docs: move cruft pack docs to gitformat-pack

Integrate the cruft packs documentation initially added in
3d89a8c1180 (Documentation/technical: add cruft-packs.txt, 2022-05-20)
to the newly created "gitformat-pack" documentation.

Like the "bitmap-format" added before it in
0d4455a3ab0 (documentation: add documentation for the bitmap format,
2013-11-14) the "cruft-packs" were documented in their own file.

As the diff move detection will show there is no change to
"Documentation/technical/cruft-packs.txt" here except to move it, and
to "indent" the existing sections by adding an extra "=" to them.

We could similarly convert the "bitmap-format.txt", but let's leave it
for now due to a conflict with the in-flight ac/bitmap-lookup-table
series.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 126 insertions, 0 deletions

diff --git a/Documentation/gitformat-pack.txt b/Documentation/gitformat-pack.txt
index 546c99f887..e06af02f21 100644
--- a/Documentation/gitformat-pack.txt
+++ b/Documentation/gitformat-pack.txt
@@ -11,6 +11,7 @@ SYNOPSIS
 [verse]
 $GIT_DIR/objects/pack/pack-*.{pack,idx}
 $GIT_DIR/objects/pack/pack-*.rev
+$GIT_DIR/objects/pack/pack-*.mtimes
 $GIT_DIR/objects/pack/multi-pack-index
 
 DESCRIPTION
@@ -507,6 +508,131 @@ packs arranged in MIDX order (with the preferred pack coming first).
 The MIDX's reverse index is stored in the optional 'RIDX' chunk within
 the MIDX itself.
 
+== cruft packs
+
+The cruft packs feature offer an alternative to Git's traditional mechanism of
+removing unreachable objects. This document provides an overview of Git's
+pruning mechanism, and how a cruft pack can be used instead to accomplish the
+same.
+
+=== Background
+
+To remove unreachable objects from your repository, Git offers `git repack -Ad`
+(see linkgit:git-repack[1]). Quoting from the documentation:
+
+----
+[...] unreachable objects in a previous pack become loose, unpacked objects,
+instead of being left in the old pack. [...] loose unreachable objects will be
+pruned according to normal expiry rules with the next 'git gc' invocation.
+----
+
+Unreachable objects aren't removed immediately, since doing so could race with
+an incoming push which may reference an object which is about to be deleted.
+Instead, those unreachable objects are stored as loose objects and stay that way
+until they are older than the expiration window, at which point they are removed
+by linkgit:git-prune[1].
+
+Git must store these unreachable objects loose in order to keep track of their
+per-object mtimes. If these unreachable objects were written into one big pack,
+then either freshening that pack (because an object contained within it was
+re-written) or creating a new pack of unreachable objects would cause the pack's
+mtime to get updated, and the objects within it would never leave the expiration
+window. Instead, objects are stored loose in order to keep track of the
+individual object mtimes and avoid a situation where all cruft objects are
+freshened at once.
+
+This can lead to undesirable situations when a repository contains many
+unreachable objects which have not yet left the grace period. Having large
+directories in the shards of `.git/objects` can lead to decreased performance in
+the repository. But given enough unreachable objects, this can lead to inode
+starvation and degrade the performance of the whole system. Since we
+can never pack those objects, these repositories often take up a large amount of
+disk space, since we can only zlib compress them, but not store them in delta
+chains.
+
+=== Cruft packs
+
+A cruft pack eliminates the need for storing unreachable objects in a loose
+state by including the per-object mtimes in a separate file alongside a single
+pack containing all loose objects.
+
+A cruft pack is written by `git repack --cruft` when generating a new pack.
+linkgit:git-pack-objects[1]'s `--cruft` option. Note that `git repack --cruft`
+is a classic all-into-one repack, meaning that everything in the resulting pack is
+reachable, and everything else is unreachable. Once written, the `--cruft`
+option instructs `git repack` to generate another pack containing only objects
+not packed in the previous step (which equates to packing all unreachable
+objects together). This progresses as follows:
+
+  1. Enumerate every object, marking any object which is (a) not contained in a
+     kept-pack, and (b) whose mtime is within the grace period as a traversal
+     tip.
+
+  2. Perform a reachability traversal based on the tips gathered in the previous
+     step, adding every object along the way to the pack.
+
+  3. Write the pack out, along with a `.mtimes` file that records the per-object
+     timestamps.
+
+This mode is invoked internally by linkgit:git-repack[1] when instructed to
+write a cruft pack. Crucially, the set of in-core kept packs is exactly the set
+of packs which will not be deleted by the repack; in other words, they contain
+all of the repository's reachable objects.
+
+When a repository already has a cruft pack, `git repack --cruft` typically only
+adds objects to it. An exception to this is when `git repack` is given the
+`--cruft-expiration` option, which allows the generated cruft pack to omit
+expired objects instead of waiting for linkgit:git-gc[1] to expire those objects
+later on.
+
+It is linkgit:git-gc[1] that is typically responsible for removing expired
+unreachable objects.
+
+=== Caution for mixed-version environments
+
+Repositories that have cruft packs in them will continue to work with any older
+version of Git. Note, however, that previous versions of Git which do not
+understand the `.mtimes` file will use the cruft pack's mtime as the mtime for
+all of the objects in it. In other words, do not expect older (pre-cruft pack)
+versions of Git to interpret or even read the contents of the `.mtimes` file.
+
+Note that having mixed versions of Git GC-ing the same repository can lead to
+unreachable objects never being completely pruned. This can happen under the
+following circumstances:
+
+  - An older version of Git running GC explodes the contents of an existing
+    cruft pack loose, using the cruft pack's mtime.
+  - A newer version running GC collects those loose objects into a cruft pack,
+    where the .mtime file reflects the loose object's actual mtimes, but the
+    cruft pack mtime is "now".
+
+Repeating this process will lead to unreachable objects not getting pruned as a
+result of repeatedly resetting the objects' mtimes to the present time.
+
+If you are GC-ing repositories in a mixed version environment, consider omitting
+the `--cruft` option when using linkgit:git-repack[1] and linkgit:git-gc[1], and
+leaving the `gc.cruftPacks` configuration unset until all writers understand
+cruft packs.
+
+=== Alternatives
+
+Notable alternatives to this design include:
+
+  - The location of the per-object mtime data, and
+  - Storing unreachable objects in multiple cruft packs.
+
+On the location of mtime data, a new auxiliary file tied to the pack was chosen
+to avoid complicating the `.idx` format. If the `.idx` format were ever to gain
+support for optional chunks of data, it may make sense to consolidate the
+`.mtimes` format into the `.idx` itself.
+
+Storing unreachable objects among multiple cruft packs (e.g., creating a new
+cruft pack during each repacking operation including only unreachable objects
+which aren't already stored in an earlier cruft pack) is significantly more
+complicated to construct, and so aren't pursued here. The obvious drawback to
+the current implementation is that the entire cruft pack must be re-written from
+scratch.
+
 GIT
 ---
 Part of the linkgit:git[1] suite