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git-commit-vandalism/Documentation/git-repack.txt
Taylor Blau 0fabafd0b9 builtin/repack.c: add '--geometric' option 
Often it is useful to both:

  - have relatively few packfiles in a repository, and

  - avoid having so few packfiles in a repository that we repack its
    entire contents regularly

This patch implements a '--geometric=<n>' option in 'git repack'. This
allows the caller to specify that they would like each pack to be at
least a factor times as large as the previous largest pack (by object
count).

Concretely, say that a repository has 'n' packfiles, labeled P1, P2,
..., up to Pn. Each packfile has an object count equal to 'objects(Pn)'.
With a geometric factor of 'r', it should be that:

  objects(Pi) > r*objects(P(i-1))

for all i in [1, n], where the packs are sorted by

  objects(P1) <= objects(P2) <= ... <= objects(Pn).

Since finding a true optimal repacking is NP-hard, we approximate it
along two directions:

  1. We assume that there is a cutoff of packs _before starting the
     repack_ where everything to the right of that cut-off already forms
     a geometric progression (or no cutoff exists and everything must be
     repacked).

  2. We assume that everything smaller than the cutoff count must be
     repacked. This forms our base assumption, but it can also cause
     even the "heavy" packs to get repacked, for e.g., if we have 6
     packs containing the following number of objects:

       1, 1, 1, 2, 4, 32

     then we would place the cutoff between '1, 1' and '1, 2, 4, 32',
     rolling up the first two packs into a pack with 2 objects. That
     breaks our progression and leaves us:

       2, 1, 2, 4, 32
         ^

     (where the '^' indicates the position of our split). To restore a
     progression, we move the split forward (towards larger packs)
     joining each pack into our new pack until a geometric progression
     is restored. Here, that looks like:

       2, 1, 2, 4, 32  ~>  3, 2, 4, 32  ~>  5, 4, 32  ~> ... ~> 9, 32
         ^                   ^                ^                   ^

This has the advantage of not repacking the heavy-side of packs too
often while also only creating one new pack at a time. Another wrinkle
is that we assume that loose, indexed, and reflog'd objects are
insignificant, and lump them into any new pack that we create. This can
lead to non-idempotent results.

Suggested-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2021-02-22 23:30:52 -08:00

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git-repack(1)
=============
NAME
----
git-repack - Pack unpacked objects in a repository
SYNOPSIS
--------
[verse]
'git repack' [-a] [-A] [-d] [-f] [-F] [-l] [-n] [-q] [-b] [--window=<n>] [--depth=<n>] [--threads=<n>] [--keep-pack=<pack-name>]
DESCRIPTION
-----------
This command is used to combine all objects that do not currently
reside in a "pack", into a pack. It can also be used to re-organize
existing packs into a single, more efficient pack.
A pack is a collection of objects, individually compressed, with
delta compression applied, stored in a single file, with an
associated index file.
Packs are used to reduce the load on mirror systems, backup
engines, disk storage, etc.
OPTIONS
-------
-a::
Instead of incrementally packing the unpacked objects,
pack everything referenced into a single pack.
Especially useful when packing a repository that is used
for private development. Use
with `-d`. This will clean up the objects that `git prune`
leaves behind, but `git fsck --full --dangling` shows as
dangling.
+
Note that users fetching over dumb protocols will have to fetch the
whole new pack in order to get any contained object, no matter how many
other objects in that pack they already have locally.
+
Promisor packfiles are repacked separately: if there are packfiles that
have an associated ".promisor" file, these packfiles will be repacked
into another separate pack, and an empty ".promisor" file corresponding
to the new separate pack will be written.
-A::
Same as `-a`, unless `-d` is used. Then any unreachable
objects in a previous pack become loose, unpacked objects,
instead of being left in the old pack. Unreachable objects
are never intentionally added to a pack, even when repacking.
This option prevents unreachable objects from being immediately
deleted by way of being left in the old pack and then
removed. Instead, the loose unreachable objects
will be pruned according to normal expiry rules
with the next 'git gc' invocation. See linkgit:git-gc[1].
-d::
After packing, if the newly created packs make some
existing packs redundant, remove the redundant packs.
Also run 'git prune-packed' to remove redundant
loose object files.
-l::
Pass the `--local` option to 'git pack-objects'. See
linkgit:git-pack-objects[1].
-f::
Pass the `--no-reuse-delta` option to `git-pack-objects`, see
linkgit:git-pack-objects[1].
-F::
Pass the `--no-reuse-object` option to `git-pack-objects`, see
linkgit:git-pack-objects[1].
-q::
Pass the `-q` option to 'git pack-objects'. See
linkgit:git-pack-objects[1].
-n::
Do not update the server information with
'git update-server-info'. This option skips
updating local catalog files needed to publish
this repository (or a direct copy of it)
over HTTP or FTP. See linkgit:git-update-server-info[1].
--window=<n>::
--depth=<n>::
These two options affect how the objects contained in the pack are
stored using delta compression. The objects are first internally
sorted by type, size and optionally names and compared against the
other objects within `--window` to see if using delta compression saves
space. `--depth` limits the maximum delta depth; making it too deep
affects the performance on the unpacker side, because delta data needs
to be applied that many times to get to the necessary object.
+
The default value for --window is 10 and --depth is 50. The maximum
depth is 4095.
--threads=<n>::
This option is passed through to `git pack-objects`.
--window-memory=<n>::
This option provides an additional limit on top of `--window`;
the window size will dynamically scale down so as to not take
up more than '<n>' bytes in memory. This is useful in
repositories with a mix of large and small objects to not run
out of memory with a large window, but still be able to take
advantage of the large window for the smaller objects. The
size can be suffixed with "k", "m", or "g".
`--window-memory=0` makes memory usage unlimited. The default
is taken from the `pack.windowMemory` configuration variable.
Note that the actual memory usage will be the limit multiplied
by the number of threads used by linkgit:git-pack-objects[1].
--max-pack-size=<n>::
Maximum size of each output pack file. The size can be suffixed with
"k", "m", or "g". The minimum size allowed is limited to 1 MiB.
If specified, multiple packfiles may be created, which also
prevents the creation of a bitmap index.
The default is unlimited, unless the config variable
`pack.packSizeLimit` is set.
-b::
--write-bitmap-index::
Write a reachability bitmap index as part of the repack. This
only makes sense when used with `-a` or `-A`, as the bitmaps
must be able to refer to all reachable objects. This option
overrides the setting of `repack.writeBitmaps`. This option
has no effect if multiple packfiles are created.
--pack-kept-objects::
Include objects in `.keep` files when repacking. Note that we
still do not delete `.keep` packs after `pack-objects` finishes.
This means that we may duplicate objects, but this makes the
option safe to use when there are concurrent pushes or fetches.
This option is generally only useful if you are writing bitmaps
with `-b` or `repack.writeBitmaps`, as it ensures that the
bitmapped packfile has the necessary objects.
--keep-pack=<pack-name>::
Exclude the given pack from repacking. This is the equivalent
of having `.keep` file on the pack. `<pack-name>` is the
pack file name without leading directory (e.g. `pack-123.pack`).
The option could be specified multiple times to keep multiple
packs.
--unpack-unreachable=<when>::
When loosening unreachable objects, do not bother loosening any
objects older than `<when>`. This can be used to optimize out
the write of any objects that would be immediately pruned by
a follow-up `git prune`.
-k::
--keep-unreachable::
When used with `-ad`, any unreachable objects from existing
packs will be appended to the end of the packfile instead of
being removed. In addition, any unreachable loose objects will
be packed (and their loose counterparts removed).
-i::
--delta-islands::
Pass the `--delta-islands` option to `git-pack-objects`, see
linkgit:git-pack-objects[1].
-g=<factor>::
--geometric=<factor>::
Arrange resulting pack structure so that each successive pack
contains at least `<factor>` times the number of objects as the
next-largest pack.
+
`git repack` ensures this by determining a "cut" of packfiles that need
to be repacked into one in order to ensure a geometric progression. It
picks the smallest set of packfiles such that as many of the larger
packfiles (by count of objects contained in that pack) may be left
intact.
+
Unlike other repack modes, the set of objects to pack is determined
uniquely by the set of packs being "rolled-up"; in other words, the
packs determined to need to be combined in order to restore a geometric
progression.
+
When `--unpacked` is specified, loose objects are implicitly included in
this "roll-up", without respect to their reachability. This is subject
to change in the future. This option (implying a drastically different
repack mode) is not guaranteed to work with all other combinations of
option to `git repack`).
Configuration
-------------
By default, the command passes `--delta-base-offset` option to
'git pack-objects'; this typically results in slightly smaller packs,
but the generated packs are incompatible with versions of Git older than
version 1.4.4. If you need to share your repository with such ancient Git
versions, either directly or via the dumb http protocol, then you
need to set the configuration variable `repack.UseDeltaBaseOffset` to
"false" and repack. Access from old Git versions over the native protocol
is unaffected by this option as the conversion is performed on the fly
as needed in that case.
SEE ALSO
--------
linkgit:git-pack-objects[1]
linkgit:git-prune-packed[1]
GIT
---
Part of the linkgit:git[1] suite
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