Merge pull request #84 Create 'expire' and 'repack' subcommands for multi-pack-index

The multi-pack-index provides a fast way to find an object among a large list of pack-files. It stores a single pack-reference for each object id, so duplicate objects are ignored. Among a list of pack-files storing the same object, the most-recently modified one is used.

Create new subcommands for the multi-pack-index builtin.

* 'git multi-pack-index expire': If we have a pack-file indexed by the multi-pack-index, but all objects in that pack are duplicated in more-recently modified packs, then delete that pack (and any others like it). Delete the reference to that pack in the multi-pack-index.

* 'git multi-pack-index repack --batch-size=<size>': Starting from the oldest pack-files covered by the multi-pack-index, find those whose on-disk size is below the batch size until we have a collection of packs whose sizes add up to the batch size. Create a new pack containing all objects that the multi-pack-index references to those packs.

This allows us to create a new pattern for repacking objects: run 'repack'. After enough time has passed that all Git commands that started before the last 'repack' are finished, run 'expire' again. This approach has some advantages over the existing "repack everything" model:

1. Incremental. We can repack a small batch of objects at a time, instead of repacking all reachable objects. We can also limit ourselves to the objects that do not appear in newer pack-files.

2. Highly Available. By adding a new pack-file (and not deleting the old pack-files) we do not interrupt concurrent Git commands, and do not suffer performance degradation. By expiring only pack-files that have no referenced objects, we know that Git commands that are doing normal object lookups* will not be interrupted.

* Note: if someone concurrently runs a Git command that uses get_all_packs(), then that command could try to read the pack-files and pack-indexes that we are deleting during an expire command. Such commands are usually related to object maintenance (i.e. fsck, gc, pack-objects) or are related to less-often-used features (i.e. fast-import, http-backend, server-info).

We plan to use this approach in VFS for Git to do background maintenance of the "shared object cache" which is a Git alternate directory filled with packfiles containing commits and trees. We currently download pack-files on an hourly basis to keep up-to-date with the central server. The cache servers supply packs on an hourly and daily basis, so most of the hourly packs become useless after a new daily pack is downloaded. The 'expire' command would clear out most of those packs, but many will still remain with fewer than 100 objects remaining. The 'repack' command (with a batch size of 1-3gb, probably) can condense the remaining packs in commands that run for 1-3 min at a time. Since the daily packs range from 100-250mb, we will also combine and condense those packs.

Author: derrickstolee

Documentation/git-multi-pack-index.txt

@@ -9,7 +9,7 @@ git-multi-pack-index - Write and verify multi-pack-indexes
 SYNOPSIS
 --------
 [verse]
-'git multi-pack-index' [--object-dir=<dir>] <verb>
+'git multi-pack-index' [--object-dir=<dir>] <subcommand>
 
 DESCRIPTION
 -----------
@@ -23,13 +23,29 @@ OPTIONS
 	`<dir>/packs/multi-pack-index` for the current MIDX file, and
 	`<dir>/packs` for the pack-files to index.
 
+The following subcommands are available:
+
 write::
-	When given as the verb, write a new MIDX file to
-	`<dir>/packs/multi-pack-index`.
+	Write a new MIDX file.
 
 verify::
-	When given as the verb, verify the contents of the MIDX file
-	at `<dir>/packs/multi-pack-index`.
+	Verify the contents of the MIDX file.
+
+expire::
+	Delete the pack-files that are tracked 	by the MIDX file, but
+	have no objects referenced by the MIDX. Rewrite the MIDX file
+	afterward to remove all references to these pack-files.
+
+repack::
+	Collect a batch of pack-files whose size are all at most the
+	size given by --batch-size, but whose sizes sum to larger
+	than --batch-size. The batch is selected by greedily adding
+	small pack-files starting with the oldest pack-files that fit
+	the size. Create a new pack-file containing the objects the
+	multi-pack-index indexes into those pack-files, and rewrite
+	the multi-pack-index to contain that pack-file. A later run
+	of 'git multi-pack-index expire' will delete the pack-files
+	that were part of this batch.
 
 
 EXAMPLES

builtin/multi-pack-index.c

@@ -5,12 +5,13 @@
 #include "midx.h"
 
 static char const * const builtin_multi_pack_index_usage[] = {
-	N_("git multi-pack-index [--object-dir=<dir>] (write|verify)"),
+	N_("git multi-pack-index [--object-dir=<dir>] (write|verify|expire|repack --batch-size=<size>)"),
 	NULL
 };
 
 static struct opts_multi_pack_index {
 	const char *object_dir;
+	unsigned long batch_size;
 } opts;
 
 int cmd_multi_pack_index(int argc, const char **argv,
@@ -19,6 +20,8 @@ int cmd_multi_pack_index(int argc, const char **argv,
 	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_MAGNITUDE(0, "batch-size", &opts.batch_size,
+		  N_("during repack, collect pack-files of smaller size into a batch that is larger than this size")),
 		OPT_END(),
 	};
 
@@ -40,10 +43,17 @@ int cmd_multi_pack_index(int argc, const char **argv,
 		return 1;
 	}
 
+	if (!strcmp(argv[0], "repack"))
+		return midx_repack(opts.object_dir, (size_t)opts.batch_size);
+	if (opts.batch_size)
+		die(_("--batch-size option is only for 'repack' verb"));
+
 	if (!strcmp(argv[0], "write"))
 		return write_midx_file(opts.object_dir);
 	if (!strcmp(argv[0], "verify"))
 		return verify_midx_file(opts.object_dir);
+	if (!strcmp(argv[0], "expire"))
+		return expire_midx_packs(opts.object_dir);
 
 	die(_("unrecognized verb: %s"), argv[0]);
 }

builtin/repack.c

@@ -129,19 +129,9 @@ static void get_non_kept_pack_filenames(struct string_list *fname_list,
 
 static void remove_redundant_pack(const char *dir_name, const char *base_name)
 {
-	const char *exts[] = {".pack", ".idx", ".keep", ".bitmap", ".promisor"};
-	int i;
 	struct strbuf buf = STRBUF_INIT;
-	size_t plen;
-
-	strbuf_addf(&buf, "%s/%s", dir_name, base_name);
-	plen = buf.len;
-
-	for (i = 0; i < ARRAY_SIZE(exts); i++) {
-		strbuf_setlen(&buf, plen);
-		strbuf_addstr(&buf, exts[i]);
-		unlink(buf.buf);
-	}
+	strbuf_addf(&buf, "%s/%s.pack", dir_name, base_name);
+	unlink_pack_path(buf.buf, 1);
 	strbuf_release(&buf);
 }