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<body class="article">
<div id="header">
</div>
<div id="content">
<div class="sect1">
<h2 id="_git_repository_format_versions">Git Repository Format Versions</h2>
<div class="sectionbody">
<div class="paragraph">
<p>Every git repository is marked with a numeric version in the
<code>core.repositoryformatversion</code> key of its <code>config</code> file. This version
specifies the rules for operating on the on-disk repository data. An
implementation of git which does not understand a particular version
advertised by an on-disk repository MUST NOT operate on that repository;
doing so risks not only producing wrong results, but actually losing
data.</p>
</div>
<div class="paragraph">
<p>Because of this rule, version bumps should be kept to an absolute
minimum. Instead, we generally prefer these strategies:</p>
</div>
<div class="ulist">
<ul>
<li>
<p>bumping format version numbers of individual data files (e.g.,
index, packfiles, etc). This restricts the incompatibilities only to
those files.</p>
</li>
<li>
<p>introducing new data that gracefully degrades when used by older
clients (e.g., pack bitmap files are ignored by older clients, which
simply do not take advantage of the optimization they provide).</p>
</li>
</ul>
</div>
<div class="paragraph">
<p>A whole-repository format version bump should only be part of a change
that cannot be independently versioned. For instance, if one were to
change the reachability rules for objects, or the rules for locking
refs, that would require a bump of the repository format version.</p>
</div>
<div class="paragraph">
<p>Note that this applies only to accessing the repository’s disk contents
directly. An older client which understands only format <code>0</code> may still
connect via <code>git://</code> to a repository using format <code>1</code>, as long as the
server process understands format <code>1</code>.</p>
</div>
<div class="paragraph">
<p>The preferred strategy for rolling out a version bump (whether whole
repository or for a single file) is to teach git to read the new format,
and allow writing the new format with a config switch or command line
option (for experimentation or for those who do not care about backwards
compatibility with older gits). Then after a long period to allow the
reading capability to become common, we may switch to writing the new
format by default.</p>
</div>
<div class="paragraph">
<p>The currently defined format versions are:</p>
</div>
<div class="sect2">
<h3 id="_version_0">Version <code>0</code></h3>
<div class="paragraph">
<p>This is the format defined by the initial version of git, including but
not limited to the format of the repository directory, the repository
configuration file, and the object and ref storage. Specifying the
complete behavior of git is beyond the scope of this document.</p>
</div>
</div>
<div class="sect2">
<h3 id="_version_1">Version <code>1</code></h3>
<div class="paragraph">
<p>This format is identical to version <code>0</code>, with the following exceptions:</p>
</div>
<div class="olist arabic">
<ol class="arabic">
<li>
<p>When reading the <code>core.repositoryformatversion</code> variable, a git
implementation which supports version 1 MUST also read any
configuration keys found in the <code>extensions</code> section of the
configuration file.</p>
</li>
<li>
<p>If a version-1 repository specifies any <code>extensions.*</code> keys that
the running git has not implemented, the operation MUST NOT
proceed. Similarly, if the value of any known key is not understood
by the implementation, the operation MUST NOT proceed.</p>
</li>
</ol>
</div>
<div class="paragraph">
<p>Note that if no extensions are specified in the config file, then
<code>core.repositoryformatversion</code> SHOULD be set to <code>0</code> (setting it to <code>1</code>
provides no benefit, and makes the repository incompatible with older
implementations of git).</p>
</div>
<div class="paragraph">
<p>The defined extensions are given in the <code>extensions.*</code> section of
<a href="../git-config.html">git-config(1)</a>. Any implementation wishing to define a new
extension should make a note of it there, in order to claim the name.</p>
</div>
</div>
</div>
</div>
</div>
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<div id="footer-text">
Last updated 2025-11-17 16:38:17 UTC
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