Documentation/technical/pack-format.txt

   1 GIT pack format
   2 ===============
   3
   4 = pack-*.pack file has the following format:
   5
   6    - The 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 = pack-*.idx file has 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 are smaller than N.  This is called the
  43     'first-level fan-out' table.
  44
  45     Observation: we would need to extend this to an array of
  46     8-byte integers to go beyond 4G objects per pack, but it is
  47     not strictly necessary.
  48
  49   - The header is followed by sorted 24-byte entries, one entry
  50     per object in the pack.  Each entry is:
  51
  52     4-byte network byte order integer, recording where the
  53     object is stored in the packfile as the offset from the
  54     beginning.
  55
  56     20-byte object name.
  57
  58     Observation: we would definitely need to extend this to
  59     8-byte integer plus 20-byte object name to handle a packfile
  60     that is larger than 4GB.
  61
  62   - The file is concluded with a trailer:
  63
  64     A copy of the 20-byte SHA1 checksum at the end of
  65     corresponding packfile.
  66
  67     20-byte SHA1-checksum of all of the above.
  68
  69 Pack Idx file:
  70
  71         idx
  72             +--------------------------------+
  73             | fanout[0] = 2                  |-.
  74             +--------------------------------+ |
  75             | fanout[1]                      | |
  76             +--------------------------------+ |
  77             | fanout[2]                      | |
  78             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
  79             | fanout[255]                    | |
  80             +--------------------------------+ |
  81 main        | offset                         | |
  82 index       | object name 00XXXXXXXXXXXXXXXX | |
  83 table       +--------------------------------+ |
  84             | offset                         | |
  85             | object name 00XXXXXXXXXXXXXXXX | |
  86             +--------------------------------+ |
  87           .-| offset                         |<+
  88           | | object name 01XXXXXXXXXXXXXXXX |
  89           | +--------------------------------+
  90           | | offset                         |
  91           | | object name 01XXXXXXXXXXXXXXXX |
  92           | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  93           | | offset                         |
  94           | | object name FFXXXXXXXXXXXXXXXX |
  95           | +--------------------------------+
  96 trailer   | | packfile checksum              |
  97           | +--------------------------------+
  98           | | idxfile checksum               |
  99           | +--------------------------------+
 100           .-------.
 101                   |
 102 Pack file entry: <+
 103
 104      packed object header:
 105         1-byte size extension bit (MSB)
 106                type (next 3 bit)
 107                size0 (lower 4-bit)
 108         n-byte sizeN (as long as MSB is set, each 7-bit)
 109                 size0..sizeN form 4+7+7+..+7 bit integer, size0
 110                 is the least significant part, and sizeN is the
 111                 most significant part.
 112      packed object data:
 113         If it is not DELTA, then deflated bytes (the size above
 114                 is the size before compression).
 115         If it is DELTA, then
 116           20-byte base object name SHA1 (the size above is the
 117                 size of the delta data that follows).
 118           delta data, deflated.