gitformat-pack(5) — Linux manual page


GITFORMAT-PACK(5)              Git Manual              GITFORMAT-PACK(5)

NAME         top

       gitformat-pack - Git pack format

SYNOPSIS         top


DESCRIPTION         top

       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 git-pack-objects(1).

       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 gitformat-bundle(5).


       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.


       •   A header appears at the beginning and consists of the

               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.

       •   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.

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

   Object types
       Valid object types are:

       •   OBJ_COMMIT (1)

       •   OBJ_TREE (2)

       •   OBJ_BLOB (3)

       •   OBJ_TAG (4)

       •   OBJ_OFS_DELTA (6)

       •   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 significant.

       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

           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

       Reserved instruction

               | 00000000 |

           This is the instruction reserved for future expansion.

       •   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.

       •   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.

       •   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:

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

       •   A 4-byte version number (= 2)

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

       •   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.

       •   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

       •   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

       •   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.

       •   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.


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

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

       •   A 4-byte hash function identifier (= 1 for SHA-1, 2 for

       •   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

       •   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.

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

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

       •   A 4-byte hash function identifier (= 1 for SHA-1, 2 for

       •   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.

       •   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 types.

       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

       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
       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’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

       •   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).

       •   If pack(o1) ≠ pack(o2), then sort the two objects in
           descending order based on the pack ID.

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

       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

       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

       •   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).

       •   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 chunk.

CRUFT PACKS         top

       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

           [...] 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:

       •   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.

GIT         top

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       system) project.  Information about the project can be found at
       ⟨⟩.  If you have a bug report for this manual
       page, see ⟨⟩.  This page was obtained
       from the project's upstream Git repository
       ⟨⟩ on 2024-06-14.  (At that time,
       the date of the most recent commit that was found in the
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Git         2024-06-12              GITFORMAT-PACK(5)

Pages that refer to this page: git(1)git-bundle(1)git-config(1)git-multi-pack-index(1)gitformat-chunk(5)gitprotocol-pack(5)