# gitformat-pack

## NAME



           gitformat-pack - Git pack format
    


## SYNOPSIS



           $GIT_DIR/objects/pack/pack-**.{pack,idx}**
           **$GIT_DIR/objects/pack/pack-**.rev
           $GIT_DIR/objects/pack/pack-*.mtimes
           $GIT_DIR/objects/pack/multi-pack-index
    


## DESCRIPTION



           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).
    


## CHECKSUMS AND OBJECT IDS



           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.
    


## PACK-*.PACK FILES HAVE THE FOLLOWING FORMAT:



           •   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.
    
           •   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
               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.
    


## ORIGINAL (VERSION 1) PACK-*.IDX FILES HAVE THE FOLLOWING FORMAT:



    
           •   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
                   beginning.
    
                   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
                   packfile.
    
                   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.
    


## VERSION 2 PACK-*.IDX FILES SUPPORT PACKS LARGER THAN 4 GIB, AND



               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
               corruption.
    
           •   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.
    
           •   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.
    


## PACK-*.REV FILES HAVE THE FORMAT:



           •   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
               SHA-256).
    
           •   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.
    
           •   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.
    


## PACK-*.MTIMES FILES HAVE THE FORMAT:



           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
               SHA-256).
    
           •   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).
    


## MULTI-PACK-INDEX (MIDX) FILES HAVE THE FOLLOWING FORMAT:



           The multi-pack-index files refer to multiple pack-files and loose
           objects.
    
           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.
    
           HEADER:
    
               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
    
           CHUNK LOOKUP:
    
               (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.
    
           TRAILER:
    
               Index checksum of the above contents.
    


## MULTI-PACK-INDEX REVERSE INDEXES



           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:
    
               |a,0|a,1|...|a,9|b,0|b,1|...|b,14|c,0|c,1|...|c,19|
    
           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
           follows:
    
           •   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
           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:
    
               |a,0|a,1|...|a,9|b,0|b,1|...|b,14|c,0|c,1|...|c,19|
    
           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:
    
           •   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:
    
           **bitmap_pos**
               The first bit position (in pseudo-pack order) in the
               multi-pack index’s reachability bitmap occupied by an object
               from **p**.
    
           **bitmap_nr**
               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



           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.
    
       **Background**
           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.
    
       **Alternatives**
           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



           Part of the git(1) suite
    


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