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1== Git Repository Format Versions
2
3Every git repository is marked with a numeric version in the
4`core.repositoryformatversion` key of its `config` file. This version
5specifies the rules for operating on the on-disk repository data. An
6implementation of git which does not understand a particular version
7advertised by an on-disk repository MUST NOT operate on that repository;
8doing so risks not only producing wrong results, but actually losing
9data.
10
11Because of this rule, version bumps should be kept to an absolute
12minimum. Instead, we generally prefer these strategies:
13
14 - bumping format version numbers of individual data files (e.g.,
15 index, packfiles, etc). This restricts the incompatibilities only to
16 those files.
17
18 - introducing new data that gracefully degrades when used by older
19 clients (e.g., pack bitmap files are ignored by older clients, which
20 simply do not take advantage of the optimization they provide).
21
22A whole-repository format version bump should only be part of a change
23that cannot be independently versioned. For instance, if one were to
24change the reachability rules for objects, or the rules for locking
25refs, that would require a bump of the repository format version.
26
27Note that this applies only to accessing the repository's disk contents
28directly. An older client which understands only format `0` may still
29connect via `git://` to a repository using format `1`, as long as the
30server process understands format `1`.
31
32The preferred strategy for rolling out a version bump (whether whole
33repository or for a single file) is to teach git to read the new format,
34and allow writing the new format with a config switch or command line
35option (for experimentation or for those who do not care about backwards
36compatibility with older gits). Then after a long period to allow the
37reading capability to become common, we may switch to writing the new
38format by default.
39
40The currently defined format versions are:
41
42=== Version `0`
43
44This is the format defined by the initial version of git, including but
45not limited to the format of the repository directory, the repository
46configuration file, and the object and ref storage. Specifying the
47complete behavior of git is beyond the scope of this document.
48
49=== Version `1`
50
51This format is identical to version `0`, with the following exceptions:
52
53 1. When reading the `core.repositoryformatversion` variable, a git
54 implementation which supports version 1 MUST also read any
55 configuration keys found in the `extensions` section of the
56 configuration file.
57
58 2. If a version-1 repository specifies any `extensions.*` keys that
59 the running git has not implemented, the operation MUST NOT
60 proceed. Similarly, if the value of any known key is not understood
61 by the implementation, the operation MUST NOT proceed.
62
63Note that if no extensions are specified in the config file, then
64`core.repositoryformatversion` SHOULD be set to `0` (setting it to `1`
65provides no benefit, and makes the repository incompatible with older
66implementations of git).
67
68The defined extensions are given in the `extensions.*` section of
69linkgit:git-config[1]. Any implementation wishing to define a new
70extension should make a note of it there, in order to claim the name.