When a multi-pack bitmap is used to implement verbatim pack reuse (that
is, when verbatim chunks from an on-disk packfile are copied
directly[^1]), it does so by using its "preferred pack" as the source
for pack-reuse.
This allows repositories to pack the majority of their objects into a
single (often large) pack, and then use it as the single source for
verbatim pack reuse. This increases the amount of objects that are
reused verbatim (and consequently, decrease the amount of time it takes
to generate many packs). But this performance comes at a cost, which is
that the preferred packfile must pace its growth with that of the entire
repository in order to maintain the utility of verbatim pack reuse.
As repositories grow beyond what we can reasonably store in a single
packfile, the utility of verbatim pack reuse diminishes. Or, at the very
least, it becomes increasingly more expensive to maintain as the pack
grows larger and larger.
It would be beneficial to be able to perform this same optimization over
multiple packs, provided some modest constraints (most importantly, that
the set of packs eligible for verbatim reuse are disjoint with respect
to the subset of their objects being sent).
If we assume that the packs which we treat as candidates for verbatim
reuse are disjoint with respect to any of their objects we may output,
we need to make only modest modifications to the verbatim pack-reuse
code itself. Most notably, we need to remove the assumption that the
bits in the reachability bitmap corresponding to objects from the single
reuse pack begin at the first bit position.
Future patches will unwind these assumptions and reimplement their
existing functionality as special cases of the more general assumptions
(e.g. that reuse bits can start anywhere within the bitset, but happen
to start at 0 for all existing cases).
This patch does not yet relax any of those assumptions. Instead, it
implements a foundational data-structure, the "Bitampped Packs" (`BTMP`)
chunk of the multi-pack index. The `BTMP` chunk's contents are described
in detail here. Importantly, the `BTMP` chunk contains information to
map regions of a multi-pack index's reachability bitmap to the packs
whose objects they represent.
For now, this chunk is only written, not read (outside of the test-tool
used in this patch to test the new chunk's behavior). Future patches
will begin to make use of this new chunk.
[^1]: Modulo patching any `OFS_DELTA`'s that cross over a region of the
pack that wasn't used verbatim.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Back in 32f3c541e3 (multi-pack-index: write pack names in chunk,
2018-07-12) the MIDX's "Packfile Names" (or "PNAM", for short) chunk was
described as containing an array of string entries. e0d1bcf825 notes
that this is the only chunk in the MIDX format's specification that is
not guaranteed to be 4-byte aligned, and so should be placed last.
This isn't quite accurate: the entries within the PNAM chunk are not
guaranteed to be 4-byte aligned since they are arbitrary strings, but
the chunk itself is 4-byte aligned since the ending is padded with NUL
bytes.
That padding has always been there since 32f3c541e3 via
midx.c::write_midx_pack_names(), which ended with:
i = MIDX_CHUNK_ALIGNMENT - (written % MIDX_CHUNK_ALIGNMENT)
if (i < MIDX_CHUNK_ALIGNMENT) {
unsigned char padding[MIDX_CHUNK_ALIGNMENT];
memset(padding, 0, sizeof(padding))
hashwrite(f, padding, i);
written += i;
}
In fact, 32f3c541e3's log message itself describes the chunk in its
first paragraph with:
Since filenames are not well structured, add padding to keep good
alignment in later chunks.
So these have always been externally aligned. Correct the corresponding
part of our documentation to reflect that.
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
e0d1bcf825 (multi-pack-index: add format details, 2018-07-12) describes
the MIDX's "PNAM" chunk as having entries which are "null-terminated
strings".
This is a typo, as strings are terminated with a NUL character, which is
a distinct concept from "NULL" or "null", which we typically reserve for
the void pointer to address 0.
Correct the documentation accordingly.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This section was added in 3d89a8c118 (Documentation/technical: add
cruft-packs.txt, 2022-05-20) to highlight a potential pitfall when
deploying cruft packs in an environment where multiple versions of Git
are GC-ing the same repository.
Now that it has been more than a year since 3d89a8c118 was written,
let's drop this section as it is no longer relevant.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This text, originally from 3d89a8c118 (Documentation/technical: add
cruft-packs.txt, 2022-05-20) lists multiple cruft packs as a potential
alternative to the design of cruft packs.
We have always supported multiple cruft packs (i.e. we use the most
recent mtime for a given object among all cruft packs which contain it,
etc.), but haven't encouraged its use.
We still aren't encouraging users to go out and generate multiple cruft
packs, but let's take a step in that direction by dropping language that
suggests we aren't capable of working with multiple cruft packs.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Back in 5b92477f89 (builtin/gc.c: conditionally avoid pruning objects
via loose, 2022-05-20), `git gc` learned the `--cruft` option and
`gc.cruftPacks` configuration to opt-in to writing cruft packs when
collecting or pruning unreachable objects.
Cruft packs were introduced with the merge in a50036da1a (Merge branch
'tb/cruft-packs', 2022-06-03). They address the problem of "loose object
explosions", where Git will write out many individual loose objects when
there is a large number of unreachable objects that have not yet aged
past `--prune=<date>`.
Instead of keeping track of those unreachable yet recent objects via
their loose object file's mtime, cruft packs collect all unreachable
objects into a single pack with a corresponding `*.mtimes` file that
acts as a table to store the mtimes of all unreachable objects. This
prevents the need to store unreachable objects as loose as they age out
of the repository, and avoids the problem of loose object explosions.
Beyond avoiding loose object explosions, cruft packs also act as a more
efficient mechanism to store unreachable objects as they age out of a
repository. This is because pairs of similar unreachable objects serve
as delta bases for one another.
In 5b92477f89, the feature was introduced as experimental. Since then,
GitHub has been running these patches in every repository generating
hundreds of millions of cruft packs along the way. The feature is
battle-tested, and avoids many pathological cases such as above. Users
who either run `git gc` manually, or via `git maintenance` can benefit
from having cruft packs.
As such, enable cruft pack generation to take place by default (by
making `gc.cruftPacks` have the default of "true" rather than "false).
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Integrate the cruft packs documentation initially added in
3d89a8c118 (Documentation/technical: add cruft-packs.txt, 2022-05-20)
to the newly created "gitformat-pack" documentation.
Like the "bitmap-format" added before it in
0d4455a3ab (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>
Continue the move of existing Documentation/technical/* protocol and
file-format documentation into our main documentation space by moving
the various documentation pertaining to the *.pack format and related
files, and updating things that refer to it to link to the new
location.
By moving these we can properly link from the newly created
gitformat-commit-graph to a gitformat-chunk-format page.
Integrating "Documentation/technical/bitmap-format.txt" and
"Documentation/technical/cruft-packs.txt" might logically be part of
this change, but as those cover parts of the wider "pack
format" (including associated files) that's documented outside of
"Documentation/technical/pack-format.txt" let's leave those for now,
subsequent commit(s) will address those.
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
