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git-commit-vandalism/Documentation/git-gc.txt
Nguyễn Thái Ngọc Duy 125f81461d gc --aggressive: make --depth configurable 
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>
2014-03-31 10:26:24 -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]
DESCRIPTION
-----------
Runs a number of housekeeping tasks within the current repository,
such as compressing file revisions (to reduce disk space and increase
performance) and removing unreachable objects which may have been
created from prior invocations of 'git add'.
Users are encouraged to run this task on a regular basis within
each repository to maintain good disk space utilization and good
operating performance.
Some git commands may automatically run 'git gc'; see the `--auto` flag
below for details. If you know what you're doing and all you want is to
disable this behavior permanently without further considerations, just do:
----------------------
$ git config --global gc.auto 0
----------------------
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
persistent, so this option only needs to be used occasionally; every
few hundred changesets or so.
--auto::
With this option, 'git gc' checks whether any housekeeping is
required; if not, it exits without performing any work.
Some git commands run `git gc --auto` after performing
operations that could create many loose objects.
+
Housekeeping is required if there are too many loose objects or
too many packs in the repository. If the number of loose objects
exceeds the value of the `gc.auto` configuration variable, then
all loose objects are combined into a single pack using
`git repack -d -l`. Setting the value of `gc.auto` to 0
disables automatic packing of loose objects.
+
If the number of packs exceeds the value of `gc.autopacklimit`,
then existing packs (except those marked with a `.keep` file)
are consolidated into a single pack by using the `-A` option of
'git repack'. Setting `gc.autopacklimit` to 0 disables
automatic consolidation of packs.
--prune=<date>::
Prune loose objects older than date (default is 2 weeks ago,
overridable by the config variable `gc.pruneExpire`).
--prune=all prunes loose objects regardless of their age.
--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.
Configuration
-------------
The optional configuration variable 'gc.reflogExpire' can be
set to indicate how long historical entries within each branch's
reflog should remain available in this repository. The setting is
expressed as a length of time, for example '90 days' or '3 months'.
It defaults to '90 days'.
The optional configuration variable 'gc.reflogExpireUnreachable'
can be set to indicate how long historical reflog entries which
are not part of the current branch should remain available in
this repository. These types of entries are generally created as
a result of using `git commit --amend` or `git rebase` and are the
commits prior to the amend or rebase occurring. Since these changes
are not part of the current project most users will want to expire
them sooner. This option defaults to '30 days'.
The above two configuration variables can be given to a pattern. For
example, this sets non-default expiry values only to remote-tracking
branches:
------------
[gc "refs/remotes/*"]
reflogExpire = never
reflogexpireUnreachable = 3 days
------------
The optional configuration variable 'gc.rerereresolved' indicates
how long records of conflicted merge you resolved earlier are
kept. This defaults to 60 days.
The optional configuration variable 'gc.rerereunresolved' indicates
how long records of conflicted merge you have not resolved are
kept. This defaults to 15 days.
The optional configuration variable 'gc.packrefs' determines if
'git gc' runs 'git pack-refs'. This can be set to "notbare" to enable
it within all non-bare repos or it can be set to a boolean value.
This defaults to true.
The optional configuration variable 'gc.aggressiveWindow' controls how
much time is spent optimizing the delta compression of the objects in
the repository when the --aggressive option is specified. The larger
the value, the more time is spent optimizing the delta compression. See
the documentation for the --window' option in linkgit:git-repack[1] for
more details. This defaults to 250.
Similarly, the optional configuration variable 'gc.aggressiveDepth'
controls --depth option in linkgit:git-repack[1]. This defaults to 250.
The optional configuration variable 'gc.pruneExpire' controls how old
the unreferenced loose objects have to be before they are pruned. The
default is "2 weeks ago".
Notes
-----
'git gc' tries very hard to be safe about the garbage it collects. 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, refs saved by 'git filter-branch' in
refs/original/, or reflogs (which may reference commits in branches
that were later amended or rewound).
If you are expecting some objects to be collected and they aren't, check
all of those locations and decide whether it makes sense in your case to
remove those references.
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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