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git-commit-vandalism/Documentation/config/pack.txt
Taylor Blau 3f267a1128 builtin/pack-objects.c: respect 'pack.preferBitmapTips' 
When writing a new pack with a bitmap, it is sometimes convenient to
indicate some reference prefixes which should receive priority when
selecting which commits to receive bitmaps.

A truly motivated caller could accomplish this by setting
'pack.islandCore', (since all commits in the core island are similarly
marked as preferred) but this requires callers to opt into using delta
islands, which they may or may not want to do.

Introduce a new multi-valued configuration, 'pack.preferBitmapTips' to
allow callers to specify a list of reference prefixes. All references
which have a prefix contained in 'pack.preferBitmapTips' will mark their
tips as "preferred" in the same way as commits are marked as preferred
for selection by 'pack.islandCore'.

The choice of the verb "prefer" is intentional: marking the NEEDS_BITMAP
flag on an object does *not* guarantee that that object will receive a
bitmap. It merely guarantees that that commit will receive a bitmap over
any *other* commit in the same window by bitmap_writer_select_commits().

The test this patch adds reflects this quirk, too. It only tests that
a commit (which didn't receive bitmaps by default) is selected for
bitmaps after changing the value of 'pack.preferBitmapTips' to include
it. Other commits may lose their bitmaps as a byproduct of how the
selection process works (bitmap_writer_select_commits() ignores the
remainder of a window after seeing a commit with the NEEDS_BITMAP flag).

This configuration will aide in selecting important references for
multi-pack bitmaps, since they do not respect the same pack.islandCore
configuration. (They could, but doing so may be confusing, since it is
packs--not bitmaps--which are influenced by the delta-islands
configuration).

In a fork network repository (one which lists all forks of a given
repository as remotes), for example, it is useful to set
pack.preferBitmapTips to 'refs/remotes/<root>/heads' and
'refs/remotes/<root>/tags', where '<root>' is an opaque identifier
referring to the repository which is at the base of the fork chain.

Suggested-by: Jeff King <peff@peff.net>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2021-03-31 23:14:03 -07:00

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pack.window::
The size of the window used by linkgit:git-pack-objects[1] when no
window size is given on the command line. Defaults to 10.
pack.depth::
The maximum delta depth used by linkgit:git-pack-objects[1] when no
maximum depth is given on the command line. Defaults to 50.
Maximum value is 4095.
pack.windowMemory::
The maximum size of memory that is consumed by each thread
in linkgit:git-pack-objects[1] for pack window memory when
no limit is given on the command line. The value can be
suffixed with "k", "m", or "g". When left unconfigured (or
set explicitly to 0), there will be no limit.
pack.compression::
An integer -1..9, indicating the compression level for objects
in a pack file. -1 is the zlib default. 0 means no
compression, and 1..9 are various speed/size tradeoffs, 9 being
slowest. If not set, defaults to core.compression. If that is
not set, defaults to -1, the zlib default, which is "a default
compromise between speed and compression (currently equivalent
to level 6)."
+
Note that changing the compression level will not automatically recompress
all existing objects. You can force recompression by passing the -F option
to linkgit:git-repack[1].
pack.allowPackReuse::
When true, and when reachability bitmaps are enabled,
pack-objects will try to send parts of the bitmapped packfile
verbatim. This can reduce memory and CPU usage to serve fetches,
but might result in sending a slightly larger pack. Defaults to
true.
pack.island::
An extended regular expression configuring a set of delta
islands. See "DELTA ISLANDS" in linkgit:git-pack-objects[1]
for details.
pack.islandCore::
Specify an island name which gets to have its objects be
packed first. This creates a kind of pseudo-pack at the front
of one pack, so that the objects from the specified island are
hopefully faster to copy into any pack that should be served
to a user requesting these objects. In practice this means
that the island specified should likely correspond to what is
the most commonly cloned in the repo. See also "DELTA ISLANDS"
in linkgit:git-pack-objects[1].
pack.deltaCacheSize::
The maximum memory in bytes used for caching deltas in
linkgit:git-pack-objects[1] before writing them out to a pack.
This cache is used to speed up the writing object phase by not
having to recompute the final delta result once the best match
for all objects is found. Repacking large repositories on machines
which are tight with memory might be badly impacted by this though,
especially if this cache pushes the system into swapping.
A value of 0 means no limit. The smallest size of 1 byte may be
used to virtually disable this cache. Defaults to 256 MiB.
pack.deltaCacheLimit::
The maximum size of a delta, that is cached in
linkgit:git-pack-objects[1]. This cache is used to speed up the
writing object phase by not having to recompute the final delta
result once the best match for all objects is found.
Defaults to 1000. Maximum value is 65535.
pack.threads::
Specifies the number of threads to spawn when searching for best
delta matches. This requires that linkgit:git-pack-objects[1]
be compiled with pthreads otherwise this option is ignored with a
warning. This is meant to reduce packing time on multiprocessor
machines. The required amount of memory for the delta search window
is however multiplied by the number of threads.
Specifying 0 will cause Git to auto-detect the number of CPU's
and set the number of threads accordingly.
pack.indexVersion::
Specify the default pack index version. Valid values are 1 for
legacy pack index used by Git versions prior to 1.5.2, and 2 for
the new pack index with capabilities for packs larger than 4 GB
as well as proper protection against the repacking of corrupted
packs. Version 2 is the default. Note that version 2 is enforced
and this config option ignored whenever the corresponding pack is
larger than 2 GB.
+
If you have an old Git that does not understand the version 2 `*.idx` file,
cloning or fetching over a non native protocol (e.g. "http")
that will copy both `*.pack` file and corresponding `*.idx` file from the
other side may give you a repository that cannot be accessed with your
older version of Git. If the `*.pack` file is smaller than 2 GB, however,
you can use linkgit:git-index-pack[1] on the *.pack file to regenerate
the `*.idx` file.
pack.packSizeLimit::
The maximum size of a pack. This setting only affects
packing to a file when repacking, i.e. the git:// protocol
is unaffected. It can be overridden by the `--max-pack-size`
option of linkgit:git-repack[1]. Reaching this limit results
in the creation of multiple packfiles; which in turn prevents
bitmaps from being created.
The minimum size allowed is limited to 1 MiB.
The default is unlimited.
Common unit suffixes of 'k', 'm', or 'g' are
supported.
pack.useBitmaps::
When true, git will use pack bitmaps (if available) when packing
to stdout (e.g., during the server side of a fetch). Defaults to
true. You should not generally need to turn this off unless
you are debugging pack bitmaps.
pack.useSparse::
When true, git will default to using the '--sparse' option in
'git pack-objects' when the '--revs' option is present. This
algorithm only walks trees that appear in paths that introduce new
objects. This can have significant performance benefits when
computing a pack to send a small change. However, it is possible
that extra objects are added to the pack-file if the included
commits contain certain types of direct renames. Default is
`true`.
pack.preferBitmapTips::
When selecting which commits will receive bitmaps, prefer a
commit at the tip of any reference that is a suffix of any value
of this configuration over any other commits in the "selection
window".
+
Note that setting this configuration to `refs/foo` does not mean that
the commits at the tips of `refs/foo/bar` and `refs/foo/baz` will
necessarily be selected. This is because commits are selected for
bitmaps from within a series of windows of variable length.
+
If a commit at the tip of any reference which is a suffix of any value
of this configuration is seen in a window, it is immediately given
preference over any other commit in that window.
pack.writeBitmaps (deprecated)::
This is a deprecated synonym for `repack.writeBitmaps`.
pack.writeBitmapHashCache::
When true, git will include a "hash cache" section in the bitmap
index (if one is written). This cache can be used to feed git's
delta heuristics, potentially leading to better deltas between
bitmapped and non-bitmapped objects (e.g., when serving a fetch
between an older, bitmapped pack and objects that have been
pushed since the last gc). The downside is that it consumes 4
bytes per object of disk space. Defaults to true.
pack.writeReverseIndex::
When true, git will write a corresponding .rev file (see:
link:../technical/pack-format.html[Documentation/technical/pack-format.txt])
for each new packfile that it writes in all places except for
linkgit:git-fast-import[1] and in the bulk checkin mechanism.
Defaults to false.
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