Create 'expire' and 'repack' verbs for git-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=': Starting from the oldest pack-files covered by the multi-pack-index, find those whose "expected size" is below the batch size until we have a collection of packs whose expected sizes add up to the batch size. We compute the expected size by multiplying the number of referenced objects by the pack-size and dividing by the total number of objects in the pack. If the batch-size is zero, then select all packs. 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 are using 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.

This series is the same as v6 of an earlier series [1].

Thanks, -Stolee

[1] https://public-inbox.org/git/[email protected]/T/#u

Derrick Stolee (11):
repack: refactor pack deletion for future use
Docs: rearrange subcommands for multi-pack-index
multi-pack-index: prepare for 'expire' subcommand
midx: simplify computation of pack name lengths
midx: refactor permutation logic and pack sorting
multi-pack-index: implement 'expire' subcommand
multi-pack-index: prepare 'repack' subcommand
midx: implement midx_repack()
multi-pack-index: test expire while adding packs
midx: add test that 'expire' respects .keep files t5319-multi-pack-index.sh: test batch size zero

 Documentation/git-multi-pack-index.txt |  32 +-
 builtin/multi-pack-index.c             |  14 +-
 builtin/repack.c                       |  14 +-
 midx.c                                 | 440 +++++++++++++++++++------
 midx.h                                 |   2 +
 packfile.c                             |  28 ++
 packfile.h                             |   7 +
 t/t5319-multi-pack-index.sh            | 184 +++++++++++
 8 files changed, 602 insertions(+), 119 deletions(-)

base-commit: af96fe3392fb078cb5447bcb94f2ed8d79d0a4a8
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-261%2Fderrickstolee%2Fmidx-expire%2Fupstream-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-261/derrickstolee/midx-expire/upstream-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/261

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