index {
+	struct multi_pack_index *next;
+
+	int fd;
+
+	const unsigned char *data;
+	size_t data_len;
+
+	uint32_t signature;
+	unsigned char version;
+	unsigned char hash_len;
+	unsigned char num_chunks;
+	uint32_t num_packs;
+	uint32_t num_objects;
+
+	int local;
+
+	const unsigned char *chunk_pack_names;
+	const uint32_t *chunk_oid_fanout;
+	const unsigned char *chunk_oid_lookup;
+	const unsigned char *chunk_object_offsets;
+	const unsigned char *chunk_large_offsets;
+
+	const char **pack_names;
+	struct packed_git **packs;
+	char object_dir[FLEX_ARRAY];
+};
+
+struct multi_pack_index *load_multi_pack_index(const char *object_dir, int local);
+int prepare_midx_pack(struct multi_pack_index *m, uint32_t pack_int_id);
+int bsearch_midx(const struct object_id *oid, struct multi_pack_index *m, uint32_t *result);
+struct object_id *nth_midxed_object_oid(struct object_id *oid,
+					struct multi_pack_index *m,
+					uint32_t n);
+int fill_midx_entry(const struct object_id *oid, struct pack_entry *e, struct multi_pack_index *m);
+int midx_contains_pack(struct multi_pack_index *m, const char *idx_name);
+int prepare_multi_pack_index_one(struct repository *r, const char *object_dir, int local);
+
+int write_midx_file(const char *object_dir);
+void clear_midx_file(const char *object_dir);
+
+#endif
diff --git a/object-store.h b/object-store.h
index 67e66227d9..63b7605a3e 100644
--- a/object-store.h
+++ b/object-store.h
@@ -88,6 +88,8 @@ struct packed_git {
 	char pack_name[FLEX_ARRAY]; /* more */
 };
 
+struct multi_pack_index;
+
 struct raw_object_store {
 	/*
 	 * Path to the repository's object store.
@@ -113,6 +115,13 @@ struct raw_object_store {
 	/*
 	 * private data
 	 *
+	 * should only be accessed directly by packfile.c and midx.c
+	 */
+	struct multi_pack_index *multi_pack_index;
+
+	/*
+	 * private data
+	 *
 	 * should only be accessed directly by packfile.c
 	 */
 
@@ -121,6 +130,12 @@ struct raw_object_store {
 	struct list_head packed_git_mru;
 
 	/*
+	 * A linked list containing all packfiles, starting with those
+	 * contained in the multi_pack_index.
+	 */
+	struct packed_git *all_packs;
+
+	/*
 	 * A fast, rough count of the number of objects in the repository.
 	 * These two fields are not meant for direct access. Use
 	 * approximate_object_count() instead.
diff --git a/pack-bitmap.c b/pack-bitmap.c
index f0a1937a1c..4e50ab391f 100644
--- a/pack-bitmap.c
+++ b/pack-bitmap.c
@@ -335,7 +335,7 @@ static int open_pack_bitmap(struct bitmap_index *bitmap_git)
 
 	assert(!bitmap_git->map && !bitmap_git->loaded);
 
-	for (p = get_packed_git(the_repository); p; p = p->next) {
+	for (p = get_all_packs(the_repository); p; p = p->next) {
 		if (open_pack_bitmap_1(bitmap_git, p) == 0)
 			ret = 0;
 	}
diff --git a/pack-objects.c b/pack-objects.c
index 6ef87e5683..d04cfa8e9f 100644
--- a/pack-objects.c
+++ b/pack-objects.c
@@ -99,7 +99,7 @@ static void prepare_in_pack_by_idx(struct packing_data *pdata)
 	 * (i.e. in_pack_idx also zero) should return NULL.
 	 */
 	mapping[cnt++] = NULL;
-	for (p = get_packed_git(the_repository); p; p = p->next, cnt++) {
+	for (p = get_all_packs(the_repository); p; p = p->next, cnt++) {
 		if (cnt == nr) {
 			free(mapping);
 			return;
diff --git a/packfile.c b/packfile.c
index ebcb5742ec..cbef7033c3 100644
--- a/packfile.c
+++ b/packfile.c
@@ -15,6 +15,7 @@
 #include "tree-walk.h"
 #include "tree.h"
 #include "object-store.h"
+#include "midx.h"
 
 char *odb_pack_name(struct strbuf *buf,
 		    const unsigned char *sha1,
@@ -196,6 +197,23 @@ int open_pack_index(struct packed_git *p)
 	return ret;
 }
 
+uint32_t get_pack_fanout(struct packed_git *p, uint32_t value)
+{
+	const uint32_t *level1_ofs = p->index_data;
+
+	if (!level1_ofs) {
+		if (open_pack_index(p))
+			return 0;
+		level1_ofs = p->index_data;
+	}
+
+	if (p->index_version > 1) {
+		level1_ofs += 2;
+	}
+
+	return ntohl(level1_ofs[value]);
+}
+
 static struct packed_git *alloc_packed_git(int extra)
 {
 	struct packed_git *p = xmalloc(st_add(sizeof(*p), extra));
@@ -451,8 +469,19 @@ static int open_packed_git_1(struct packed_git *p)
 	ssize_t read_result;
 	const unsigned hashsz = the_hash_algo->rawsz;
 
-	if (!p->index_data && open_pack_index(p))
-		return error("packfile %s index unavailable", p->pack_name);
+	if (!p->index_data) {
+		struct multi_pack_index *m;
+		const char *pack_name = strrchr(p->pack_name, '/');
+
+		for (m = the_repository->objects->multi_pack_index;
+		     m; m = m->next) {
+			if (midx_contains_pack(m, pack_name))
+				break;
+		}
+
+		if (!m && open_pack_index(p))
+			return error("packfile %s index unavailable", p->pack_name);
+	}
 
 	if (!pack_max_fds) {
 		unsigned int max_fds = get_max_fd_limit();
@@ -503,6 +532,10 @@ static int open_packed_git_1(struct packed_git *p)
 			" supported (try upgrading GIT to a newer version)",
 			p->pack_name, ntohl(hdr.hdr_version));
 
+	/* Skip index checking if in multi-pack-index */
+	if (!p->index_data)
+		return 0;
+
 	/* Verify the pack matches its index. */
 	if (p->num_objects != ntohl(hdr.hdr_entries))
 		return error("packfile %s claims to have %"PRIu32" objects"
@@ -738,13 +771,14 @@ static void report_pack_garbage(struct string_list *list)
 	report_helper(list, seen_bits, first, list->nr);
 }
 
-static void prepare_packed_git_one(struct repository *r, char *objdir, int local)
+void for_each_file_in_pack_dir(const char *objdir,
+			       each_file_in_pack_dir_fn fn,
+			       void *data)
 {
 	struct strbuf path = STRBUF_INIT;
 	size_t dirnamelen;
 	DIR *dir;
 	struct dirent *de;
-	struct string_list garbage = STRING_LIST_INIT_DUP;
 
 	strbuf_addstr(&path, objdir);
 	strbuf_addstr(&path, "/pack");
@@ -759,53 +793,87 @@ static void prepare_packed_git_one(struct repository *r, char *objdir, int local
 	strbuf_addch(&path, '/');
 	dirnamelen = path.len;
 	while ((de = readdir(dir)) != NULL) {
-		struct packed_git *p;
-		size_t base_len;
-
 		if (is_dot_or_dotdot(de->d_name))
 			continue;
 
 		strbuf_setlen(&path, dirnamelen);
 		strbuf_addstr(&path, de->d_name);
 
-		base_len = path.len;
-		if (strip_suffix_mem(path.buf, &base_len, ".idx")) {
-			/* Don't reopen a pack we already have. */
-			for (p = r->objects->packed_git; p;
-			     p = p->next) {
-				size_t len;
-				if (strip_suffix(p->pack_name, ".pack", &len) &&
-				    len == base_len &&
-				    !memcmp(p->pack_name, path.buf, len))
-					break;
-			}
-			if (p == NULL &&
-			    /*
-			     * See if it really is a valid .idx file with
-			     * corresponding .pack file that we can map.
-			     */
-			    (p = add_packed_git(path.buf, path.len, local)) != NULL)
-				install_packed_git(r, p);
-		}
-
-		if (!report_garbage)
-			continue;
-
-		if (ends_with(de->d_name, ".idx") ||
-		    ends_with(de->d_name, ".pack") ||
-		    ends_with(de->d_name, ".bitmap") ||
-		    ends_with(de->d_name, ".keep") ||
-		    ends_with(de->d_name, ".promisor"))
-			string_list_append(&garbage, path.buf);
-		else
-			report_garbage(PACKDIR_FILE_GARBAGE, path.buf);
+		fn(path.buf, path.len, de->d_name, data);
 	}
+
 	closedir(dir);
-	report_pack_garbage(&garbage);
-	string_list_clear(&garbage, 0);
 	strbuf_release(&path);
 }
 
+struct prepare_pack_data {
+	struct repository *r;
+	struct string_list *garbage;
+	int local;
+	struct multi_pack_index *m;
+};
+
+static void prepare_pack(const char *full_name, size_t full_name_len,
+			 const char *file_name, void *_data)
+{
+	struct prepare_pack_data *data = (struct prepare_pack_data *)_data;
+	struct packed_git *p;
+	size_t base_len = full_name_len;
+
+	if (strip_suffix_mem(full_name, &base_len, ".idx") &&
+	    !(data->m && midx_contains_pack(data->m, file_name))) {
+		/* Don't reopen a pack we already have. */
+		for (p = data->r->objects->packed_git; p; p = p->next) {
+			size_t len;
+			if (strip_suffix(p->pack_name, ".pack", &len) &&
+			    len == base_len &&
+			    !memcmp(p->pack_name, full_name, len))
+				break;
+		}
+
+		if (!p) {
+			p = add_packed_git(full_name, full_name_len, data->local);
+			if (p)
+				install_packed_git(data->r, p);
+		}
+	}
+
+	if (!report_garbage)
+		return;
+
+	if (!strcmp(file_name, "multi-pack-index"))
+		return;
+	if (ends_with(file_name, ".idx") ||
+	    ends_with(file_name, ".pack") ||
+	    ends_with(file_name, ".bitmap") ||
+	    ends_with(file_name, ".keep") ||
+	    ends_with(file_name, ".promisor"))
+		string_list_append(data->garbage, full_name);
+	else
+		report_garbage(PACKDIR_FILE_GARBAGE, full_name);
+}
+
+static void prepare_packed_git_one(struct repository *r, char *objdir, int local)
+{
+	struct prepare_pack_data data;
+	struct string_list garbage = STRING_LIST_INIT_DUP;
+
+	data.m = r->objects->multi_pack_index;
+
+	/* look for the multi-pack-index for this object directory */
+	while (data.m && strcmp(data.m->object_dir, objdir))
+		data.m = data.m->next;
+
+	data.r = r;
+	data.garbage = &garbage;
+	data.local = local;
+
+	for_each_file_in_pack_dir(objdir, prepare_pack, &data);
+
+	report_pack_garbage(data.garbage);
+	string_list_clear(data.garbage, 0);
+}
+
 static void prepare_packed_git(struct repository *r);
 /*
  * Give a fast, rough count of the number of objects in the repository. This
@@ -818,10 +886,13 @@ unsigned long approximate_object_count(void)
 {
 	if (!the_repository->objects->approximate_object_count_valid) {
 		unsigned long count;
+		struct multi_pack_index *m;
 		struct packed_git *p;
 
 		prepare_packed_git(the_repository);
 		count = 0;
+		for (m = get_multi_pack_index(the_repository); m; m = m->next)
+			count += m->num_objects;
 		for (p = the_repository->objects->packed_git; p; p = p->next) {
 			if (open_pack_index(p))
 				continue;
@@ -893,11 +964,17 @@ static void prepare_packed_git(struct repository *r)
 
 	if (r->objects->packed_git_initialized)
 		return;
+	prepare_multi_pack_index_one(r, r->objects->objectdir, 1);
 	prepare_packed_git_one(r, r->objects->objectdir, 1);
 	prepare_alt_odb(r);
-	for (alt = r->objects->alt_odb_list; alt; alt = alt->next)
+	for (alt = r->objects->alt_odb_list; alt; alt = alt->next) {
+		prepare_multi_pack_index_one(r, alt->path, 0);
 		prepare_packed_git_one(r, alt->path, 0);
+	}
 	rearrange_packed_git(r);
+
+	r->objects->all_packs = NULL;
+
 	prepare_packed_git_mru(r);
 	r->objects->packed_git_initialized = 1;
 }
@@ -915,6 +992,36 @@ struct packed_git *get_packed_git(struct repository *r)
 	return r->objects->packed_git;
 }
 
+struct multi_pack_index *get_multi_pack_index(struct repository *r)
+{
+	prepare_packed_git(r);
+	return r->objects->multi_pack_index;
+}
+
+struct packed_git *get_all_packs(struct repository *r)
+{
+	prepare_packed_git(r);
+
+	if (!r->objects->all_packs) {
+		struct packed_git *p = r->objects->packed_git;
+		struct multi_pack_index *m;
+
+		for (m = r->objects->multi_pack_index; m; m = m->next) {
+			uint32_t i;
+			for (i = 0; i < m->num_packs; i++) {
+				if (!prepare_midx_pack(m, i)) {
+					m->packs[i]->next = p;
+					p = m->packs[i];
+				}
+			}
+		}
+
+		r->objects->all_packs = p;
+	}
+
+	return r->objects->all_packs;
+}
+
 struct list_head *get_packed_git_mru(struct repository *r)
 {
 	prepare_packed_git(r);
@@ -1856,11 +1963,17 @@ static int fill_pack_entry(const struct object_id *oid,
 int find_pack_entry(struct repository *r, const struct object_id *oid, struct pack_entry *e)
 {
 	struct list_head *pos;
+	struct multi_pack_index *m;
 
 	prepare_packed_git(r);
-	if (!r->objects->packed_git)
+	if (!r->objects->packed_git && !r->objects->multi_pack_index)
 		return 0;
 
+	for (m = r->objects->multi_pack_index; m; m = m->next) {
+		if (fill_midx_entry(oid, e, m))
+			return 1;
+	}
+
 	list_for_each(pos, &r->objects->packed_git_mru) {
 		struct packed_git *p = list_entry(pos, struct packed_git, mru);
 		if (fill_pack_entry(oid, e, p)) {
@@ -1923,7 +2036,7 @@ int for_each_packed_object(each_packed_object_fn cb, void *data,
 	int pack_errors = 0;
 
 	prepare_packed_git(the_repository);
-	for (p = the_repository->objects->packed_git; p; p = p->next) {
+	for (p = get_all_packs(the_repository); p; p = p->next) {
 		if ((flags & FOR_EACH_OBJECT_LOCAL_ONLY) && !p->pack_local)
 			continue;
 		if ((flags & FOR_EACH_OBJECT_PROMISOR_ONLY) &&
diff --git a/packfile.h b/packfile.h
index 630f35cf31..442625723d 100644
--- a/packfile.h
+++ b/packfile.h
@@ -33,6 +33,12 @@ extern char *sha1_pack_index_name(const unsigned char *sha1);
 
 extern struct packed_git *parse_pack_index(unsigned char *sha1, const char *idx_path);
 
+typedef void each_file_in_pack_dir_fn(const char *full_path, size_t full_path_len,
+				      const char *file_pach, void *data);
+void for_each_file_in_pack_dir(const char *objdir,
+			       each_file_in_pack_dir_fn fn,
+			       void *data);
+
 /* A hook to report invalid files in pack directory */
 #define PACKDIR_FILE_PACK 1
 #define PACKDIR_FILE_IDX 2
@@ -44,6 +50,8 @@ extern void install_packed_git(struct repository *r, struct packed_git *pack);
 
 struct packed_git *get_packed_git(struct repository *r);
 struct list_head *get_packed_git_mru(struct repository *r);
+struct multi_pack_index *get_multi_pack_index(struct repository *r);
+struct packed_git *get_all_packs(struct repository *r);
 
 /*
  * Give a rough count of objects in the repository. This sacrifices accuracy
@@ -68,6 +76,8 @@ extern int open_pack_index(struct packed_git *);
  */
 extern void close_pack_index(struct packed_git *);
 
+extern uint32_t get_pack_fanout(struct packed_git *p, uint32_t value);
+
 extern unsigned char *use_pack(struct packed_git *, struct pack_window **, off_t, unsigned long *);
 extern void close_pack_windows(struct packed_git *);
 extern void close_pack(struct packed_git *);
diff --git a/server-info.c b/server-info.c
index 41050c2449..e2b2d6a27a 100644
--- a/server-info.c
+++ b/server-info.c
@@ -199,7 +199,7 @@ static void init_pack_info(const char *infofile, int force)
 	objdir = get_object_directory();
 	objdirlen = strlen(objdir);
 
-	for (p = get_packed_git(the_repository); p; p = p->next) {
+	for (p = get_all_packs(the_repository); p; p = p->next) {
 		/* we ignore things on alternate path since they are
 		 * not available to the pullers in general.
 		 */
@@ -209,7 +209,7 @@ static void init_pack_info(const char *infofile, int force)
 	}
 	num_pack = i;
 	info = xcalloc(num_pack, sizeof(struct pack_info *));
-	for (i = 0, p = get_packed_git(the_repository); p; p = p->next) {
+	for (i = 0, p = get_all_packs(the_repository); p; p = p->next) {
 		if (!p->pack_local)
 			continue;
 		info[i] = xcalloc(1, sizeof(struct pack_info));
diff --git a/sha1-name.c b/sha1-name.c
index c9cc1318b7..6cccfbbfbf 100644
--- a/sha1-name.c
+++ b/sha1-name.c
@@ -12,6 +12,7 @@
 #include "packfile.h"
 #include "object-store.h"
 #include "repository.h"
+#include "midx.h"
 
 static int get_oid_oneline(const char *, struct object_id *, struct commit_list *);
 
@@ -149,6 +150,32 @@ static int match_sha(unsigned len, const unsigned char *a, const unsigned char *
 	return 1;
 }
 
+static void unique_in_midx(struct multi_pack_index *m,
+			   struct disambiguate_state *ds)
+{
+	uint32_t num, i, first = 0;
+	const struct object_id *current = NULL;
+	num = m->num_objects;
+
+	if (!num)
+		return;
+
+	bsearch_midx(&ds->bin_pfx, m, &first);
+
+	/*
+	 * At this point, "first" is the location of the lowest object
+	 * with an object name that could match "bin_pfx".  See if we have
+	 * 0, 1 or more objects that actually match(es).
+	 */
+	for (i = first; i < num && !ds->ambiguous; i++) {
+		struct object_id oid;
+		current = nth_midxed_object_oid(&oid, m, i);
+		if (!match_sha(ds->len, ds->bin_pfx.hash, current->hash))
+			break;
+		update_candidates(ds, current);
+	}
+}
+
 static void unique_in_pack(struct packed_git *p,
 			   struct disambiguate_state *ds)
 {
@@ -177,8 +204,12 @@ static void unique_in_pack(struct packed_git *p,
 
 static void find_short_packed_object(struct disambiguate_state *ds)
 {
+	struct multi_pack_index *m;
 	struct packed_git *p;
 
+	for (m = get_multi_pack_index(the_repository); m && !ds->ambiguous;
+	     m = m->next)
+		unique_in_midx(m, ds);
 	for (p = get_packed_git(the_repository); p && !ds->ambiguous;
 	     p = p->next)
 		unique_in_pack(p, ds);
@@ -529,6 +560,42 @@ static int extend_abbrev_len(const struct object_id *oid, void *cb_data)
 	return 0;
 }
 
+static void find_abbrev_len_for_midx(struct multi_pack_index *m,
+				     struct min_abbrev_data *mad)
+{
+	int match = 0;
+	uint32_t num, first = 0;
+	struct object_id oid;
+	const struct object_id *mad_oid;
+
+	if (!m->num_objects)
+		return;
+
+	num = m->num_objects;
+	mad_oid = mad->oid;
+	match = bsearch_midx(mad_oid, m, &first);
+
+	/*
+	 * first is now the position in the packfile where we would insert
+	 * mad->hash if it does not exist (or the position of mad->hash if
+	 * it does exist). Hence, we consider a maximum of two objects
+	 * nearby for the abbreviation length.
+	 */
+	mad->init_len = 0;
+	if (!match) {
+		if (nth_midxed_object_oid(&oid, m, first))
+			extend_abbrev_len(&oid, mad);
+	} else if (first < num - 1) {
+		if (nth_midxed_object_oid(&oid, m, first + 1))
+			extend_abbrev_len(&oid, mad);
+	}
+	if (first > 0) {
+		if (nth_midxed_object_oid(&oid, m, first - 1))
+			extend_abbrev_len(&oid, mad);
+	}
+	mad->init_len = mad->cur_len;
+}
+
 static void find_abbrev_len_for_pack(struct packed_git *p,
 				     struct min_abbrev_data *mad)
 {
@@ -567,8 +634,11 @@ static void find_abbrev_len_for_pack(struct packed_git *p,
 
 static void find_abbrev_len_packed(struct min_abbrev_data *mad)
 {
+	struct multi_pack_index *m;
 	struct packed_git *p;
 
+	for (m = get_multi_pack_index(the_repository); m; m = m->next)
+		find_abbrev_len_for_midx(m, mad);
 	for (p = get_packed_git(the_repository); p; p = p->next)
 		find_abbrev_len_for_pack(p, mad);
 }
diff --git a/t/helper/test-read-midx.c b/t/helper/test-read-midx.c
new file mode 100644
index 0000000000..831b586d02
--- /dev/null
+++ b/t/helper/test-read-midx.c
@@ -0,0 +1,51 @@
+#include "test-tool.h"
+#include "cache.h"
+#include "midx.h"
+#include "repository.h"
+#include "object-store.h"
+
+static int read_midx_file(const char *object_dir)
+{
+	uint32_t i;
+	struct multi_pack_index *m = load_multi_pack_index(object_dir, 1);
+
+	if (!m)
+		return 1;
+
+	printf("header: %08x %d %d %d\n",
+	       m->signature,
+	       m->version,
+	       m->num_chunks,
+	       m->num_packs);
+
+	printf("chunks:");
+
+	if (m->chunk_pack_names)
+		printf(" pack-names");
+	if (m->chunk_oid_fanout)
+		printf(" oid-fanout");
+	if (m->chunk_oid_lookup)
+		printf(" oid-lookup");
+	if (m->chunk_object_offsets)
+		printf(" object-offsets");
+	if (m->chunk_large_offsets)
+		printf(" large-offsets");
+
+	printf("\nnum_objects: %d\n", m->num_objects);
+
+	printf("packs:\n");
+	for (i = 0; i < m->num_packs; i++)
+		printf("%s\n", m->pack_names[i]);
+
+	printf("object-dir: %s\n", m->object_dir);
+
+	return 0;
+}
+
+int cmd__read_midx(int argc, const char **argv)
+{
+	if (argc != 2)
+		usage("read-midx <object-dir>");
+
+	return read_midx_file(argv[1]);
+}
diff --git a/t/helper/test-tool.c b/t/helper/test-tool.c
index 0edafcfd65..3276701710 100644
--- a/t/helper/test-tool.c
+++ b/t/helper/test-tool.c
@@ -28,6 +28,7 @@ static struct test_cmd cmds[] = {
 	{ "path-utils", cmd__path_utils },
 	{ "prio-queue", cmd__prio_queue },
 	{ "read-cache", cmd__read_cache },
+	{ "read-midx", cmd__read_midx },
 	{ "ref-store", cmd__ref_store },
 	{ "regex", cmd__regex },
 	{ "repository", cmd__repository },
diff --git a/t/helper/test-tool.h b/t/helper/test-tool.h
index e954e8c522..710fb1b286 100644
--- a/t/helper/test-tool.h
+++ b/t/helper/test-tool.h
@@ -24,6 +24,7 @@ int cmd__online_cpus(int argc, const char **argv);
 int cmd__path_utils(int argc, const char **argv);
 int cmd__prio_queue(int argc, const char **argv);
 int cmd__read_cache(int argc, const char **argv);
+int cmd__read_midx(int argc, const char **argv);
 int cmd__ref_store(int argc, const char **argv);
 int cmd__regex(int argc, const char **argv);
 int cmd__repository(int argc, const char **argv);
diff --git a/t/t5319-multi-pack-index.sh b/t/t5319-multi-pack-index.sh
new file mode 100755
index 0000000000..6f56b38674
--- /dev/null
+++ b/t/t5319-multi-pack-index.sh
@@ -0,0 +1,217 @@
+#!/bin/sh
+
+test_description='multi-pack-indexes'
+. ./test-lib.sh
+
+objdir=.git/objects
+
+midx_read_expect () {
+	NUM_PACKS=$1
+	NUM_OBJECTS=$2
+	NUM_CHUNKS=$3
+	OBJECT_DIR=$4
+	EXTRA_CHUNKS="$5"
+	{
+		cat <<-EOF &&
+		header: 4d494458 1 $NUM_CHUNKS $NUM_PACKS
+		chunks: pack-names oid-fanout oid-lookup object-offsets$EXTRA_CHUNKS
+		num_objects: $NUM_OBJECTS
+		packs:
+		EOF
+		if test $NUM_PACKS -ge 1
+		then
+			ls $OBJECT_DIR/pack/ | grep idx | sort
+		fi &&
+		printf "object-dir: $OBJECT_DIR\n"
+	} >expect &&
+	test-tool read-midx $OBJECT_DIR >actual &&
+	test_cmp expect actual
+}
+
+test_expect_success 'write midx with no packs' '
+	test_when_finished rm -f pack/multi-pack-index &&
+	git multi-pack-index --object-dir=. write &&
+	midx_read_expect 0 0 4 .
+'
+
+generate_objects () {
+	i=$1
+	iii=$(printf '%03i' $i)
+	{
+		test-tool genrandom "bar" 200 &&
+		test-tool genrandom "baz $iii" 50
+	} >wide_delta_$iii &&
+	{
+		test-tool genrandom "foo"$i 100 &&
+		test-tool genrandom "foo"$(( $i + 1 )) 100 &&
+		test-tool genrandom "foo"$(( $i + 2 )) 100
+	} >deep_delta_$iii &&
+	{
+		echo $iii &&
+		test-tool genrandom "$iii" 8192
+	} >file_$iii &&
+	git update-index --add file_$iii deep_delta_$iii wide_delta_$iii
+}
+
+commit_and_list_objects () {
+	{
+		echo 101 &&
+		test-tool genrandom 100 8192;
+	} >file_101 &&
+	git update-index --add file_101 &&
+	tree=$(git write-tree) &&
+	commit=$(git commit-tree $tree -p HEAD</dev/null) &&
+	{
+		echo $tree &&
+		git ls-tree $tree | sed -e "s/.* \\([0-9a-f]*\\)	.*/\\1/"
+	} >obj-list &&
+	git reset --hard $commit
+}
+
+test_expect_success 'create objects' '
+	test_commit initial &&
+	for i in $(test_seq 1 5)
+	do
+		generate_objects $i
+	done &&
+	commit_and_list_objects
+'
+
+test_expect_success 'write midx with one v1 pack' '
+	pack=$(git pack-objects --index-version=1 $objdir/pack/test <obj-list) &&
+	test_when_finished rm $objdir/pack/test-$pack.pack \
+		$objdir/pack/test-$pack.idx $objdir/pack/multi-pack-index &&
+	git multi-pack-index --object-dir=$objdir write &&
+	midx_read_expect 1 18 4 $objdir
+'
+
+midx_git_two_modes () {
+	if [ "$2" = "sorted" ]
+	then
+		git -c core.multiPackIndex=false $1 | sort >expect &&
+		git -c core.multiPackIndex=true $1 | sort >actual
+	else
+		git -c core.multiPackIndex=false $1 >expect &&
+		git -c core.multiPackIndex=true $1 >actual
+	fi &&
+	test_cmp expect actual
+}
+
+compare_results_with_midx () {
+	MSG=$1
+	test_expect_success "check normal git operations: $MSG" '
+		midx_git_two_modes "rev-list --objects --all" &&
+		midx_git_two_modes "log --raw" &&
+		midx_git_two_modes "count-objects --verbose" &&
+		midx_git_two_modes "cat-file --batch-all-objects --buffer --batch-check" &&
+		midx_git_two_modes "cat-file --batch-all-objects --buffer --batch-check --unsorted" sorted
+	'
+}
+
+test_expect_success 'write midx with one v2 pack' '
+	git pack-objects --index-version=2,0x40 $objdir/pack/test <obj-list &&
+	git multi-pack-index --object-dir=$objdir write &&
+	midx_read_expect 1 18 4 $objdir
+'
+
+compare_results_with_midx "one v2 pack"
+
+test_expect_success 'add more objects' '
+	for i in $(test_seq 6 10)
+	do
+		generate_objects $i
+	done &&
+	commit_and_list_objects
+'
+
+test_expect_success 'write midx with two packs' '
+	git pack-objects --index-version=1 $objdir/pack/test-2 <obj-list &&
+	git multi-pack-index --object-dir=$objdir write &&
+	midx_read_expect 2 34 4 $objdir
+'
+
+compare_results_with_midx "two packs"
+
+test_expect_success 'add more packs' '
+	for j in $(test_seq 11 20)
+	do
+		generate_objects $j &&
+		commit_and_list_objects &&
+		git pack-objects --index-version=2 $objdir/pack/test-pack <obj-list
+	done
+'
+
+compare_results_with_midx "mixed mode (two packs + extra)"
+
+test_expect_success 'write midx with twelve packs' '
+	git multi-pack-index --object-dir=$objdir write &&
+	midx_read_expect 12 74 4 $objdir
+'
+
+compare_results_with_midx "twelve packs"
+
+test_expect_success 'repack removes multi-pack-index' '
+	test_path_is_file $objdir/pack/multi-pack-index &&
+	git repack -adf &&
+	test_path_is_missing $objdir/pack/multi-pack-index
+'
+
+compare_results_with_midx "after repack"
+
+test_expect_success 'multi-pack-index and pack-bitmap' '
+	git -c repack.writeBitmaps=true repack -ad &&
+	git multi-pack-index write &&
+	git rev-list --test-bitmap HEAD
+'
+
+test_expect_success 'multi-pack-index and alternates' '
+	git init --bare alt.git &&
+	echo $(pwd)/alt.git/objects >.git/objects/info/alternates &&
+	echo content1 >file1 &&
+	altblob=$(GIT_DIR=alt.git git hash-object -w file1) &&
+	git cat-file blob $altblob &&
+	git rev-list --all
+'
+
+compare_results_with_midx "with alternate (local midx)"
+
+test_expect_success 'multi-pack-index in an alternate' '
+	mv .git/objects/pack/* alt.git/objects/pack &&
+	test_commit add_local_objects &&
+	git repack --local &&
+	git multi-pack-index write &&
+	midx_read_expect 1 3 4 $objdir &&
+	git reset --hard HEAD~1 &&
+	rm -f .git/objects/pack/*
+'
+
+compare_results_with_midx "with alternate (remote midx)"
+
+
+# usage: corrupt_data <file> <pos> [<data>]
+corrupt_data () {
+	file=$1
+	pos=$2
+	data="${3:-\0}"
+	printf "$data" | dd of="$file" bs=1 seek="$pos" conv=notrunc
+}
+
+# Force 64-bit offsets by manipulating the idx file.
+# This makes the IDX file _incorrect_ so be careful to clean up after!
+test_expect_success 'force some 64-bit offsets with pack-objects' '
+	mkdir objects64 &&
+	mkdir objects64/pack &&
+	for i in $(test_seq 1 11)
+	do
+		generate_objects 11
+	done &&
+	commit_and_list_objects &&
+	pack64=$(git pack-objects --index-version=2,0x40 objects64/pack/test-64 <obj-list) &&
+	idx64=objects64/pack/test-64-$pack64.idx &&
+	chmod u+w $idx64 &&
+	corrupt_data $idx64 2999 "\02" &&
+	midx64=$(git multi-pack-index --object-dir=objects64 write) &&
+	midx_read_expect 1 63 5 objects64 " large-offsets"
+'
+
+test_done