Andrew's git - gitweb.git/blob - Documentation/technical/pack-format.txt

   1GIT pack format
   2===============
   3
   4= pack-*.pack files have the following format:
   5
   6   - A header appears at the beginning and consists of the following:
   7
   8     4-byte signature:
   9         The signature is: {'P', 'A', 'C', 'K'}
  10
  11     4-byte version number (network byte order):
  12         GIT currently accepts version number 2 or 3 but
  13         generates version 2 only.
  14
  15     4-byte number of objects contained in the pack (network byte order)
  16
  17     Observation: we cannot have more than 4G versions ;-) and
  18     more than 4G objects in a pack.
  19
  20   - The header is followed by number of object entries, each of
  21     which looks like this:
  22
  23     (undeltified representation)
  24     n-byte type and length (3-bit type, (n-1)*7+4-bit length)
  25     compressed data
  26
  27     (deltified representation)
  28     n-byte type and length (3-bit type, (n-1)*7+4-bit length)
  29     20-byte base object name
  30     compressed delta data
  31
  32     Observation: length of each object is encoded in a variable
  33     length format and is not constrained to 32-bit or anything.
  34
  35  - The trailer records 20-byte SHA1 checksum of all of the above.
  36
  37= Original (version 1) pack-*.idx files have the following format:
  38
  39  - The header consists of 256 4-byte network byte order
  40    integers.  N-th entry of this table records the number of
  41    objects in the corresponding pack, the first byte of whose
  42    object name is less than or equal to N.  This is called the
  43    'first-level fan-out' table.
  44
  45  - The header is followed by sorted 24-byte entries, one entry
  46    per object in the pack.  Each entry is:
  47
  48    4-byte network byte order integer, recording where the
  49    object is stored in the packfile as the offset from the
  50    beginning.
  51
  52    20-byte object name.
  53
  54  - The file is concluded with a trailer:
  55
  56    A copy of the 20-byte SHA1 checksum at the end of
  57    corresponding packfile.
  58
  59    20-byte SHA1-checksum of all of the above.
  60
  61Pack Idx file:
  62
  63        --  +--------------------------------+
  64fanout      | fanout[0] = 2 (for example)    |-.
  65table       +--------------------------------+ |
  66            | fanout[1]                      | |
  67            +--------------------------------+ |
  68            | fanout[2]                      | |
  69            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
  70            | fanout[255] = total objects    |---.
  71        --  +--------------------------------+ | |
  72main        | offset                         | | |
  73index       | object name 00XXXXXXXXXXXXXXXX | | |
  74table       +--------------------------------+ | |
  75            | offset                         | | |
  76            | object name 00XXXXXXXXXXXXXXXX | | |
  77            +--------------------------------+<+ |
  78          .-| offset                         |   |
  79          | | object name 01XXXXXXXXXXXXXXXX |   |
  80          | +--------------------------------+   |
  81          | | offset                         |   |
  82          | | object name 01XXXXXXXXXXXXXXXX |   |
  83          | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~   |
  84          | | offset                         |   |
  85          | | object name FFXXXXXXXXXXXXXXXX |   |
  86        --| +--------------------------------+<--+
  87trailer   | | packfile checksum              |
  88          | +--------------------------------+
  89          | | idxfile checksum               |
  90          | +--------------------------------+
  91          .-------.
  92                  |
  93Pack file entry: <+
  94
  95     packed object header:
  96        1-byte size extension bit (MSB)
  97               type (next 3 bit)
  98               size0 (lower 4-bit)
  99        n-byte sizeN (as long as MSB is set, each 7-bit)
 100                size0..sizeN form 4+7+7+..+7 bit integer, size0
 101                is the least significant part, and sizeN is the
 102                most significant part.
 103     packed object data:
 104        If it is not DELTA, then deflated bytes (the size above
 105                is the size before compression).
 106        If it is DELTA, then
 107          20-byte base object name SHA1 (the size above is the
 108                size of the delta data that follows).
 109          delta data, deflated.
 110
 111
 112= Version 2 pack-*.idx files support packs larger than 4 GiB, and
 113  have some other reorganizations.  They have the format:
 114
 115  - A 4-byte magic number '\377tOc' which is an unreasonable
 116    fanout[0] value.
 117
 118  - A 4-byte version number (= 2)
 119
 120  - A 256-entry fan-out table just like v1.
 121
 122  - A table of sorted 20-byte SHA1 object names.  These are
 123    packed together without offset values to reduce the cache
 124    footprint of the binary search for a specific object name.
 125
 126  - A table of 4-byte CRC32 values of the packed object data.
 127    This is new in v2 so compressed data can be copied directly
 128    from pack to pack during repacking withough undetected
 129    data corruption.
 130
 131  - A table of 4-byte offset values (in network byte order).
 132    These are usually 31-bit pack file offsets, but large
 133    offsets are encoded as an index into the next table with
 134    the msbit set.
 135
 136  - A table of 8-byte offset entries (empty for pack files less
 137    than 2 GiB).  Pack files are organized with heavily used
 138    objects toward the front, so most object references should
 139    not need to refer to this table.
 140
 141  - The same trailer as a v1 pack file:
 142
 143    A copy of the 20-byte SHA1 checksum at the end of
 144    corresponding packfile.
 145
 146    20-byte SHA1-checksum of all of the above.