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gitformat-pack(5)                 Git Manual                 gitformat-pack(5)


       gitformat-pack - Git pack format




       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 git-gc(1) and

       The pack format is also used over-the-wire, see e.g. gitprotocol-v2(5),
       as well as being a part of other container formats in the case of


       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.


       o   A header appears at the beginning and consists of the following:

               4-byte signature:
                   The signature is: {'P', 'A', 'C', 'K'}

               4-byte version number (network byte order):
                   Git currently accepts version number 2 or 3 but
                   generates version 2 only.

               4-byte number of objects contained in the pack (network byte order)

               Observation: we cannot have more than 4G versions ;-) and
               more than 4G objects in a pack.

       o   The header is followed by a number of object entries, each of which
           looks like this:

               (undeltified representation)
               n-byte type and length (3-bit type, (n-1)*7+4-bit length)
               compressed data

               (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

               Observation: the length of each object is encoded in a variable
               length format and is not constrained to 32-bit or anything.

       o   The trailer records a pack checksum of all of the above.

   Object types
       Valid object types are:

       o   OBJ_COMMIT (1)

       o   OBJ_TREE (2)

       o   OBJ_BLOB (3)

       o   OBJ_TAG (4)

       o   OBJ_OFS_DELTA (6)

       o   OBJ_REF_DELTA (7)

       Type 5 is reserved for future expansion. Type 0 is invalid.

   Size encoding
       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

       This size encoding should not be confused with the "offset encoding",
       which is also used in this document.

   Deltified representation
       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.

       Both ofs-delta and ref-delta store the "delta" to be applied to another
       object (called base object) 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.

       The base object could also be deltified if it'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.

       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'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.

       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).

       Instruction to copy from base object

               | 1xxxxxxx | offset1 | offset2 | offset3 | offset4 | size1 | size2 | size3 |

           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.

           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.

           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's right next to offset1.

               | 10000101 | offset1 | offset3 |

           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.

       Instruction to add new data

               | 0xxxxxxx |    data    |

           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.

       Reserved instruction

               | 00000000 |

           This is the instruction reserved for future expansion.


       o   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 first-level fan-out table.

       o   The header is followed by sorted 24-byte entries, one entry per
           object in the pack. Each entry is:

               4-byte network byte order integer, recording where the
               object is stored in the packfile as the offset from the

               one object name of the appropriate size.

       o   The file is concluded with a trailer:

               A copy of the pack checksum at the end of the corresponding

               Index checksum of all of the above.

       Pack Idx file:

                   --  +--------------------------------+
           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 | | |
                       +--------------------------------+<+ |
                     .-| offset                         |   |
                     | | object name 01XXXXXXXXXXXXXXXX |   |
                     | +--------------------------------+   |
                     | | offset                         |   |
                     | | object name 01XXXXXXXXXXXXXXXX |   |
                     | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~   |
                     | | offset                         |   |
                     | | object name FFXXXXXXXXXXXXXXXX |   |
                   --| +--------------------------------+<--+
           trailer   | | packfile checksum              |
                     | +--------------------------------+
                     | | idxfile checksum               |
                     | +--------------------------------+
           Pack file entry: <+

           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.

           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 >= 2 adding 2^7 + 2^14 + ... + 2^(7*(n-1))
                to the result.


           have some other reorganizations.  They have the format:

       o   A 4-byte magic number \377tOc which is an unreasonable fanout[0]

       o   A 4-byte version number (= 2)

       o   A 256-entry fan-out table just like v1.

       o   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.

       o   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.

       o   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.

       o   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

       o   The same trailer as a v1 pack file:

               A copy of the pack checksum at the end of the
               corresponding packfile.

               Index checksum of all of the above.


       o   A 4-byte magic number 0x52494458 (RIDX).

       o   A 4-byte version identifier (= 1).

       o   A 4-byte hash function identifier (= 1 for SHA-1, 2 for SHA-256).

       o   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.

       o   A trailer, containing a:

               checksum of the corresponding packfile, and

               a checksum of all of the above.

       All 4-byte numbers are in network order.


       All 4-byte numbers are in network byte order.

       o   A 4-byte magic number 0x4d544d45 (MTME).

       o   A 4-byte version identifier (= 1).

       o   A 4-byte hash function identifier (= 1 for SHA-1, 2 for SHA-256).

       o   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.

       o   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).


       The multi-pack-index files refer to multiple pack-files and loose

       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

       All 4-byte numbers are in network order.


           4-byte signature:
               The signature is: {'M', 'I', 'D', 'X'}

           1-byte version number:
               Git only writes or recognizes version 1.

           1-byte Object Id Version
               We infer the length of object IDs (OIDs) from this value:
                   1 => SHA-1
                   2 => 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.

           1-byte number of "chunks"

           1-byte number of base multi-pack-index files:
               This value is currently always zero.

           4-byte number of pack files


           (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.)

           The CHUNK LOOKUP matches the table of contents from
           the chunk-based file format, see linkgit:gitformat-chunk[5].

           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.

       CHUNK DATA:

           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.

           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.

           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.

           OID Lookup (ID: {'O', 'I', 'D', 'L'})
               The OIDs for all objects in the MIDX are stored in lexicographic
               order in this chunk.

           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.

           [Optional] Object Large Offsets (ID: {'L', 'O', 'F', 'F'})
               8-byte offsets into large packfiles.

           [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.


           Index checksum of the above contents.


       Similar to the pack-based reverse index, the multi-pack index can also
       be used to generate a reverse index.

       Instead of mapping between offset, pack-, and index position, this
       reverse index maps between an object's position within the MIDX, and
       that object'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).

       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.

       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.)

       So we'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:


       where the ordering of the packs is defined by the MIDX's pack list, and
       then the ordering of objects within each pack is the same as the order
       in the actual packfile.

       Given the list of packs and their counts of objects, you can naively
       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's a catch. Objects may be duplicated between packs, in which case
       the MIDX only stores one pointer to the object (and thus we'd want only
       one slot in the bitmap).

       Callers could handle duplicates themselves by reading objects in order
       of their bit-position, but that'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's reuse it for the MIDX's
       pseudo-pack, too.

       Objects from the MIDX are ordered as follows to string together the
       pseudo-pack. Let pack(o) return the pack from which o was selected by
       the MIDX, and define an ordering of packs based on their numeric ID (as
       stored by the MIDX). Let offset(o) return the object offset of o within
       pack(o). Then, compare o1 and o2 as follows:

       o   If one of pack(o1) and pack(o2) is preferred and the other is not,
           then the preferred one sorts first.

           (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).

       o   If pack(o1) != pack(o2), then sort the two objects in descending
           order based on the pack ID.

       o   Otherwise, pack(o1) = pack(o2), and the objects are sorted in
           pack-order (i.e., o1 sorts ahead of o2 exactly when offset(o1) <

       In short, a MIDX'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).

       The MIDX's reverse index is stored in the optional RIDX chunk within
       the MIDX itself.

   BTMP chunk
       The Bitmapped Packfiles (BTMP) chunk encodes additional information
       about the objects in the multi-pack index's reachability bitmap. Recall
       that objects from the MIDX are arranged in "pseudo-pack" order (see
       above) for reachability bitmaps.

       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:


       When working with single-pack bitmaps (or, equivalently, multi-pack
       reachability bitmaps with a preferred pack), git-pack-objects(1)
       performs "verbatim" reuse, attempting to reuse chunks of the bitmapped
       or preferred packfile instead of adding objects to the packing list.

       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:

       o   The chunk contains only objects which were requested by the caller
           (i.e. does not contain any objects which the caller didn't ask for
           explicitly or implicitly).

       o   All objects stored in non-thin packs as offset- or reference-deltas
           also include their base object in the resulting pack.

       The BTMP chunk encodes the necessary information in order to implement
       multi-pack reuse over a set of packfiles as described above.
       Specifically, the BTMP chunk encodes three pieces of information (all
       32-bit unsigned integers in network byte-order) for each packfile p
       that is stored in the MIDX, as follows:

           The first bit position (in pseudo-pack order) in the multi-pack
           index's reachability bitmap occupied by an object from p.

           The number of bit positions (including the one at bitmap_pos) that
           encode objects from that pack p.

       For example, the BTMP chunk corresponding to the above example (with
       packs "a", "b", and "c") would look like:

       |             | bitmap_pos | bitmap_nr |
       |packfile "a" | 0          | 10        |
       |packfile "b" | 10         | 15        |
       |packfile "c" | 25         | 20        |

       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 BTMP


       The cruft packs feature offer an alternative to Git's traditional
       mechanism of removing unreachable objects. This document provides an
       overview of Git's pruning mechanism, and how a cruft pack can be used
       instead to accomplish the same.

       To remove unreachable objects from your repository, Git offers git
       repack -Ad (see git-repack(1)). Quoting from the documentation:

           [...] 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.

       Unreachable objects aren'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 git-prune(1).

       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'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.

       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 .git/objects 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.

   Cruft packs
       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.

       A cruft pack is written by git repack --cruft when generating a new
       pack. git-pack-objects(1)'s --cruft option. Note that git repack
       --cruft is a classic all-into-one repack, meaning that everything in
       the resulting pack is reachable, and everything else is unreachable.
       Once written, the --cruft option instructs git repack 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:

        1. 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.

        2. Perform a reachability traversal based on the tips gathered in the
           previous step, adding every object along the way to the pack.

        3. Write the pack out, along with a .mtimes file that records the
           per-object timestamps.

       This mode is invoked internally by git-repack(1) 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's reachable objects.

       When a repository already has a cruft pack, git repack --cruft
       typically only adds objects to it. An exception to this is when git
       repack is given the --cruft-expiration option, which allows the
       generated cruft pack to omit expired objects instead of waiting for
       git-gc(1) to expire those objects later on.

       It is git-gc(1) that is typically responsible for removing expired
       unreachable objects.

       Notable alternatives to this design include:

       o   The location of the per-object mtime data.

       On the location of mtime data, a new auxiliary file tied to the pack
       was chosen to avoid complicating the .idx format. If the .idx format
       were ever to gain support for optional chunks of data, it may make
       sense to consolidate the .mtimes format into the .idx itself.


       Part of the git(1) suite

Git 2.44.0                        2024-02-22                 gitformat-pack(5)

git 2.44.0 - Generated Sat Feb 24 18:03:57 CST 2024
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