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When 1c192f3 (gc --aggressive: make it really aggressive - 2007-12-06)
made --depth=250 the default value, it didn't really explain the
reason behind, especially the pros and cons of --depth=250.
An old mail from Linus below explains it at length. Long story short,
--depth=250 is a disk saver and a performance killer. Not everybody
agrees on that aggressiveness. Let the user configure it.
From: Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH] gc --aggressive: make it really aggressive
Date: Thu, 6 Dec 2007 08:19:24 -0800 (PST)
Message-ID: <alpine.LFD.0.9999.0712060803430.13796@woody.linux-foundation.org>
Gmane-URL: http://article.gmane.org/gmane.comp.gcc.devel/94637
On Thu, 6 Dec 2007, Harvey Harrison wrote:
>
> 7:41:25elapsed 86%CPU
Heh. And this is why you want to do it exactly *once*, and then just
export the end result for others ;)
> -r--r--r-- 1 hharrison hharrison 324094684 2007-12-06 07:26 pack-1d46...pack
But yeah, especially if you allow longer delta chains, the end result can
be much smaller (and what makes the one-time repack more expensive is the
window size, not the delta chain - you could make the delta chains longer
with no cost overhead at packing time)
HOWEVER.
The longer delta chains do make it potentially much more expensive to then
use old history. So there's a trade-off. And quite frankly, a delta depth
of 250 is likely going to cause overflows in the delta cache (which is
only 256 entries in size *and* it's a hash, so it's going to start having
hash conflicts long before hitting the 250 depth limit).
So when I said "--depth=250 --window=250", I chose those numbers more as
an example of extremely aggressive packing, and I'm not at all sure that
the end result is necessarily wonderfully usable. It's going to save disk
space (and network bandwidth - the delta's will be re-used for the network
protocol too!), but there are definitely downsides too, and using long
delta chains may simply not be worth it in practice.
(And some of it might just want to have git tuning, ie if people think
that long deltas are worth it, we could easily just expand on the delta
hash, at the cost of some more memory used!)
That said, the good news is that working with *new* history will not be
affected negatively, and if you want to be _really_ sneaky, there are ways
to say "create a pack that contains the history up to a version one year
ago, and be very aggressive about those old versions that we still want to
have around, but do a separate pack for newer stuff using less aggressive
parameters"
So this is something that can be tweaked, although we don't really have
any really nice interfaces for stuff like that (ie the git delta cache
size is hardcoded in the sources and cannot be set in the config file, and
the "pack old history more aggressively" involves some manual scripting
and knowing how "git pack-objects" works rather than any nice simple
command line switch).
So the thing to take away from this is:
- git is certainly flexible as hell
- .. but to get the full power you may need to tweak things
- .. happily you really only need to have one person to do the tweaking,
and the tweaked end results will be available to others that do not
need to know/care.
And whether the difference between 320MB and 500MB is worth any really
involved tweaking (considering the potential downsides), I really don't
know. Only testing will tell.
Linus
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
165 lines
5.7 KiB
Text
165 lines
5.7 KiB
Text
git-gc(1)
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=========
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NAME
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----
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git-gc - Cleanup unnecessary files and optimize the local repository
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SYNOPSIS
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--------
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[verse]
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'git gc' [--aggressive] [--auto] [--quiet] [--prune=<date> | --no-prune] [--force]
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DESCRIPTION
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-----------
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Runs a number of housekeeping tasks within the current repository,
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such as compressing file revisions (to reduce disk space and increase
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performance) and removing unreachable objects which may have been
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created from prior invocations of 'git add'.
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Users are encouraged to run this task on a regular basis within
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each repository to maintain good disk space utilization and good
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operating performance.
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Some git commands may automatically run 'git gc'; see the `--auto` flag
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below for details. If you know what you're doing and all you want is to
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disable this behavior permanently without further considerations, just do:
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----------------------
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$ git config --global gc.auto 0
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----------------------
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OPTIONS
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-------
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--aggressive::
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Usually 'git gc' runs very quickly while providing good disk
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space utilization and performance. This option will cause
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'git gc' to more aggressively optimize the repository at the expense
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of taking much more time. The effects of this optimization are
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persistent, so this option only needs to be used occasionally; every
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few hundred changesets or so.
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--auto::
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With this option, 'git gc' checks whether any housekeeping is
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required; if not, it exits without performing any work.
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Some git commands run `git gc --auto` after performing
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operations that could create many loose objects.
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+
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Housekeeping is required if there are too many loose objects or
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too many packs in the repository. If the number of loose objects
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exceeds the value of the `gc.auto` configuration variable, then
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all loose objects are combined into a single pack using
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`git repack -d -l`. Setting the value of `gc.auto` to 0
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disables automatic packing of loose objects.
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+
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If the number of packs exceeds the value of `gc.autopacklimit`,
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then existing packs (except those marked with a `.keep` file)
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are consolidated into a single pack by using the `-A` option of
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'git repack'. Setting `gc.autopacklimit` to 0 disables
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automatic consolidation of packs.
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--prune=<date>::
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Prune loose objects older than date (default is 2 weeks ago,
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overridable by the config variable `gc.pruneExpire`).
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--prune=all prunes loose objects regardless of their age.
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--prune is on by default.
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--no-prune::
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Do not prune any loose objects.
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--quiet::
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Suppress all progress reports.
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--force::
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Force `git gc` to run even if there may be another `git gc`
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instance running on this repository.
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Configuration
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-------------
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The optional configuration variable 'gc.reflogExpire' can be
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set to indicate how long historical entries within each branch's
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reflog should remain available in this repository. The setting is
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expressed as a length of time, for example '90 days' or '3 months'.
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It defaults to '90 days'.
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The optional configuration variable 'gc.reflogExpireUnreachable'
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can be set to indicate how long historical reflog entries which
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are not part of the current branch should remain available in
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this repository. These types of entries are generally created as
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a result of using `git commit --amend` or `git rebase` and are the
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commits prior to the amend or rebase occurring. Since these changes
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are not part of the current project most users will want to expire
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them sooner. This option defaults to '30 days'.
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The above two configuration variables can be given to a pattern. For
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example, this sets non-default expiry values only to remote-tracking
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branches:
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------------
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[gc "refs/remotes/*"]
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reflogExpire = never
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reflogexpireUnreachable = 3 days
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------------
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The optional configuration variable 'gc.rerereresolved' indicates
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how long records of conflicted merge you resolved earlier are
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kept. This defaults to 60 days.
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The optional configuration variable 'gc.rerereunresolved' indicates
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how long records of conflicted merge you have not resolved are
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kept. This defaults to 15 days.
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The optional configuration variable 'gc.packrefs' determines if
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'git gc' runs 'git pack-refs'. This can be set to "notbare" to enable
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it within all non-bare repos or it can be set to a boolean value.
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This defaults to true.
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The optional configuration variable 'gc.aggressiveWindow' controls how
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much time is spent optimizing the delta compression of the objects in
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the repository when the --aggressive option is specified. The larger
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the value, the more time is spent optimizing the delta compression. See
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the documentation for the --window' option in linkgit:git-repack[1] for
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more details. This defaults to 250.
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Similarly, the optional configuration variable 'gc.aggressiveDepth'
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controls --depth option in linkgit:git-repack[1]. This defaults to 250.
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The optional configuration variable 'gc.pruneExpire' controls how old
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the unreferenced loose objects have to be before they are pruned. The
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default is "2 weeks ago".
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Notes
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-----
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'git gc' tries very hard to be safe about the garbage it collects. In
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particular, it will keep not only objects referenced by your current set
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of branches and tags, but also objects referenced by the index,
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remote-tracking branches, refs saved by 'git filter-branch' in
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refs/original/, or reflogs (which may reference commits in branches
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that were later amended or rewound).
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If you are expecting some objects to be collected and they aren't, check
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all of those locations and decide whether it makes sense in your case to
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remove those references.
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HOOKS
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-----
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The 'git gc --auto' command will run the 'pre-auto-gc' hook. See
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linkgit:githooks[5] for more information.
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SEE ALSO
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--------
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linkgit:git-prune[1]
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linkgit:git-reflog[1]
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linkgit:git-repack[1]
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linkgit:git-rerere[1]
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GIT
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---
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Part of the linkgit:git[1] suite
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