From: Derrick Stolee <dstolee@xxxxxxxxxxxxx> The previous change cleaned up loose objects using the 'loose-objects' that can be run safely in the background. Add a similar job that performs similar cleanups for pack-files. One issue with running 'git repack' is that it is designed to repack all pack-files into a single pack-file. While this is the most space-efficient way to store object data, it is not time or memory efficient. This becomes extremely important if the repo is so large that a user struggles to store two copies of the pack on their disk. Instead, perform an "incremental" repack by collecting a few small pack-files into a new pack-file. The multi-pack-index facilitates this process ever since 'git multi-pack-index expire' was added in 19575c7 (multi-pack-index: implement 'expire' subcommand, 2019-06-10) and 'git multi-pack-index repack' was added in ce1e4a1 (midx: implement midx_repack(), 2019-06-10). The 'incremental-repack' task runs the following steps: 1. 'git multi-pack-index write' creates a multi-pack-index file if one did not exist, and otherwise will update the multi-pack-index with any new pack-files that appeared since the last write. This is particularly relevant with the background fetch job. When the multi-pack-index sees two copies of the same object, it stores the offset data into the newer pack-file. This means that some old pack-files could become "unreferenced" which I will use to mean "a pack-file that is in the pack-file list of the multi-pack-index but none of the objects in the multi-pack-index reference a location inside that pack-file." 2. 'git multi-pack-index expire' deletes any unreferenced pack-files and updaes the multi-pack-index to drop those pack-files from the list. This is safe to do as concurrent Git processes will see the multi-pack-index and not open those packs when looking for object contents. (Similar to the 'loose-objects' job, there are some Git commands that open pack-files regardless of the multi-pack-index, but they are rarely used. Further, a user that self-selects to use background operations would likely refrain from using those commands.) 3. 'git multi-pack-index repack --bacth-size=<size>' collects a set of pack-files that are listed in the multi-pack-index and creates a new pack-file containing the objects whose offsets are listed by the multi-pack-index to be in those objects. The set of pack- files is selected greedily by sorting the pack-files by modified time and adding a pack-file to the set if its "expected size" is smaller than the batch size until the total expected size of the selected pack-files is at least the batch size. The "expected size" is calculated by taking the size of the pack-file divided by the number of objects in the pack-file and multiplied by the number of objects from the multi-pack-index with offset in that pack-file. The expected size approximates how much data from that pack-file will contribute to the resulting pack-file size. The intention is that the resulting pack-file will be close in size to the provided batch size. The next run of the incremental-repack task will delete these repacked pack-files during the 'expire' step. In this version, the batch size is set to "0" which ignores the size restrictions when selecting the pack-files. It instead selects all pack-files and repacks all packed objects into a single pack-file. This will be updated in the next change, but it requires doing some calculations that are better isolated to a separate change. These steps are based on a similar background maintenance step in Scalar (and VFS for Git) [1]. This was incredibly effective for users of the Windows OS repository. After using the same VFS for Git repository for over a year, some users had _thousands_ of pack-files that combined to up to 250 GB of data. We noticed a few users were running into the open file descriptor limits (due in part to a bug in the multi-pack-index fixed by af96fe3 (midx: add packs to packed_git linked list, 2019-04-29). These pack-files were mostly small since they contained the commits and trees that were pushed to the origin in a given hour. The GVFS protocol includes a "prefetch" step that asks for pre-computed pack- files containing commits and trees by timestamp. These pack-files were grouped into "daily" pack-files once a day for up to 30 days. If a user did not request prefetch packs for over 30 days, then they would get the entire history of commits and trees in a new, large pack-file. This led to a large number of pack-files that had poor delta compression. By running this pack-file maintenance step once per day, these repos with thousands of packs spanning 200+ GB dropped to dozens of pack- files spanning 30-50 GB. This was done all without removing objects from the system and using a constant batch size of two gigabytes. Once the work was done to reduce the pack-files to small sizes, the batch size of two gigabytes means that not every run triggers a repack operation, so the following run will not expire a pack-file. This has kept these repos in a "clean" state. [1] https://github.com/microsoft/scalar/blob/master/Scalar.Common/Maintenance/PackfileMaintenanceStep.cs Signed-off-by: Derrick Stolee <dstolee@xxxxxxxxxxxxx> --- Documentation/git-maintenance.txt | 18 ++++++++ builtin/gc.c | 76 +++++++++++++++++++++++++++++++ t/t5319-multi-pack-index.sh | 1 + t/t7900-maintenance.sh | 38 ++++++++++++++++ 4 files changed, 133 insertions(+) diff --git a/Documentation/git-maintenance.txt b/Documentation/git-maintenance.txt index fc95eb594f..3f5d8946b4 100644 --- a/Documentation/git-maintenance.txt +++ b/Documentation/git-maintenance.txt @@ -85,6 +85,24 @@ loose-objects:: advisable to enable both the `loose-objects` and `gc` tasks at the same time. +incremental-repack:: + The `incremental-repack` job repacks the object directory + using the `multi-pack-index` feature. In order to prevent race + conditions with concurrent Git commands, it follows a two-step + process. First, it calls `git multi-pack-index expire` to delete + pack-files unreferenced by the `multi-pack-index` file. Second, it + calls `git multi-pack-index repack` to select several small + pack-files and repack them into a bigger one, and then update the + `multi-pack-index` entries that refer to the small pack-files to + refer to the new pack-file. This prepares those small pack-files + for deletion upon the next run of `git multi-pack-index expire`. + The selection of the small pack-files is such that the expected + size of the big pack-file is at least the batch size; see the + `--batch-size` option for the `repack` subcommand in + linkgit:git-multi-pack-index[1]. The default batch-size is zero, + which is a special case that attempts to repack all pack-files + into a single pack-file. + OPTIONS ------- --auto:: diff --git a/builtin/gc.c b/builtin/gc.c index 4403827481..5f877b097a 100644 --- a/builtin/gc.c +++ b/builtin/gc.c @@ -1001,6 +1001,77 @@ static int maintenance_task_loose_objects(struct maintenance_run_opts *opts) return prune_packed(opts) || pack_loose(opts); } +static int multi_pack_index_write(struct maintenance_run_opts *opts) +{ + struct child_process child = CHILD_PROCESS_INIT; + + child.git_cmd = 1; + strvec_pushl(&child.args, "multi-pack-index", "write", NULL); + + if (opts->quiet) + strvec_push(&child.args, "--no-progress"); + + if (run_command(&child)) + return error(_("failed to write multi-pack-index")); + + return 0; +} + +static int multi_pack_index_expire(struct maintenance_run_opts *opts) +{ + struct child_process child = CHILD_PROCESS_INIT; + + child.git_cmd = 1; + strvec_pushl(&child.args, "multi-pack-index", "expire", NULL); + + if (opts->quiet) + strvec_push(&child.args, "--no-progress"); + + close_object_store(the_repository->objects); + + if (run_command(&child)) + return error(_("'git multi-pack-index expire' failed")); + + return 0; +} + +static int multi_pack_index_repack(struct maintenance_run_opts *opts) +{ + struct child_process child = CHILD_PROCESS_INIT; + + child.git_cmd = 1; + strvec_pushl(&child.args, "multi-pack-index", "repack", NULL); + + if (opts->quiet) + strvec_push(&child.args, "--no-progress"); + + strvec_push(&child.args, "--batch-size=0"); + + close_object_store(the_repository->objects); + + if (run_command(&child)) + return error(_("'git multi-pack-index repack' failed")); + + return 0; +} + +static int maintenance_task_incremental_repack(struct maintenance_run_opts *opts) +{ + prepare_repo_settings(the_repository); + if (!the_repository->settings.core_multi_pack_index) { + warning(_("skipping incremental-repack task because core.multiPackIndex is disabled")); + return 0; + } + + if (multi_pack_index_write(opts)) + return 1; + if (multi_pack_index_expire(opts)) + return 1; + if (multi_pack_index_repack(opts)) + return 1; + return 0; +} + typedef int maintenance_task_fn(struct maintenance_run_opts *opts); /* @@ -1023,6 +1094,7 @@ struct maintenance_task { enum maintenance_task_label { TASK_PREFETCH, TASK_LOOSE_OBJECTS, + TASK_INCREMENTAL_REPACK, TASK_GC, TASK_COMMIT_GRAPH, @@ -1040,6 +1112,10 @@ static struct maintenance_task tasks[] = { maintenance_task_loose_objects, loose_object_auto_condition, }, + [TASK_INCREMENTAL_REPACK] = { + "incremental-repack", + maintenance_task_incremental_repack, + }, [TASK_GC] = { "gc", maintenance_task_gc, diff --git a/t/t5319-multi-pack-index.sh b/t/t5319-multi-pack-index.sh index ec87f616c6..2f942ee1fa 100755 --- a/t/t5319-multi-pack-index.sh +++ b/t/t5319-multi-pack-index.sh @@ -3,6 +3,7 @@ test_description='multi-pack-indexes' . ./test-lib.sh +GIT_TEST_MULTI_PACK_INDEX=0 objdir=.git/objects midx_read_expect () { diff --git a/t/t7900-maintenance.sh b/t/t7900-maintenance.sh index 27565c55a2..a2db2291b0 100755 --- a/t/t7900-maintenance.sh +++ b/t/t7900-maintenance.sh @@ -5,6 +5,7 @@ test_description='git maintenance builtin' . ./test-lib.sh GIT_TEST_COMMIT_GRAPH=0 +GIT_TEST_MULTI_PACK_INDEX=0 test_expect_success 'help text' ' test_expect_code 129 git maintenance -h 2>err && @@ -149,4 +150,41 @@ test_expect_success 'maintenance.loose-objects.auto' ' test_subcommand git prune-packed --quiet <trace-loC ' +test_expect_success 'incremental-repack task' ' + packDir=.git/objects/pack && + for i in $(test_seq 1 5) + do + test_commit $i || return 1 + done && + + # Create three disjoint pack-files with size BIG, small, small. + echo HEAD~2 | git pack-objects --revs $packDir/test-1 && + test_tick && + git pack-objects --revs $packDir/test-2 <<-\EOF && + HEAD~1 + ^HEAD~2 + EOF + test_tick && + git pack-objects --revs $packDir/test-3 <<-\EOF && + HEAD + ^HEAD~1 + EOF + rm -f $packDir/pack-* && + rm -f $packDir/loose-* && + ls $packDir/*.pack >packs-before && + test_line_count = 3 packs-before && + + # the job repacks the two into a new pack, but does not + # delete the old ones. + git maintenance run --task=incremental-repack && + ls $packDir/*.pack >packs-between && + test_line_count = 4 packs-between && + + # the job deletes the two old packs, and does not write + # a new one because only one pack remains. + git maintenance run --task=incremental-repack && + ls .git/objects/pack/*.pack >packs-after && + test_line_count = 1 packs-after +' + test_done -- gitgitgadget
