technical/pack-format.html - external/gitster/git-htmldocs - Git at Google

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 <body class="article">
 <div id="header">
 <h1>Git pack format</h1>
 </div>
 <div id="content">
 <div class="sect1">
 <h2 id="_pack_pack_files_have_the_following_format">pack-*.pack files have the following format:</h2>
 <div class="sectionbody">
 <div class="ulist"><ul>
 <li>
 <p>
 A header appears at the beginning and consists of the following:
 </p>
 <div class="literalblock">
 <div class="content">
 <pre><code>4-byte signature:
  The signature is: {'P', 'A', 'C', 'K'}</code></pre>
 </div></div>
 <div class="literalblock">
 <div class="content">
 <pre><code>4-byte version number (network byte order):
  Git currently accepts version number 2 or 3 but
  generates version 2 only.</code></pre>
 </div></div>
 <div class="literalblock">
 <div class="content">
 <pre><code>4-byte number of objects contained in the pack (network byte order)</code></pre>
 </div></div>
 <div class="literalblock">
 <div class="content">
 <pre><code>Observation: we cannot have more than 4G versions ;-) and
 more than 4G objects in a pack.</code></pre>
 </div></div>
 </li>
 <li>
 <p>
 The header is followed by number of object entries, each of
  which looks like this:
 </p>
 <div class="literalblock">
 <div class="content">
 <pre><code>(undeltified representation)
 n-byte type and length (3-bit type, (n-1)*7+4-bit length)
 compressed data</code></pre>
 </div></div>
 <div class="literalblock">
 <div class="content">
 <pre><code>(deltified representation)
 n-byte type and length (3-bit type, (n-1)*7+4-bit length)
 20-byte base object name if OBJ_REF_DELTA or a negative relative
  offset from the delta object's position in the pack if this
  is an OBJ_OFS_DELTA object
 compressed delta data</code></pre>
 </div></div>
 <div class="literalblock">
 <div class="content">
 <pre><code>Observation: length of each object is encoded in a variable
 length format and is not constrained to 32-bit or anything.</code></pre>
 </div></div>
 </li>
 <li>
 <p>
 The trailer records 20-byte SHA-1 checksum of all of the above.
 </p>
 </li>
 </ul></div>
 </div>
 </div>
 <div class="sect1">
 <h2 id="_original_version_1_pack_idx_files_have_the_following_format">Original (version 1) pack-*.idx files have the following format:</h2>
 <div class="sectionbody">
 <div class="ulist"><ul>
 <li>
 <p>
 The header consists of 256 4-byte network byte order
  integers. N-th entry of this table records the number of
  objects in the corresponding pack, the first byte of whose
  object name is less than or equal to N. This is called the
  <em>first-level fan-out</em> table.
 </p>
 </li>
 <li>
 <p>
 The header is followed by sorted 24-byte entries, one entry
  per object in the pack. Each entry is:
 </p>
 <div class="literalblock">
 <div class="content">
 <pre><code>4-byte network byte order integer, recording where the
 object is stored in the packfile as the offset from the
 beginning.</code></pre>
 </div></div>
 <div class="literalblock">
 <div class="content">
 <pre><code>20-byte object name.</code></pre>
 </div></div>
 </li>
 <li>
 <p>
 The file is concluded with a trailer:
 </p>
 <div class="literalblock">
 <div class="content">
 <pre><code>A copy of the 20-byte SHA-1 checksum at the end of
 corresponding packfile.</code></pre>
 </div></div>
 <div class="literalblock">
 <div class="content">
 <pre><code>20-byte SHA-1-checksum of all of the above.</code></pre>
 </div></div>
 </li>
 </ul></div>
 <div class="paragraph"><p>Pack Idx file:</p></div>
 <div class="literalblock">
 <div class="content">
 <pre><code> -- +--------------------------------+
 fanout | fanout[0] = 2 (for example) |-.
 table +--------------------------------+ |
  | fanout[1] | |
  +--------------------------------+ |
  | fanout[2] | |
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
  | fanout[255] = total objects |---.
  -- +--------------------------------+ | |
 main | offset | | |
 index | object name 00XXXXXXXXXXXXXXXX | | |
 table +--------------------------------+ | |
  | offset | | |
  | object name 00XXXXXXXXXXXXXXXX | | |
  +--------------------------------+&lt;+ |
  .-| offset | |
  | | object name 01XXXXXXXXXXXXXXXX | |
  | +--------------------------------+ |
  | | offset | |
  | | object name 01XXXXXXXXXXXXXXXX | |
  | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
  | | offset | |
  | | object name FFXXXXXXXXXXXXXXXX | |
  --| +--------------------------------+&lt;--+
 trailer | | packfile checksum |
  | +--------------------------------+
  | | idxfile checksum |
  | +--------------------------------+
  .-------.
  |
 Pack file entry: &lt;+</code></pre>
 </div></div>
 <div class="literalblock">
 <div class="content">
 <pre><code>packed object header:
  1-byte size extension bit (MSB)
  type (next 3 bit)
  size0 (lower 4-bit)
  n-byte sizeN (as long as MSB is set, each 7-bit)
  size0..sizeN form 4+7+7+..+7 bit integer, size0
  is the least significant part, and sizeN is the
  most significant part.
 packed object data:
  If it is not DELTA, then deflated bytes (the size above
  is the size before compression).
  If it is REF_DELTA, then
  20-byte base object name SHA-1 (the size above is the
  size of the delta data that follows).
  delta data, deflated.
  If it is OFS_DELTA, then
  n-byte offset (see below) interpreted as a negative
  offset from the type-byte of the header of the
  ofs-delta entry (the size above is the size of
  the delta data that follows).
  delta data, deflated.</code></pre>
 </div></div>
 <div class="literalblock">
 <div class="content">
 <pre><code>offset encoding:
  n bytes with MSB set in all but the last one.
  The offset is then the number constructed by
  concatenating the lower 7 bit of each byte, and
  for n &gt;= 2 adding 2^7 + 2^14 + ... + 2^(7*(n-1))
  to the result.</code></pre>
 </div></div>
 </div>
 </div>
 <div class="sect1">
 <h2 id="_version_2_pack_idx_files_support_packs_larger_than_4_gib_and">Version 2 pack-*.idx files support packs larger than 4 GiB, and</h2>
 <div class="sectionbody">
 <div class="literalblock">
 <div class="content">
 <pre><code>have some other reorganizations. They have the format:</code></pre>
 </div></div>
 <div class="ulist"><ul>
 <li>
 <p>
 A 4-byte magic number <em>\377tOc</em> which is an unreasonable
  fanout[0] value.
 </p>
 </li>
 <li>
 <p>
 A 4-byte version number (= 2)
 </p>
 </li>
 <li>
 <p>
 A 256-entry fan-out table just like v1.
 </p>
 </li>
 <li>
 <p>
 A table of sorted 20-byte SHA-1 object names. These are
  packed together without offset values to reduce the cache
  footprint of the binary search for a specific object name.
 </p>
 </li>
 <li>
 <p>
 A table of 4-byte CRC32 values of the packed object data.
  This is new in v2 so compressed data can be copied directly
  from pack to pack during repacking without undetected
  data corruption.
 </p>
 </li>
 <li>
 <p>
 A table of 4-byte offset values (in network byte order).
  These are usually 31-bit pack file offsets, but large
  offsets are encoded as an index into the next table with
  the msbit set.
 </p>
 </li>
 <li>
 <p>
 A table of 8-byte offset entries (empty for pack files less
  than 2 GiB). Pack files are organized with heavily used
  objects toward the front, so most object references should
  not need to refer to this table.
 </p>
 </li>
 <li>
 <p>
 The same trailer as a v1 pack file:
 </p>
 <div class="literalblock">
 <div class="content">
 <pre><code>A copy of the 20-byte SHA-1 checksum at the end of
 corresponding packfile.</code></pre>
 </div></div>
 <div class="literalblock">
 <div class="content">
 <pre><code>20-byte SHA-1-checksum of all of the above.</code></pre>
 </div></div>
 </li>
 </ul></div>
 </div>
 </div>
 </div>
 <div id="footnotes"><hr /></div>
 <div id="footer">
 <div id="footer-text">
 Last updated 2013-08-20 08:40:27 PDT
 </div>
 </div>
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