37dc6d8104
Cruft packs are an alternative mechanism for storing a collection of unreachable objects whose mtimes are recent enough to avoid being pruned out of the repository. When cruft packs were first introduced back inb757353676(builtin/pack-objects.c: --cruft without expiration, 2022-05-20) anda7d493833f(builtin/pack-objects.c: --cruft with expiration, 2022-05-20), the recommended workflow consisted of: - Repacking periodically, either by packing anything loose in the repository (via `git repack -d`) or producing a geometric sequence of packs (via `git repack --geometric=<d> -d`). - Every so often, splitting the repository into two packs, one cruft to store the unreachable objects, and another non-cruft pack to store the reachable objects. Repositories may (out of band with the above) choose periodically to prune out some unreachable objects which have aged out of the grace period by generating a pack with `--cruft-expiration=<approxidate>`. This allowed repositories to maintain relatively few packs on average, and quarantine unreachable objects together in a cruft pack, avoiding the pitfalls of holding unreachable objects as loose while they age out (for more, see some of the details in3d89a8c118(Documentation/technical: add cruft-packs.txt, 2022-05-20)). This all works, but can be costly from an I/O-perspective when frequently repacking a repository that has many unreachable objects. This problem is exacerbated when those unreachable objects are rarely (if every) pruned. Since there is at most one cruft pack in the above scheme, each time we update the cruft pack it must be rewritten from scratch. Because much of the pack is reused, this is a relatively inexpensive operation from a CPU-perspective, but is very costly in terms of I/O since we end up rewriting basically the same pack (plus any new unreachable objects that have entered the repository since the last time a cruft pack was generated). At the time, we decided against implementing more robust support for multiple cruft packs. This patch implements that support which we were lacking. Introduce a new option `--max-cruft-size` which allows repositories to accumulate cruft packs up to a given size, after which point a new generation of cruft packs can accumulate until it reaches the maximum size, and so on. To generate a new cruft pack, the process works like so: - Sort a list of any existing cruft packs in ascending order of pack size. - Starting from the beginning of the list, group cruft packs together while the accumulated size is smaller than the maximum specified pack size. - Combine the objects in these cruft packs together into a new cruft pack, along with any other unreachable objects which have since entered the repository. Once a cruft pack grows beyond the size specified via `--max-cruft-size` the pack is effectively frozen. This limits the I/O churn up to a quadratic function of the value specified by the `--max-cruft-size` option, instead of behaving quadratically in the number of total unreachable objects. When pruning unreachable objects, we bypass the new code paths which combine small cruft packs together, and instead start from scratch, passing in the appropriate `--max-pack-size` down to `pack-objects`, putting it in charge of keeping the resulting set of cruft packs sized correctly. This may seem like further I/O churn, but in practice it isn't so bad. We could prune old cruft packs for whom all or most objects are removed, and then generate a new cruft pack with just the remaining set of objects. But this additional complexity buys us relatively little, because most objects end up being pruned anyway, so the I/O churn is well contained. Signed-off-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>
Git - fast, scalable, distributed revision control system
Git is a fast, scalable, distributed revision control system with an unusually rich command set that provides both high-level operations and full access to internals.
Git is an Open Source project covered by the GNU General Public License version 2 (some parts of it are under different licenses, compatible with the GPLv2). It was originally written by Linus Torvalds with help of a group of hackers around the net.
Please read the file INSTALL for installation instructions.
Many Git online resources are accessible from https://git-scm.com/ including full documentation and Git related tools.
See Documentation/gittutorial.txt to get started, then see
Documentation/giteveryday.txt for a useful minimum set of commands, and
Documentation/git-<commandname>.txt for documentation of each command.
If git has been correctly installed, then the tutorial can also be
read with man gittutorial or git help tutorial, and the
documentation of each command with man git-<commandname> or git help <commandname>.
CVS users may also want to read Documentation/gitcvs-migration.txt
(man gitcvs-migration or git help cvs-migration if git is
installed).
The user discussion and development of Git take place on the Git mailing list -- everyone is welcome to post bug reports, feature requests, comments and patches to git@vger.kernel.org (read Documentation/SubmittingPatches for instructions on patch submission and Documentation/CodingGuidelines).
Those wishing to help with error message, usage and informational message
string translations (localization l10) should see po/README.md
(a po file is a Portable Object file that holds the translations).
To subscribe to the list, send an email with just "subscribe git" in the body to majordomo@vger.kernel.org (not the Git list). The mailing list archives are available at https://lore.kernel.org/git/, http://marc.info/?l=git and other archival sites.
Issues which are security relevant should be disclosed privately to the Git Security mailing list git-security@googlegroups.com.
The maintainer frequently sends the "What's cooking" reports that list the current status of various development topics to the mailing list. The discussion following them give a good reference for project status, development direction and remaining tasks.
The name "git" was given by Linus Torvalds when he wrote the very first version. He described the tool as "the stupid content tracker" and the name as (depending on your mood):
- random three-letter combination that is pronounceable, and not actually used by any common UNIX command. The fact that it is a mispronunciation of "get" may or may not be relevant.
- stupid. contemptible and despicable. simple. Take your pick from the dictionary of slang.
- "global information tracker": you're in a good mood, and it actually works for you. Angels sing, and a light suddenly fills the room.
- "goddamn idiotic truckload of sh*t": when it breaks