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![Ævar Arnfjörð Bjarmason](/assets/img/avatar_default.png)
Integrate the cruft packs documentation initially added in3d89a8c118
(Documentation/technical: add cruft-packs.txt, 2022-05-20) to the newly created "gitformat-pack" documentation. Like the "bitmap-format" added before it in0d4455a3ab
(documentation: add documentation for the bitmap format, 2013-11-14) the "cruft-packs" were documented in their own file. As the diff move detection will show there is no change to "Documentation/technical/cruft-packs.txt" here except to move it, and to "indent" the existing sections by adding an extra "=" to them. We could similarly convert the "bitmap-format.txt", but let's leave it for now due to a conflict with the in-flight ac/bitmap-lookup-table series. Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>
639 lines
25 KiB
Plaintext
639 lines
25 KiB
Plaintext
gitformat-pack(5)
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=================
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NAME
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----
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gitformat-pack - Git pack format
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SYNOPSIS
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--------
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[verse]
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$GIT_DIR/objects/pack/pack-*.{pack,idx}
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$GIT_DIR/objects/pack/pack-*.rev
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$GIT_DIR/objects/pack/pack-*.mtimes
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$GIT_DIR/objects/pack/multi-pack-index
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DESCRIPTION
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-----------
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The Git pack format is now Git stores most of its primary repository
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data. Over the lietime af a repository loose objects (if any) and
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smaller packs are consolidated into larger pack(s). See
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linkgit:git-gc[1] and linkgit:git-pack-objects[1].
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The pack format is also used over-the-wire, see
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e.g. linkgit:gitprotocol-v2[5], as well as being a part of
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other container formats in the case of linkgit:gitformat-bundle[5].
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== Checksums and object IDs
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In a repository using the traditional SHA-1, pack checksums, index checksums,
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and object IDs (object names) mentioned below are all computed using SHA-1.
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Similarly, in SHA-256 repositories, these values are computed using SHA-256.
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== pack-*.pack files have the following format:
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- A header appears at the beginning and consists of the following:
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4-byte signature:
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The signature is: {'P', 'A', 'C', 'K'}
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4-byte version number (network byte order):
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Git currently accepts version number 2 or 3 but
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generates version 2 only.
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4-byte number of objects contained in the pack (network byte order)
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Observation: we cannot have more than 4G versions ;-) and
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more than 4G objects in a pack.
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- The header is followed by number of object entries, each of
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which looks like this:
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(undeltified representation)
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n-byte type and length (3-bit type, (n-1)*7+4-bit length)
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compressed data
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(deltified representation)
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n-byte type and length (3-bit type, (n-1)*7+4-bit length)
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base object name if OBJ_REF_DELTA or a negative relative
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offset from the delta object's position in the pack if this
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is an OBJ_OFS_DELTA object
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compressed delta data
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Observation: length of each object is encoded in a variable
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length format and is not constrained to 32-bit or anything.
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- The trailer records a pack checksum of all of the above.
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=== Object types
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Valid object types are:
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- OBJ_COMMIT (1)
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- OBJ_TREE (2)
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- OBJ_BLOB (3)
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- OBJ_TAG (4)
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- OBJ_OFS_DELTA (6)
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- OBJ_REF_DELTA (7)
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Type 5 is reserved for future expansion. Type 0 is invalid.
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=== Size encoding
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This document uses the following "size encoding" of non-negative
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integers: From each byte, the seven least significant bits are
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used to form the resulting integer. As long as the most significant
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bit is 1, this process continues; the byte with MSB 0 provides the
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last seven bits. The seven-bit chunks are concatenated. Later
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values are more significant.
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This size encoding should not be confused with the "offset encoding",
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which is also used in this document.
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=== Deltified representation
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Conceptually there are only four object types: commit, tree, tag and
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blob. However to save space, an object could be stored as a "delta" of
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another "base" object. These representations are assigned new types
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ofs-delta and ref-delta, which is only valid in a pack file.
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Both ofs-delta and ref-delta store the "delta" to be applied to
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another object (called 'base object') to reconstruct the object. The
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difference between them is, ref-delta directly encodes base object
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name. If the base object is in the same pack, ofs-delta encodes
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the offset of the base object in the pack instead.
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The base object could also be deltified if it's in the same pack.
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Ref-delta can also refer to an object outside the pack (i.e. the
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so-called "thin pack"). When stored on disk however, the pack should
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be self contained to avoid cyclic dependency.
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The delta data starts with the size of the base object and the
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size of the object to be reconstructed. These sizes are
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encoded using the size encoding from above. The remainder of
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the delta data is a sequence of instructions to reconstruct the object
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from the base object. If the base object is deltified, it must be
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converted to canonical form first. Each instruction appends more and
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more data to the target object until it's complete. There are two
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supported instructions so far: one for copy a byte range from the
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source object and one for inserting new data embedded in the
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instruction itself.
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Each instruction has variable length. Instruction type is determined
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by the seventh bit of the first octet. The following diagrams follow
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the convention in RFC 1951 (Deflate compressed data format).
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==== Instruction to copy from base object
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+----------+---------+---------+---------+---------+-------+-------+-------+
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| 1xxxxxxx | offset1 | offset2 | offset3 | offset4 | size1 | size2 | size3 |
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+----------+---------+---------+---------+---------+-------+-------+-------+
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This is the instruction format to copy a byte range from the source
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object. It encodes the offset to copy from and the number of bytes to
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copy. Offset and size are in little-endian order.
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All offset and size bytes are optional. This is to reduce the
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instruction size when encoding small offsets or sizes. The first seven
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bits in the first octet determines which of the next seven octets is
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present. If bit zero is set, offset1 is present. If bit one is set
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offset2 is present and so on.
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Note that a more compact instruction does not change offset and size
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encoding. For example, if only offset2 is omitted like below, offset3
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still contains bits 16-23. It does not become offset2 and contains
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bits 8-15 even if it's right next to offset1.
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+----------+---------+---------+
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| 10000101 | offset1 | offset3 |
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+----------+---------+---------+
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In its most compact form, this instruction only takes up one byte
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(0x80) with both offset and size omitted, which will have default
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values zero. There is another exception: size zero is automatically
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converted to 0x10000.
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==== Instruction to add new data
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+----------+============+
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| 0xxxxxxx | data |
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+----------+============+
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This is the instruction to construct target object without the base
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object. The following data is appended to the target object. The first
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seven bits of the first octet determines the size of data in
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bytes. The size must be non-zero.
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==== Reserved instruction
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+----------+============
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| 00000000 |
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+----------+============
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This is the instruction reserved for future expansion.
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== Original (version 1) pack-*.idx files have the following format:
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- The header consists of 256 4-byte network byte order
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integers. N-th entry of this table records the number of
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objects in the corresponding pack, the first byte of whose
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object name is less than or equal to N. This is called the
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'first-level fan-out' table.
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- The header is followed by sorted 24-byte entries, one entry
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per object in the pack. Each entry is:
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4-byte network byte order integer, recording where the
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object is stored in the packfile as the offset from the
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beginning.
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one object name of the appropriate size.
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- The file is concluded with a trailer:
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A copy of the pack checksum at the end of the corresponding
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packfile.
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Index checksum of all of the above.
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Pack Idx file:
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-- +--------------------------------+
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fanout | fanout[0] = 2 (for example) |-.
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table +--------------------------------+ |
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| fanout[1] | |
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+--------------------------------+ |
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| fanout[2] | |
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
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| fanout[255] = total objects |---.
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-- +--------------------------------+ | |
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main | offset | | |
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index | object name 00XXXXXXXXXXXXXXXX | | |
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table +--------------------------------+ | |
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| offset | | |
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| object name 00XXXXXXXXXXXXXXXX | | |
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+--------------------------------+<+ |
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.-| offset | |
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| | object name 01XXXXXXXXXXXXXXXX | |
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| +--------------------------------+ |
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| | offset | |
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| | object name 01XXXXXXXXXXXXXXXX | |
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| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
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| | offset | |
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| | object name FFXXXXXXXXXXXXXXXX | |
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--| +--------------------------------+<--+
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trailer | | packfile checksum |
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| +--------------------------------+
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| | idxfile checksum |
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| +--------------------------------+
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.-------.
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Pack file entry: <+
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packed object header:
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1-byte size extension bit (MSB)
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type (next 3 bit)
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size0 (lower 4-bit)
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n-byte sizeN (as long as MSB is set, each 7-bit)
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size0..sizeN form 4+7+7+..+7 bit integer, size0
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is the least significant part, and sizeN is the
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most significant part.
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packed object data:
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If it is not DELTA, then deflated bytes (the size above
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is the size before compression).
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If it is REF_DELTA, then
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base object name (the size above is the
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size of the delta data that follows).
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delta data, deflated.
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If it is OFS_DELTA, then
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n-byte offset (see below) interpreted as a negative
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offset from the type-byte of the header of the
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ofs-delta entry (the size above is the size of
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the delta data that follows).
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delta data, deflated.
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offset encoding:
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n bytes with MSB set in all but the last one.
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The offset is then the number constructed by
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concatenating the lower 7 bit of each byte, and
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for n >= 2 adding 2^7 + 2^14 + ... + 2^(7*(n-1))
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to the result.
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== Version 2 pack-*.idx files support packs larger than 4 GiB, and
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have some other reorganizations. They have the format:
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- A 4-byte magic number '\377tOc' which is an unreasonable
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fanout[0] value.
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- A 4-byte version number (= 2)
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- A 256-entry fan-out table just like v1.
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- A table of sorted object names. These are packed together
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without offset values to reduce the cache footprint of the
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binary search for a specific object name.
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- A table of 4-byte CRC32 values of the packed object data.
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This is new in v2 so compressed data can be copied directly
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from pack to pack during repacking without undetected
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data corruption.
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- A table of 4-byte offset values (in network byte order).
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These are usually 31-bit pack file offsets, but large
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offsets are encoded as an index into the next table with
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the msbit set.
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- A table of 8-byte offset entries (empty for pack files less
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than 2 GiB). Pack files are organized with heavily used
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objects toward the front, so most object references should
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not need to refer to this table.
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- The same trailer as a v1 pack file:
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A copy of the pack checksum at the end of
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corresponding packfile.
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Index checksum of all of the above.
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== pack-*.rev files have the format:
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- A 4-byte magic number '0x52494458' ('RIDX').
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- A 4-byte version identifier (= 1).
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- A 4-byte hash function identifier (= 1 for SHA-1, 2 for SHA-256).
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- A table of index positions (one per packed object, num_objects in
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total, each a 4-byte unsigned integer in network order), sorted by
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their corresponding offsets in the packfile.
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- A trailer, containing a:
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checksum of the corresponding packfile, and
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a checksum of all of the above.
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All 4-byte numbers are in network order.
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== pack-*.mtimes files have the format:
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All 4-byte numbers are in network byte order.
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- A 4-byte magic number '0x4d544d45' ('MTME').
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- A 4-byte version identifier (= 1).
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- A 4-byte hash function identifier (= 1 for SHA-1, 2 for SHA-256).
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- A table of 4-byte unsigned integers. The ith value is the
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modification time (mtime) of the ith object in the corresponding
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pack by lexicographic (index) order. The mtimes count standard
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epoch seconds.
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- A trailer, containing a checksum of the corresponding packfile,
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and a checksum of all of the above (each having length according
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to the specified hash function).
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== multi-pack-index (MIDX) files have the following format:
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The multi-pack-index files refer to multiple pack-files and loose objects.
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In order to allow extensions that add extra data to the MIDX, we organize
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the body into "chunks" and provide a lookup table at the beginning of the
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body. The header includes certain length values, such as the number of packs,
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the number of base MIDX files, hash lengths and types.
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All 4-byte numbers are in network order.
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HEADER:
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4-byte signature:
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The signature is: {'M', 'I', 'D', 'X'}
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1-byte version number:
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Git only writes or recognizes version 1.
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1-byte Object Id Version
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We infer the length of object IDs (OIDs) from this value:
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1 => SHA-1
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2 => SHA-256
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If the hash type does not match the repository's hash algorithm,
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the multi-pack-index file should be ignored with a warning
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presented to the user.
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1-byte number of "chunks"
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1-byte number of base multi-pack-index files:
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This value is currently always zero.
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4-byte number of pack files
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CHUNK LOOKUP:
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(C + 1) * 12 bytes providing the chunk offsets:
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First 4 bytes describe chunk id. Value 0 is a terminating label.
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Other 8 bytes provide offset in current file for chunk to start.
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(Chunks are provided in file-order, so you can infer the length
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using the next chunk position if necessary.)
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The CHUNK LOOKUP matches the table of contents from
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the chunk-based file format, see linkgit:gitformat-chunk[5].
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The remaining data in the body is described one chunk at a time, and
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these chunks may be given in any order. Chunks are required unless
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otherwise specified.
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CHUNK DATA:
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Packfile Names (ID: {'P', 'N', 'A', 'M'})
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Stores the packfile names as concatenated, null-terminated strings.
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Packfiles must be listed in lexicographic order for fast lookups by
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name. This is the only chunk not guaranteed to be a multiple of four
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bytes in length, so should be the last chunk for alignment reasons.
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OID Fanout (ID: {'O', 'I', 'D', 'F'})
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The ith entry, F[i], stores the number of OIDs with first
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byte at most i. Thus F[255] stores the total
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number of objects.
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OID Lookup (ID: {'O', 'I', 'D', 'L'})
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The OIDs for all objects in the MIDX are stored in lexicographic
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order in this chunk.
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Object Offsets (ID: {'O', 'O', 'F', 'F'})
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Stores two 4-byte values for every object.
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1: The pack-int-id for the pack storing this object.
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2: The offset within the pack.
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If all offsets are less than 2^32, then the large offset chunk
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will not exist and offsets are stored as in IDX v1.
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If there is at least one offset value larger than 2^32-1, then
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the large offset chunk must exist, and offsets larger than
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2^31-1 must be stored in it instead. If the large offset chunk
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exists and the 31st bit is on, then removing that bit reveals
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the row in the large offsets containing the 8-byte offset of
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this object.
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[Optional] Object Large Offsets (ID: {'L', 'O', 'F', 'F'})
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8-byte offsets into large packfiles.
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[Optional] Bitmap pack order (ID: {'R', 'I', 'D', 'X'})
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A list of MIDX positions (one per object in the MIDX, num_objects in
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total, each a 4-byte unsigned integer in network byte order), sorted
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according to their relative bitmap/pseudo-pack positions.
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TRAILER:
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Index checksum of the above contents.
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== multi-pack-index reverse indexes
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Similar to the pack-based reverse index, the multi-pack index can also
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be used to generate a reverse index.
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Instead of mapping between offset, pack-, and index position, this
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reverse index maps between an object's position within the MIDX, and
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that object's position within a pseudo-pack that the MIDX describes
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(i.e., the ith entry of the multi-pack reverse index holds the MIDX
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position of ith object in pseudo-pack order).
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To clarify the difference between these orderings, consider a multi-pack
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reachability bitmap (which does not yet exist, but is what we are
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building towards here). Each bit needs to correspond to an object in the
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MIDX, and so we need an efficient mapping from bit position to MIDX
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position.
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One solution is to let bits occupy the same position in the oid-sorted
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index stored by the MIDX. But because oids are effectively random, their
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resulting reachability bitmaps would have no locality, and thus compress
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poorly. (This is the reason that single-pack bitmaps use the pack
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ordering, and not the .idx ordering, for the same purpose.)
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So we'd like to define an ordering for the whole MIDX based around
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pack ordering, which has far better locality (and thus compresses more
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efficiently). We can think of a pseudo-pack created by the concatenation
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of all of the packs in the MIDX. E.g., if we had a MIDX with three packs
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(a, b, c), with 10, 15, and 20 objects respectively, we can imagine an
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ordering of the objects like:
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|a,0|a,1|...|a,9|b,0|b,1|...|b,14|c,0|c,1|...|c,19|
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where the ordering of the packs is defined by the MIDX's pack list,
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and then the ordering of objects within each pack is the same as the
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order in the actual packfile.
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Given the list of packs and their counts of objects, you can
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naïvely reconstruct that pseudo-pack ordering (e.g., the object at
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position 27 must be (c,1) because packs "a" and "b" consumed 25 of the
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slots). But there's a catch. Objects may be duplicated between packs, in
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which case the MIDX only stores one pointer to the object (and thus we'd
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want only one slot in the bitmap).
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Callers could handle duplicates themselves by reading objects in order
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of their bit-position, but that's linear in the number of objects, and
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much too expensive for ordinary bitmap lookups. Building a reverse index
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solves this, since it is the logical inverse of the index, and that
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index has already removed duplicates. But, building a reverse index on
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the fly can be expensive. Since we already have an on-disk format for
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pack-based reverse indexes, let's reuse it for the MIDX's pseudo-pack,
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too.
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Objects from the MIDX are ordered as follows to string together the
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pseudo-pack. Let `pack(o)` return the pack from which `o` was selected
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by the MIDX, and define an ordering of packs based on their numeric ID
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(as stored by the MIDX). Let `offset(o)` return the object offset of `o`
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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.
|
|
|
|
== 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 linkgit: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 linkgit: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.
|
|
linkgit: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 linkgit: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 linkgit:git-gc[1] to expire those objects
|
|
later on.
|
|
|
|
It is linkgit:git-gc[1] that is typically responsible for removing expired
|
|
unreachable objects.
|
|
|
|
=== Caution for mixed-version environments
|
|
|
|
Repositories that have cruft packs in them will continue to work with any older
|
|
version of Git. Note, however, that previous versions of Git which do not
|
|
understand the `.mtimes` file will use the cruft pack's mtime as the mtime for
|
|
all of the objects in it. In other words, do not expect older (pre-cruft pack)
|
|
versions of Git to interpret or even read the contents of the `.mtimes` file.
|
|
|
|
Note that having mixed versions of Git GC-ing the same repository can lead to
|
|
unreachable objects never being completely pruned. This can happen under the
|
|
following circumstances:
|
|
|
|
- An older version of Git running GC explodes the contents of an existing
|
|
cruft pack loose, using the cruft pack's mtime.
|
|
- A newer version running GC collects those loose objects into a cruft pack,
|
|
where the .mtime file reflects the loose object's actual mtimes, but the
|
|
cruft pack mtime is "now".
|
|
|
|
Repeating this process will lead to unreachable objects not getting pruned as a
|
|
result of repeatedly resetting the objects' mtimes to the present time.
|
|
|
|
If you are GC-ing repositories in a mixed version environment, consider omitting
|
|
the `--cruft` option when using linkgit:git-repack[1] and linkgit:git-gc[1], and
|
|
leaving the `gc.cruftPacks` configuration unset until all writers understand
|
|
cruft packs.
|
|
|
|
=== Alternatives
|
|
|
|
Notable alternatives to this design include:
|
|
|
|
- The location of the per-object mtime data, and
|
|
- Storing unreachable objects in multiple cruft packs.
|
|
|
|
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.
|
|
|
|
Storing unreachable objects among multiple cruft packs (e.g., creating a new
|
|
cruft pack during each repacking operation including only unreachable objects
|
|
which aren't already stored in an earlier cruft pack) is significantly more
|
|
complicated to construct, and so aren't pursued here. The obvious drawback to
|
|
the current implementation is that the entire cruft pack must be re-written from
|
|
scratch.
|
|
|
|
GIT
|
|
---
|
|
Part of the linkgit:git[1] suite
|