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GITFORMAT-PACK(5) Git Manual GITFORMAT-PACK(5)
gitformat-pack - Git pack format
$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
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
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:
%%%SH%%%
• 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.
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.
• 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.
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).
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.
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.
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.
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.
Part of the git(1) suite
This page is part of the git (Git distributed version control
system) project. Information about the project can be found at
⟨http://git-scm.com/⟩. If you have a bug report for this manual
page, see ⟨http://git-scm.com/community⟩. This page was obtained
from the project's upstream Git repository
⟨https://github.com/git/git.git⟩ on 2023-12-22. (At that time,
the date of the most recent commit that was found in the
repository was 2023-12-20.) If you discover any rendering
problems in this HTML version of the page, or you believe there
is a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
man-pages@man7.org
Git 2.43.0.174.g055bb6 2023-12-20 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)
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HTML rendering created 2023-12-22 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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