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<body class="manpage">
<div id="header">
<h1>gitformat-pack(5) Manual Page</h1>
<h2 id="_name">NAME</h2>
<div class="sectionbody">
<p>gitformat-pack - Git pack format</p>
</div>
</div>
<div id="content">
<div class="sect1">
<h2 id="_synopsis">SYNOPSIS</h2>
<div class="sectionbody">
<div class="verseblock">
<pre class="content">$GIT_DIR/objects/pack/pack-<strong>.{pack,idx}
$GIT_DIR/objects/pack/pack-</strong>.rev
$GIT_DIR/objects/pack/pack-*.mtimes
$GIT_DIR/objects/pack/multi-pack-index</pre>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_description">DESCRIPTION</h2>
<div class="sectionbody">
<div class="paragraph">
<p>The Git pack format is how Git stores most of its primary repository
data. Over the lifetime of a repository, loose objects (if any) and
smaller packs are consolidated into larger pack(s). See
<a href="git-gc.html">git-gc(1)</a> and <a href="git-pack-objects.html">git-pack-objects(1)</a>.</p>
</div>
<div class="paragraph">
<p>The pack format is also used over-the-wire, see
e.g. <a href="gitprotocol-v2.html">gitprotocol-v2(5)</a>, as well as being a part of
other container formats in the case of <a href="gitformat-bundle.html">gitformat-bundle(5)</a>.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_checksums_and_object_ids">Checksums and object IDs</h2>
<div class="sectionbody">
<div class="paragraph">
<p>In a repository using the traditional SHA-1, pack checksums, index checksums,
and object IDs (object names) mentioned below are all computed using SHA-1.
Similarly, in SHA-256 repositories, these values are computed using SHA-256.</p>
</div>
<div class="paragraph">
<p>CRC32 checksums are always computed over the entire packed object, including
the header (n-byte type and length); the base object name or offset, if any;
and the entire compressed object.  The CRC32 algorithm used is that of zlib.</p>
</div>
</div>
</div>
<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>4-byte signature:
    The signature is: {'P', 'A', 'C', 'K'}</pre>
</div>
</div>
<div class="literalblock">
<div class="content">
<pre>   4-byte version number (network byte order):
Git currently accepts version number 2 or 3 but
       generates version 2 only.</pre>
</div>
</div>
<div class="literalblock">
<div class="content">
<pre>4-byte number of objects contained in the pack (network byte order)</pre>
</div>
</div>
<div class="literalblock">
<div class="content">
<pre>Observation: we cannot have more than 4G versions ;-) and
more than 4G objects in a pack.</pre>
</div>
</div>
</li>
<li>
<p>The header is followed by a number of object entries, each of
which looks like this:</p>
<div class="literalblock">
<div class="content">
<pre>(undeltified representation)
n-byte type and length (3-bit type, (n-1)*7+4-bit length)
compressed data</pre>
</div>
</div>
<div class="literalblock">
<div class="content">
<pre>   (deltified representation)
   n-byte type and length (3-bit type, (n-1)*7+4-bit length)
   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</pre>
</div>
</div>
<div class="literalblock">
<div class="content">
<pre>Observation: the length of each object is encoded in a variable
length format and is not constrained to 32-bit or anything.</pre>
</div>
</div>
</li>
<li>
<p>The trailer records a pack checksum of all of the above.</p>
</li>
</ul>
</div>
<div class="sect2">
<h3 id="_object_types">Object types</h3>
<div class="paragraph">
<p>Valid object types are:</p>
</div>
<div class="ulist">
<ul>
<li>
<p>OBJ_COMMIT (1)</p>
</li>
<li>
<p>OBJ_TREE (2)</p>
</li>
<li>
<p>OBJ_BLOB (3)</p>
</li>
<li>
<p>OBJ_TAG (4)</p>
</li>
<li>
<p>OBJ_OFS_DELTA (6)</p>
</li>
<li>
<p>OBJ_REF_DELTA (7)</p>
</li>
</ul>
</div>
<div class="paragraph">
<p>Type 5 is reserved for future expansion. Type 0 is invalid.</p>
</div>
</div>
<div class="sect2">
<h3 id="_object_encoding">Object encoding</h3>
<div class="paragraph">
<p>Unlike loose objects, packed objects do not have a prefix containing the type,
size, and a NUL byte. These are not necessary because they can be determined by
the n-byte type and length that prefixes the data and so they are omitted from
the compressed and deltified data.</p>
</div>
<div class="paragraph">
<p>The computation of the object ID still uses this prefix by reconstructing it
from the type and length as needed.</p>
</div>
</div>
<div class="sect2">
<h3 id="_size_encoding">Size encoding</h3>
<div class="paragraph">
<p>This document uses the following "size encoding" of non-negative
integers: From each byte, the seven least significant bits are
used to form the resulting integer. As long as the most significant
bit is 1, this process continues; the byte with MSB 0 provides the
last seven bits.  The seven-bit chunks are concatenated. Later
values are more significant.</p>
</div>
<div class="paragraph">
<p>This size encoding should not be confused with the "offset encoding",
which is also used in this document.</p>
</div>
<div class="paragraph">
<p>When encoding the size of an undeltified object in a pack, the size is that of
the uncompressed raw object. For deltified objects, it is the size of the
uncompressed delta.  The base object name or offset is not included in the size
computation.</p>
</div>
</div>
<div class="sect2">
<h3 id="_deltified_representation">Deltified representation</h3>
<div class="paragraph">
<p>Conceptually there are only four object types: commit, tree, tag and
blob. However to save space, an object could be stored as a "delta" of
another "base" object. These representations are assigned new types
ofs-delta and ref-delta, which is only valid in a pack file.</p>
</div>
<div class="paragraph">
<p>Both ofs-delta and ref-delta store the "delta" to be applied to
another object (called <em>base object</em>) to reconstruct the object. The
difference between them is, ref-delta directly encodes base object
name. If the base object is in the same pack, ofs-delta encodes
the offset of the base object in the pack instead.</p>
</div>
<div class="paragraph">
<p>The base object could also be deltified if it&#8217;s in the same pack.
Ref-delta can also refer to an object outside the pack (i.e. the
so-called "thin pack"). When stored on disk however, the pack should
be self contained to avoid cyclic dependency.</p>
</div>
<div class="paragraph">
<p>The delta data starts with the size of the base object and the
size of the object to be reconstructed. These sizes are
encoded using the size encoding from above.  The remainder of
the delta data is a sequence of instructions to reconstruct the object
from the base object. If the base object is deltified, it must be
converted to canonical form first. Each instruction appends more and
more data to the target object until it&#8217;s complete. There are two
supported instructions so far: one for copying a byte range from the
source object and one for inserting new data embedded in the
instruction itself.</p>
</div>
<div class="paragraph">
<p>Each instruction has variable length. Instruction type is determined
by the seventh bit of the first octet. The following diagrams follow
the convention in RFC 1951 (Deflate compressed data format).</p>
</div>
<div class="sect3">
<h4 id="_instruction_to_copy_from_base_object">Instruction to copy from base object</h4>
<div class="literalblock">
<div class="content">
<pre>+----------+---------+---------+---------+---------+-------+-------+-------+
| 1xxxxxxx | offset1 | offset2 | offset3 | offset4 | size1 | size2 | size3 |
+----------+---------+---------+---------+---------+-------+-------+-------+</pre>
</div>
</div>
<div class="paragraph">
<p>This is the instruction format to copy a byte range from the source
object. It encodes the offset to copy from and the number of bytes to
copy. Offset and size are in little-endian order.</p>
</div>
<div class="paragraph">
<p>All offset and size bytes are optional. This is to reduce the
instruction size when encoding small offsets or sizes. The first seven
bits in the first octet determine which of the next seven octets is
present. If bit zero is set, offset1 is present. If bit one is set
offset2 is present and so on.</p>
</div>
<div class="paragraph">
<p>Note that a more compact instruction does not change offset and size
encoding. For example, if only offset2 is omitted like below, offset3
still contains bits 16-23. It does not become offset2 and contains
bits 8-15 even if it&#8217;s right next to offset1.</p>
</div>
<div class="literalblock">
<div class="content">
<pre>+----------+---------+---------+
| 10000101 | offset1 | offset3 |
+----------+---------+---------+</pre>
</div>
</div>
<div class="paragraph">
<p>In its most compact form, this instruction only takes up one byte
(0x80) with both offset and size omitted, which will have default
values zero. There is another exception: size zero is automatically
converted to 0x10000.</p>
</div>
</div>
<div class="sect3">
<h4 id="_instruction_to_add_new_data">Instruction to add new data</h4>
<div class="literalblock">
<div class="content">
<pre>+----------+============+
| 0xxxxxxx |    data    |
+----------+============+</pre>
</div>
</div>
<div class="paragraph">
<p>This is the instruction to construct the target object without the base
object. The following data is appended to the target object. The first
seven bits of the first octet determine the size of data in
bytes. The size must be non-zero.</p>
</div>
</div>
<div class="sect3">
<h4 id="_reserved_instruction">Reserved instruction</h4>
<div class="literalblock">
<div class="content">
<pre>+----------+============
| 00000000 |
+----------+============</pre>
</div>
</div>
<div class="paragraph">
<p>This is the instruction reserved for future expansion.</p>
</div>
</div>
</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>4-byte network byte order integer, recording where the
object is stored in the packfile as the offset from the
beginning.</pre>
</div>
</div>
<div class="literalblock">
<div class="content">
<pre>one object name of the appropriate size.</pre>
</div>
</div>
</li>
<li>
<p>The file is concluded with a trailer:</p>
<div class="literalblock">
<div class="content">
<pre>A copy of the pack checksum at the end of the corresponding
packfile.</pre>
</div>
</div>
<div class="literalblock">
<div class="content">
<pre>Index checksum of all of the above.</pre>
</div>
</div>
</li>
</ul>
</div>
<div class="paragraph">
<p>Pack Idx file:</p>
</div>
<div class="literalblock">
<div class="content">
<pre>	--  +--------------------------------+
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;+</pre>
</div>
</div>
<div class="literalblock">
<div class="content">
<pre>    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
  base object name (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.</pre>
</div>
</div>
<div class="literalblock">
<div class="content">
<pre>  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.</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>have some other reorganizations.  They have the format:</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 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>A copy of the pack checksum at the end of the
corresponding packfile.</pre>
</div>
</div>
<div class="literalblock">
<div class="content">
<pre>Index checksum of all of the above.</pre>
</div>
</div>
</li>
</ul>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_pack_rev_files_have_the_format">pack-*.rev files have the format:</h2>
<div class="sectionbody">
<div class="ulist">
<ul>
<li>
<p>A 4-byte magic number <em>0x52494458</em> (<em>RIDX</em>).</p>
</li>
<li>
<p>A 4-byte version identifier (= 1).</p>
</li>
<li>
<p>A 4-byte hash function identifier (= 1 for SHA-1, 2 for SHA-256).</p>
</li>
<li>
<p>A table of index positions (one per packed object, num_objects in
total, each a 4-byte unsigned integer in network order), sorted by
their corresponding offsets in the packfile.</p>
</li>
<li>
<p>A trailer, containing a:</p>
<div class="literalblock">
<div class="content">
<pre>checksum of the corresponding packfile, and</pre>
</div>
</div>
<div class="literalblock">
<div class="content">
<pre>a checksum of all of the above.</pre>
</div>
</div>
</li>
</ul>
</div>
<div class="paragraph">
<p>All 4-byte numbers are in network order.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_pack_mtimes_files_have_the_format">pack-*.mtimes files have the format:</h2>
<div class="sectionbody">
<div class="paragraph">
<p>All 4-byte numbers are in network byte order.</p>
</div>
<div class="ulist">
<ul>
<li>
<p>A 4-byte magic number <em>0x4d544d45</em> (<em>MTME</em>).</p>
</li>
<li>
<p>A 4-byte version identifier (= 1).</p>
</li>
<li>
<p>A 4-byte hash function identifier (= 1 for SHA-1, 2 for SHA-256).</p>
</li>
<li>
<p>A table of 4-byte unsigned integers. The ith value is the
modification time (mtime) of the ith object in the corresponding
pack by lexicographic (index) order. The mtimes count standard
epoch seconds.</p>
</li>
<li>
<p>A trailer, containing a checksum of the corresponding packfile,
and a checksum of all of the above (each having length according
to the specified hash function).</p>
</li>
</ul>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_multi_pack_index_midx_files_have_the_following_format">multi-pack-index (MIDX) files have the following format:</h2>
<div class="sectionbody">
<div class="paragraph">
<p>The multi-pack-index files refer to multiple pack-files and loose objects.</p>
</div>
<div class="paragraph">
<p>In order to allow extensions that add extra data to the MIDX, we organize
the body into "chunks" and provide a lookup table at the beginning of the
body. The header includes certain length values, such as the number of packs,
the number of base MIDX files, hash lengths and types.</p>
</div>
<div class="paragraph">
<p>All 4-byte numbers are in network order.</p>
</div>
<div class="paragraph">
<p>HEADER:</p>
</div>
<div class="literalblock">
<div class="content">
<pre>4-byte signature:
    The signature is: {'M', 'I', 'D', 'X'}</pre>
</div>
</div>
<div class="literalblock">
<div class="content">
<pre>1-byte version number:
    Git writes the version specified by the "midx.version"
    configuration option, which defaults to 2. It recognizes
    both versions 1 and 2.</pre>
</div>
</div>
<div class="literalblock">
<div class="content">
<pre>1-byte Object Id Version
    We infer the length of object IDs (OIDs) from this value:
	1 =&gt; SHA-1
	2 =&gt; SHA-256
    If the hash type does not match the repository's hash algorithm,
    the multi-pack-index file should be ignored with a warning
    presented to the user.</pre>
</div>
</div>
<div class="literalblock">
<div class="content">
<pre>1-byte number of "chunks"</pre>
</div>
</div>
<div class="literalblock">
<div class="content">
<pre>1-byte number of base multi-pack-index files:
    This value is currently always zero.</pre>
</div>
</div>
<div class="literalblock">
<div class="content">
<pre>4-byte number of pack files</pre>
</div>
</div>
<div class="paragraph">
<p>CHUNK LOOKUP:</p>
</div>
<div class="literalblock">
<div class="content">
<pre>(C + 1) * 12 bytes providing the chunk offsets:
    First 4 bytes describe chunk id. Value 0 is a terminating label.
    Other 8 bytes provide offset in current file for chunk to start.
    (Chunks are provided in file-order, so you can infer the length
    using the next chunk position if necessary.)</pre>
</div>
</div>
<div class="literalblock">
<div class="content">
<pre>The CHUNK LOOKUP matches the table of contents from
the chunk-based file format, see linkgit:gitformat-chunk[5].</pre>
</div>
</div>
<div class="literalblock">
<div class="content">
<pre>The remaining data in the body is described one chunk at a time, and
these chunks may be given in any order. Chunks are required unless
otherwise specified.</pre>
</div>
</div>
<div class="paragraph">
<p>CHUNK DATA:</p>
</div>
<div class="literalblock">
<div class="content">
<pre>Packfile Names (ID: {'P', 'N', 'A', 'M'})
    Store the names of packfiles as a sequence of NUL-terminated
    strings. There is no extra padding between the filenames,
    and they are listed in lexicographic order. The chunk itself
    is padded at the end with between 0 and 3 NUL bytes to make the
    chunk size a multiple of 4 bytes. Version 1 MIDXs are required to
    list their packs in lexicographic order, but version 2 MIDXs may
    list their packs in any arbitrary order.</pre>
</div>
</div>
<div class="literalblock">
<div class="content">
<pre>Bitmapped Packfiles (ID: {'B', 'T', 'M', 'P'})
    Stores a table of two 4-byte unsigned integers in network order.
    Each table entry corresponds to a single pack (in the order that
    they appear above in the `PNAM` chunk). The values for each table
    entry are as follows:
    - The first bit position (in pseudo-pack order, see below) to
      contain an object from that pack.
    - The number of bits whose objects are selected from that pack.</pre>
</div>
</div>
<div class="literalblock">
<div class="content">
<pre>OID Fanout (ID: {'O', 'I', 'D', 'F'})
    The ith entry, F[i], stores the number of OIDs with first
    byte at most i. Thus F[255] stores the total
    number of objects.</pre>
</div>
</div>
<div class="literalblock">
<div class="content">
<pre>OID Lookup (ID: {'O', 'I', 'D', 'L'})
    The OIDs for all objects in the MIDX are stored in lexicographic
    order in this chunk.</pre>
</div>
</div>
<div class="literalblock">
<div class="content">
<pre>Object Offsets (ID: {'O', 'O', 'F', 'F'})
    Stores two 4-byte values for every object.
    1: The pack-int-id for the pack storing this object.
    2: The offset within the pack.
	If all offsets are less than 2^32, then the large offset chunk
	will not exist and offsets are stored as in IDX v1.
	If there is at least one offset value larger than 2^32-1, then
	the large offset chunk must exist, and offsets larger than
	2^31-1 must be stored in it instead. If the large offset chunk
	exists and the 31st bit is on, then removing that bit reveals
	the row in the large offsets containing the 8-byte offset of
	this object.</pre>
</div>
</div>
<div class="literalblock">
<div class="content">
<pre>[Optional] Object Large Offsets (ID: {'L', 'O', 'F', 'F'})
    8-byte offsets into large packfiles.</pre>
</div>
</div>
<div class="literalblock">
<div class="content">
<pre>[Optional] Bitmap pack order (ID: {'R', 'I', 'D', 'X'})
    A list of MIDX positions (one per object in the MIDX, num_objects in
    total, each a 4-byte unsigned integer in network byte order), sorted
    according to their relative bitmap/pseudo-pack positions.</pre>
</div>
</div>
<div class="paragraph">
<p>TRAILER:</p>
</div>
<div class="literalblock">
<div class="content">
<pre>Index checksum of the above contents.</pre>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_multi_pack_index_reverse_indexes">multi-pack-index reverse indexes</h2>
<div class="sectionbody">
<div class="paragraph">
<p>Similar to the pack-based reverse index, the multi-pack index can also
be used to generate a reverse index.</p>
</div>
<div class="paragraph">
<p>Instead of mapping between offset, pack-, and index position, this
reverse index maps between an object&#8217;s position within the MIDX, and
that object&#8217;s position within a pseudo-pack that the MIDX describes
(i.e., the ith entry of the multi-pack reverse index holds the MIDX
position of ith object in pseudo-pack order).</p>
</div>
<div class="paragraph">
<p>To clarify the difference between these orderings, consider a multi-pack
reachability bitmap (which does not yet exist, but is what we are
building towards here). Each bit needs to correspond to an object in the
MIDX, and so we need an efficient mapping from bit position to MIDX
position.</p>
</div>
<div class="paragraph">
<p>One solution is to let bits occupy the same position in the oid-sorted
index stored by the MIDX. But because oids are effectively random, their
resulting reachability bitmaps would have no locality, and thus compress
poorly. (This is the reason that single-pack bitmaps use the pack
ordering, and not the .idx ordering, for the same purpose.)</p>
</div>
<div class="paragraph">
<p>So we&#8217;d like to define an ordering for the whole MIDX based around
pack ordering, which has far better locality (and thus compresses more
efficiently). We can think of a pseudo-pack created by the concatenation
of all of the packs in the MIDX. E.g., if we had a MIDX with three packs
(a, b, c), with 10, 15, and 20 objects respectively, we can imagine an
ordering of the objects like:</p>
</div>
<div class="literalblock">
<div class="content">
<pre>|a,0|a,1|...|a,9|b,0|b,1|...|b,14|c,0|c,1|...|c,19|</pre>
</div>
</div>
<div class="paragraph">
<p>where the ordering of the packs is defined by the MIDX&#8217;s pack list,
and then the ordering of objects within each pack is the same as the
order in the actual packfile.</p>
</div>
<div class="paragraph">
<p>Given the list of packs and their counts of objects, you can
naïvely reconstruct that pseudo-pack ordering (e.g., the object at
position 27 must be (c,1) because packs "a" and "b" consumed 25 of the
slots). But there&#8217;s a catch. Objects may be duplicated between packs, in
which case the MIDX only stores one pointer to the object (and thus we&#8217;d
want only one slot in the bitmap).</p>
</div>
<div class="paragraph">
<p>Callers could handle duplicates themselves by reading objects in order
of their bit-position, but that&#8217;s linear in the number of objects, and
much too expensive for ordinary bitmap lookups. Building a reverse index
solves this, since it is the logical inverse of the index, and that
index has already removed duplicates. But, building a reverse index on
the fly can be expensive. Since we already have an on-disk format for
pack-based reverse indexes, let&#8217;s reuse it for the MIDX&#8217;s pseudo-pack,
too.</p>
</div>
<div class="paragraph">
<p>Objects from the MIDX are ordered as follows to string together the
pseudo-pack. Let <code>pack</code>(<code>o</code>) return the pack from which <code>o</code> was selected
by the MIDX, and define an ordering of packs based on their numeric ID
(as stored by the MIDX). Let <code>offset</code>(<code>o</code>) return the object offset of <code>o</code>
within <code>pack</code>(<code>o</code>). Then, compare <code>o1</code> and <code>o2</code> as follows:</p>
</div>
<div class="ulist">
<ul>
<li>
<p>If one of <code>pack</code>(<code>o1</code>) and <code>pack</code>(<code>o2</code>) is preferred and the other
is not, then the preferred one sorts first.</p>
<div class="paragraph">
<p>(This is a detail that allows the MIDX bitmap to determine which
pack should be used by the pack-reuse mechanism, since it can ask
the MIDX for the pack containing the object at bit position 0).</p>
</div>
</li>
<li>
<p>If <code>pack</code>(<code>o1</code>) ≠ <code>pack</code>(<code>o2</code>), then sort the two objects in descending
order based on the pack ID.</p>
</li>
<li>
<p>Otherwise, <code>pack</code>(<code>o1</code>) <code>=</code> <code>pack</code>(<code>o2</code>), and the objects are sorted in
pack-order (i.e., <code>o1</code> sorts ahead of <code>o2</code> exactly when <code>offset</code>(<code>o1</code>)
&lt; <code>offset</code>(<code>o2</code>)).</p>
</li>
</ul>
</div>
<div class="paragraph">
<p>In short, a MIDX&#8217;s pseudo-pack is the de-duplicated concatenation of
objects in packs stored by the MIDX, laid out in pack order, and the
packs arranged in MIDX order (with the preferred pack coming first).</p>
</div>
<div class="paragraph">
<p>The MIDX&#8217;s reverse index is stored in the optional <em>RIDX</em> chunk within
the MIDX itself.</p>
</div>
<div class="sect2">
<h3 id="_btmp_chunk"><code>BTMP</code> chunk</h3>
<div class="paragraph">
<p>The Bitmapped Packfiles (<code>BTMP</code>) chunk encodes additional information
about the objects in the multi-pack index&#8217;s reachability bitmap. Recall
that objects from the MIDX are arranged in "pseudo-pack" order (see
above) for reachability bitmaps.</p>
</div>
<div class="paragraph">
<p>From the example above, suppose we have packs "a", "b", and "c", with
10, 15, and 20 objects, respectively. In pseudo-pack order, those would
be arranged as follows:</p>
</div>
<div class="literalblock">
<div class="content">
<pre>|a,0|a,1|...|a,9|b,0|b,1|...|b,14|c,0|c,1|...|c,19|</pre>
</div>
</div>
<div class="paragraph">
<p>When working with single-pack bitmaps (or, equivalently, multi-pack
reachability bitmaps with a preferred pack), <a href="git-pack-objects.html">git-pack-objects(1)</a>
performs &#8220;verbatim&#8221; reuse, attempting to reuse chunks of the bitmapped
or preferred packfile instead of adding objects to the packing list.</p>
</div>
<div class="paragraph">
<p>When a chunk of bytes is reused from an existing pack, any objects
contained therein do not need to be added to the packing list, saving
memory and CPU time. But a chunk from an existing packfile can only be
reused when the following conditions are met:</p>
</div>
<div class="ulist">
<ul>
<li>
<p>The chunk contains only objects which were requested by the caller
(i.e. does not contain any objects which the caller didn&#8217;t ask for
explicitly or implicitly).</p>
</li>
<li>
<p>All objects stored in non-thin packs as offset- or reference-deltas
also include their base object in the resulting pack.</p>
</li>
</ul>
</div>
<div class="paragraph">
<p>The <code>BTMP</code> chunk encodes the necessary information in order to implement
multi-pack reuse over a set of packfiles as described above.
Specifically, the <code>BTMP</code> chunk encodes three pieces of information (all
32-bit unsigned integers in network byte-order) for each packfile <code>p</code>
that is stored in the MIDX, as follows:</p>
</div>
<div class="dlist">
<dl>
<dt class="hdlist1"><code>bitmap_pos</code></dt>
<dd>
<p>The first bit position (in pseudo-pack order) in the
multi-pack index&#8217;s reachability bitmap occupied by an object from <code>p</code>.</p>
</dd>
<dt class="hdlist1"><code>bitmap_nr</code></dt>
<dd>
<p>The number of bit positions (including the one at
<code>bitmap_pos</code>) that encode objects from that pack <code>p</code>.</p>
</dd>
</dl>
</div>
<div class="paragraph">
<p>For example, the <code>BTMP</code> chunk corresponding to the above example (with
packs &#8220;a&#8221;, &#8220;b&#8221;, and &#8220;c&#8221;) would look like:</p>
</div>
<table class="tableblock frame-all grid-all stretch">
<colgroup>
<col style="width: 20%;"/>
<col style="width: 40%;"/>
<col style="width: 40%;"/>
</colgroup>
<thead>
<tr>
<th class="tableblock halign-left valign-top"></th>
<th class="tableblock halign-left valign-top"><code>bitmap_pos</code></th>
<th class="tableblock halign-left valign-top"><code>bitmap_nr</code></th>
</tr>
</thead>
<tbody>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock">packfile &#8220;a&#8221;</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock"><code>0</code></p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock"><code>10</code></p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock">packfile &#8220;b&#8221;</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock"><code>10</code></p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock"><code>15</code></p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock">packfile &#8220;c&#8221;</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock"><code>25</code></p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock"><code>20</code></p></td>
</tr>
</tbody>
</table>
<div class="paragraph">
<p>With this information in place, we can treat each packfile as
individually reusable in the same fashion as verbatim pack reuse is
performed on individual packs prior to the implementation of the <code>BTMP</code>
chunk.</p>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_cruft_packs">cruft packs</h2>
<div class="sectionbody">
<div class="paragraph">
<p>The cruft packs feature offer an alternative to Git&#8217;s traditional mechanism of
removing unreachable objects. This document provides an overview of Git&#8217;s
pruning mechanism, and how a cruft pack can be used instead to accomplish the
same.</p>
</div>
<div class="sect2">
<h3 id="_background">Background</h3>
<div class="paragraph">
<p>To remove unreachable objects from your repository, Git offers <code>git</code> <code>repack</code> <code>-Ad</code>
(see <a href="git-repack.html">git-repack(1)</a>). Quoting from the documentation:</p>
</div>
<div class="listingblock">
<div class="content">
<pre>[...] unreachable objects in a previous pack become loose, unpacked objects,
instead of being left in the old pack. [...] loose unreachable objects will be
pruned according to normal expiry rules with the next 'git gc' invocation.</pre>
</div>
</div>
<div class="paragraph">
<p>Unreachable objects aren&#8217;t removed immediately, since doing so could race with
an incoming push which may reference an object which is about to be deleted.
Instead, those unreachable objects are stored as loose objects and stay that way
until they are older than the expiration window, at which point they are removed
by <a href="git-prune.html">git-prune(1)</a>.</p>
</div>
<div class="paragraph">
<p>Git must store these unreachable objects loose in order to keep track of their
per-object mtimes. If these unreachable objects were written into one big pack,
then either freshening that pack (because an object contained within it was
re-written) or creating a new pack of unreachable objects would cause the pack&#8217;s
mtime to get updated, and the objects within it would never leave the expiration
window. Instead, objects are stored loose in order to keep track of the
individual object mtimes and avoid a situation where all cruft objects are
freshened at once.</p>
</div>
<div class="paragraph">
<p>This can lead to undesirable situations when a repository contains many
unreachable objects which have not yet left the grace period. Having large
directories in the shards of .<code>git/objects</code> can lead to decreased performance in
the repository. But given enough unreachable objects, this can lead to inode
starvation and degrade the performance of the whole system. Since we
can never pack those objects, these repositories often take up a large amount of
disk space, since we can only zlib compress them, but not store them in delta
chains.</p>
</div>
</div>
<div class="sect2">
<h3 id="_cruft_packs_2">Cruft packs</h3>
<div class="paragraph">
<p>A cruft pack eliminates the need for storing unreachable objects in a loose
state by including the per-object mtimes in a separate file alongside a single
pack containing all loose objects.</p>
</div>
<div class="paragraph">
<p>A cruft pack is written by <code>git</code> <code>repack</code> <code>--cruft</code> when generating a new pack.
<a href="git-pack-objects.html">git-pack-objects(1)</a>'s <code>--cruft</code> option. Note that <code>git</code> <code>repack</code> <code>--cruft</code>
is a classic all-into-one repack, meaning that everything in the resulting pack is
reachable, and everything else is unreachable. Once written, the <code>--cruft</code>
option instructs <code>git</code> <code>repack</code> to generate another pack containing only objects
not packed in the previous step (which equates to packing all unreachable
objects together). This progresses as follows:</p>
</div>
<div class="olist arabic">
<ol class="arabic">
<li>
<p>Enumerate every object, marking any object which is (a) not contained in a
kept-pack, and (b) whose mtime is within the grace period as a traversal
tip.</p>
</li>
<li>
<p>Perform a reachability traversal based on the tips gathered in the previous
step, adding every object along the way to the pack.</p>
</li>
<li>
<p>Write the pack out, along with a .<code>mtimes</code> file that records the per-object
timestamps.</p>
</li>
</ol>
</div>
<div class="paragraph">
<p>This mode is invoked internally by <a href="git-repack.html">git-repack(1)</a> when instructed to
write a cruft pack. Crucially, the set of in-core kept packs is exactly the set
of packs which will not be deleted by the repack; in other words, they contain
all of the repository&#8217;s reachable objects.</p>
</div>
<div class="paragraph">
<p>When a repository already has a cruft pack, <code>git</code> <code>repack</code> <code>--cruft</code> typically only
adds objects to it. An exception to this is when <code>git</code> <code>repack</code> is given the
<code>--cruft-expiration</code> option, which allows the generated cruft pack to omit
expired objects instead of waiting for <a href="git-gc.html">git-gc(1)</a> to expire those objects
later on.</p>
</div>
<div class="paragraph">
<p>It is <a href="git-gc.html">git-gc(1)</a> that is typically responsible for removing expired
unreachable objects.</p>
</div>
</div>
<div class="sect2">
<h3 id="_alternatives">Alternatives</h3>
<div class="paragraph">
<p>Notable alternatives to this design include:</p>
</div>
<div class="ulist">
<ul>
<li>
<p>The location of the per-object mtime data.</p>
</li>
</ul>
</div>
<div class="paragraph">
<p>On the location of mtime data, a new auxiliary file tied to the pack was chosen
to avoid complicating the .<code>idx</code> format. If the .<code>idx</code> format were ever to gain
support for optional chunks of data, it may make sense to consolidate the
.<code>mtimes</code> format into the .<code>idx</code> itself.</p>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_git">GIT</h2>
<div class="sectionbody">
<div class="paragraph">
<p>Part of the <a href="git.html">git(1)</a> suite</p>
</div>
</div>
</div>
</div>
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<div id="footer-text">
Last updated 2026-03-25 15:34:02 -0700
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