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git/Documentation/git-gc.txt
Taylor Blau 37dc6d8104 builtin/repack.c: implement support for `--max-cruft-size` 
Cruft packs are an alternative mechanism for storing a collection of
unreachable objects whose mtimes are recent enough to avoid being
pruned out of the repository.

When cruft packs were first introduced back in b757353676
(builtin/pack-objects.c: --cruft without expiration, 2022-05-20) and
a7d493833f (builtin/pack-objects.c: --cruft with expiration,
2022-05-20), the recommended workflow consisted of:

  - Repacking periodically, either by packing anything loose in the
    repository (via `git repack -d`) or producing a geometric sequence
    of packs (via `git repack --geometric=<d> -d`).

  - Every so often, splitting the repository into two packs, one cruft
    to store the unreachable objects, and another non-cruft pack to
    store the reachable objects.

Repositories may (out of band with the above) choose periodically to
prune out some unreachable objects which have aged out of the grace
period by generating a pack with `--cruft-expiration=<approxidate>`.

This allowed repositories to maintain relatively few packs on average,
and quarantine unreachable objects together in a cruft pack, avoiding
the pitfalls of holding unreachable objects as loose while they age out
(for more, see some of the details in 3d89a8c118
(Documentation/technical: add cruft-packs.txt, 2022-05-20)).

This all works, but can be costly from an I/O-perspective when
frequently repacking a repository that has many unreachable objects.
This problem is exacerbated when those unreachable objects are rarely
(if every) pruned.

Since there is at most one cruft pack in the above scheme, each time we
update the cruft pack it must be rewritten from scratch. Because much of
the pack is reused, this is a relatively inexpensive operation from a
CPU-perspective, but is very costly in terms of I/O since we end up
rewriting basically the same pack (plus any new unreachable objects that
have entered the repository since the last time a cruft pack was
generated).

At the time, we decided against implementing more robust support for
multiple cruft packs. This patch implements that support which we were
lacking.

Introduce a new option `--max-cruft-size` which allows repositories to
accumulate cruft packs up to a given size, after which point a new
generation of cruft packs can accumulate until it reaches the maximum
size, and so on. To generate a new cruft pack, the process works like
so:

  - Sort a list of any existing cruft packs in ascending order of pack
    size.

  - Starting from the beginning of the list, group cruft packs together
    while the accumulated size is smaller than the maximum specified
    pack size.

  - Combine the objects in these cruft packs together into a new cruft
    pack, along with any other unreachable objects which have since
    entered the repository.

Once a cruft pack grows beyond the size specified via `--max-cruft-size`
the pack is effectively frozen. This limits the I/O churn up to a
quadratic function of the value specified by the `--max-cruft-size`
option, instead of behaving quadratically in the number of total
unreachable objects.

When pruning unreachable objects, we bypass the new code paths which
combine small cruft packs together, and instead start from scratch,
passing in the appropriate `--max-pack-size` down to `pack-objects`,
putting it in charge of keeping the resulting set of cruft packs sized
correctly.

This may seem like further I/O churn, but in practice it isn't so bad.
We could prune old cruft packs for whom all or most objects are removed,
and then generate a new cruft pack with just the remaining set of
objects. But this additional complexity buys us relatively little,
because most objects end up being pruned anyway, so the I/O churn is
well contained.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2023-10-05 13:26:11 -07:00

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git-gc(1)
=========
NAME
----
git-gc - Cleanup unnecessary files and optimize the local repository
SYNOPSIS
--------
[verse]
'git gc' [--aggressive] [--auto] [--quiet] [--prune=<date> | --no-prune] [--force] [--keep-largest-pack]
DESCRIPTION
-----------
Runs a number of housekeeping tasks within the current repository,
such as compressing file revisions (to reduce disk space and increase
performance), removing unreachable objects which may have been
created from prior invocations of 'git add', packing refs, pruning
reflog, rerere metadata or stale working trees. May also update ancillary
indexes such as the commit-graph.
When common porcelain operations that create objects are run, they
will check whether the repository has grown substantially since the
last maintenance, and if so run `git gc` automatically. See `gc.auto`
below for how to disable this behavior.
Running `git gc` manually should only be needed when adding objects to
a repository without regularly running such porcelain commands, to do
a one-off repository optimization, or e.g. to clean up a suboptimal
mass-import. See the "PACKFILE OPTIMIZATION" section in
linkgit:git-fast-import[1] for more details on the import case.
OPTIONS
-------
--aggressive::
Usually 'git gc' runs very quickly while providing good disk
space utilization and performance. This option will cause
'git gc' to more aggressively optimize the repository at the expense
of taking much more time. The effects of this optimization are
mostly persistent. See the "AGGRESSIVE" section below for details.
--auto::
With this option, 'git gc' checks whether any housekeeping is
required; if not, it exits without performing any work.
+
See the `gc.auto` option in the "CONFIGURATION" section below for how
this heuristic works.
+
Once housekeeping is triggered by exceeding the limits of
configuration options such as `gc.auto` and `gc.autoPackLimit`, all
other housekeeping tasks (e.g. rerere, working trees, reflog...) will
be performed as well.
--[no-]cruft::
When expiring unreachable objects, pack them separately into a
cruft pack instead of storing them as loose objects. `--cruft`
is on by default.
--max-cruft-size=<n>::
When packing unreachable objects into a cruft pack, limit the
size of new cruft packs to be at most `<n>` bytes. Overrides any
value specified via the `gc.maxCruftSize` configuration. See
the `--max-cruft-size` option of linkgit:git-repack[1] for
more.
--prune=<date>::
Prune loose objects older than date (default is 2 weeks ago,
overridable by the config variable `gc.pruneExpire`).
--prune=now prunes loose objects regardless of their age and
increases the risk of corruption if another process is writing to
the repository concurrently; see "NOTES" below. --prune is on by
default.
--no-prune::
Do not prune any loose objects.
--quiet::
Suppress all progress reports.
--force::
Force `git gc` to run even if there may be another `git gc`
instance running on this repository.
--keep-largest-pack::
All packs except the largest non-cruft pack, any packs marked
with a `.keep` file, and any cruft pack(s) are consolidated into
a single pack. When this option is used, `gc.bigPackThreshold`
is ignored.
AGGRESSIVE
----------
When the `--aggressive` option is supplied, linkgit:git-repack[1] will
be invoked with the `-f` flag, which in turn will pass
`--no-reuse-delta` to linkgit:git-pack-objects[1]. This will throw
away any existing deltas and re-compute them, at the expense of
spending much more time on the repacking.
The effects of this are mostly persistent, e.g. when packs and loose
objects are coalesced into one another pack the existing deltas in
that pack might get re-used, but there are also various cases where we
might pick a sub-optimal delta from a newer pack instead.
Furthermore, supplying `--aggressive` will tweak the `--depth` and
`--window` options passed to linkgit:git-repack[1]. See the
`gc.aggressiveDepth` and `gc.aggressiveWindow` settings below. By
using a larger window size we're more likely to find more optimal
deltas.
It's probably not worth it to use this option on a given repository
without running tailored performance benchmarks on it. It takes a lot
more time, and the resulting space/delta optimization may or may not
be worth it. Not using this at all is the right trade-off for most
users and their repositories.
CONFIGURATION
-------------
include::includes/cmd-config-section-all.txt[]
include::config/gc.txt[]
NOTES
-----
'git gc' tries very hard not to delete objects that are referenced
anywhere in your repository. In particular, it will keep not only
objects referenced by your current set of branches and tags, but also
objects referenced by the index, remote-tracking branches, reflogs
(which may reference commits in branches that were later amended or
rewound), and anything else in the refs/* namespace. Note that a note
(of the kind created by 'git notes') attached to an object does not
contribute in keeping the object alive. If you are expecting some
objects to be deleted and they aren't, check all of those locations
and decide whether it makes sense in your case to remove those
references.
On the other hand, when 'git gc' runs concurrently with another process,
there is a risk of it deleting an object that the other process is using
but hasn't created a reference to. This may just cause the other process
to fail or may corrupt the repository if the other process later adds a
reference to the deleted object. Git has two features that significantly
mitigate this problem:
. Any object with modification time newer than the `--prune` date is kept,
along with everything reachable from it.
. Most operations that add an object to the database update the
modification time of the object if it is already present so that #1
applies.
However, these features fall short of a complete solution, so users who
run commands concurrently have to live with some risk of corruption (which
seems to be low in practice).
HOOKS
-----
The 'git gc --auto' command will run the 'pre-auto-gc' hook. See
linkgit:githooks[5] for more information.
SEE ALSO
--------
linkgit:git-prune[1]
linkgit:git-reflog[1]
linkgit:git-repack[1]
linkgit:git-rerere[1]
GIT
---
Part of the linkgit:git[1] suite
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