Git - pack-format Documentation

pack-format last updated in 2.34.1

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

The trailer records 20-byte SHA-1 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.

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 20-byte 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 is a sequence of instructions to reconstruct an 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 copy 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 determines 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 target object without the base object. The following data is appended to the target object. The first seven bits of the first octet determines 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.

20-byte object name.

The file is concluded with a trailer:

A copy of the 20-byte SHA-1 checksum at the end of
corresponding packfile.

20-byte SHA-1-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
  20-byte base object name SHA-1 (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 20-byte SHA-1 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 20-byte SHA-1 checksum at the end of
corresponding packfile.

20-byte SHA-1-checksum of all of the above.

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