Documentation/technical/pack-format.txt

   1 Git 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 if OBJ_REF_DELTA or a negative relative
  30          offset from the delta object's position in the pack if this
  31          is an OBJ_OFS_DELTA object
  32      compressed delta data
  33
  34      Observation: length of each object is encoded in a variable
  35      length format and is not constrained to 32-bit or anything.
  36
  37   - The trailer records 20-byte SHA-1 checksum of all of the above.
  38
  39 === Object types
  40
  41 Valid object types are:
  42
  43 - OBJ_COMMIT (1)
  44 - OBJ_TREE (2)
  45 - OBJ_BLOB (3)
  46 - OBJ_TAG (4)
  47 - OBJ_OFS_DELTA (6)
  48 - OBJ_REF_DELTA (7)
  49
  50 Type 5 is reserved for future expansion. Type 0 is invalid.
  51
  52 === Deltified representation
  53
  54 Conceptually there are only four object types: commit, tree, tag and
  55 blob. However to save space, an object could be stored as a "delta" of
  56 another "base" object. These representations are assigned new types
  57 ofs-delta and ref-delta, which is only valid in a pack file.
  58
  59 Both ofs-delta and ref-delta store the "delta" to be applied to
  60 another object (called 'base object') to reconstruct the object. The
  61 difference between them is, ref-delta directly encodes 20-byte base
  62 object name. If the base object is in the same pack, ofs-delta encodes
  63 the offset of the base object in the pack instead.
  64
  65 The base object could also be deltified if it's in the same pack.
  66 Ref-delta can also refer to an object outside the pack (i.e. the
  67 so-called "thin pack"). When stored on disk however, the pack should
  68 be self contained to avoid cyclic dependency.
  69
  70 The delta data is a sequence of instructions to reconstruct an object
  71 from the base object. If the base object is deltified, it must be
  72 converted to canonical form first. Each instruction appends more and
  73 more data to the target object until it's complete. There are two
  74 supported instructions so far: one for copy a byte range from the
  75 source object and one for inserting new data embedded in the
  76 instruction itself.
  77
  78 Each instruction has variable length. Instruction type is determined
  79 by the seventh bit of the first octet. The following diagrams follow
  80 the convention in RFC 1951 (Deflate compressed data format).
  81
  82 ==== Instruction to copy from base object
  83
  84   +----------+---------+---------+---------+---------+-------+-------+-------+
  85   | 1xxxxxxx | offset1 | offset2 | offset3 | offset4 | size1 | size2 | size3 |
  86   +----------+---------+---------+---------+---------+-------+-------+-------+
  87
  88 This is the instruction format to copy a byte range from the source
  89 object. It encodes the offset to copy from and the number of bytes to
  90 copy. Offset and size are in little-endian order.
  91
  92 All offset and size bytes are optional. This is to reduce the
  93 instruction size when encoding small offsets or sizes. The first seven
  94 bits in the first octet determines which of the next seven octets is
  95 present. If bit zero is set, offset1 is present. If bit one is set
  96 offset2 is present and so on.
  97
  98 Note that a more compact instruction does not change offset and size
  99 encoding. For example, if only offset2 is omitted like below, offset3
 100 still contains bits 16-23. It does not become offset2 and contains
 101 bits 8-15 even if it's right next to offset1.
 102
 103   +----------+---------+---------+
 104   | 10000101 | offset1 | offset3 |
 105   +----------+---------+---------+
 106
 107 In its most compact form, this instruction only takes up one byte
 108 (0x80) with both offset and size omitted, which will have default
 109 values zero. There is another exception: size zero is automatically
 110 converted to 0x10000.
 111
 112 ==== Instruction to add new data
 113
 114   +----------+============+
 115   | 0xxxxxxx |    data    |
 116   +----------+============+
 117
 118 This is the instruction to construct target object without the base
 119 object. The following data is appended to the target object. The first
 120 seven bits of the first octet determines the size of data in
 121 bytes. The size must be non-zero.
 122
 123 ==== Reserved instruction
 124
 125   +----------+============
 126   | 00000000 |
 127   +----------+============
 128
 129 This is the instruction reserved for future expansion.
 130
 131 == Original (version 1) pack-*.idx files have the following format:
 132
 133   - The header consists of 256 4-byte network byte order
 134     integers.  N-th entry of this table records the number of
 135     objects in the corresponding pack, the first byte of whose
 136     object name is less than or equal to N.  This is called the
 137     'first-level fan-out' table.
 138
 139   - The header is followed by sorted 24-byte entries, one entry
 140     per object in the pack.  Each entry is:
 141
 142     4-byte network byte order integer, recording where the
 143     object is stored in the packfile as the offset from the
 144     beginning.
 145
 146     20-byte object name.
 147
 148   - The file is concluded with a trailer:
 149
 150     A copy of the 20-byte SHA-1 checksum at the end of
 151     corresponding packfile.
 152
 153     20-byte SHA-1-checksum of all of the above.
 154
 155 Pack Idx file:
 156
 157         --  +--------------------------------+
 158 fanout      | fanout[0] = 2 (for example)    |-.
 159 table       +--------------------------------+ |
 160             | fanout[1]                      | |
 161             +--------------------------------+ |
 162             | fanout[2]                      | |
 163             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
 164             | fanout[255] = total objects    |---.
 165         --  +--------------------------------+ | |
 166 main        | offset                         | | |
 167 index       | object name 00XXXXXXXXXXXXXXXX | | |
 168 table       +--------------------------------+ | |
 169             | offset                         | | |
 170             | object name 00XXXXXXXXXXXXXXXX | | |
 171             +--------------------------------+<+ |
 172           .-| offset                         |   |
 173           | | object name 01XXXXXXXXXXXXXXXX |   |
 174           | +--------------------------------+   |
 175           | | offset                         |   |
 176           | | object name 01XXXXXXXXXXXXXXXX |   |
 177           | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~   |
 178           | | offset                         |   |
 179           | | object name FFXXXXXXXXXXXXXXXX |   |
 180         --| +--------------------------------+<--+
 181 trailer   | | packfile checksum              |
 182           | +--------------------------------+
 183           | | idxfile checksum               |
 184           | +--------------------------------+
 185           .-------.
 186                   |
 187 Pack file entry: <+
 188
 189      packed object header:
 190         1-byte size extension bit (MSB)
 191                type (next 3 bit)
 192                size0 (lower 4-bit)
 193         n-byte sizeN (as long as MSB is set, each 7-bit)
 194                 size0..sizeN form 4+7+7+..+7 bit integer, size0
 195                 is the least significant part, and sizeN is the
 196                 most significant part.
 197      packed object data:
 198         If it is not DELTA, then deflated bytes (the size above
 199                 is the size before compression).
 200         If it is REF_DELTA, then
 201           20-byte base object name SHA-1 (the size above is the
 202                 size of the delta data that follows).
 203           delta data, deflated.
 204         If it is OFS_DELTA, then
 205           n-byte offset (see below) interpreted as a negative
 206                 offset from the type-byte of the header of the
 207                 ofs-delta entry (the size above is the size of
 208                 the delta data that follows).
 209           delta data, deflated.
 210
 211      offset encoding:
 212           n bytes with MSB set in all but the last one.
 213           The offset is then the number constructed by
 214           concatenating the lower 7 bit of each byte, and
 215           for n >= 2 adding 2^7 + 2^14 + ... + 2^(7*(n-1))
 216           to the result.
 217
 218
 219
 220 == Version 2 pack-*.idx files support packs larger than 4 GiB, and
 221    have some other reorganizations.  They have the format:
 222
 223   - A 4-byte magic number '\377tOc' which is an unreasonable
 224     fanout[0] value.
 225
 226   - A 4-byte version number (= 2)
 227
 228   - A 256-entry fan-out table just like v1.
 229
 230   - A table of sorted 20-byte SHA-1 object names.  These are
 231     packed together without offset values to reduce the cache
 232     footprint of the binary search for a specific object name.
 233
 234   - A table of 4-byte CRC32 values of the packed object data.
 235     This is new in v2 so compressed data can be copied directly
 236     from pack to pack during repacking without undetected
 237     data corruption.
 238
 239   - A table of 4-byte offset values (in network byte order).
 240     These are usually 31-bit pack file offsets, but large
 241     offsets are encoded as an index into the next table with
 242     the msbit set.
 243
 244   - A table of 8-byte offset entries (empty for pack files less
 245     than 2 GiB).  Pack files are organized with heavily used
 246     objects toward the front, so most object references should
 247     not need to refer to this table.
 248
 249   - The same trailer as a v1 pack file:
 250
 251     A copy of the 20-byte SHA-1 checksum at the end of
 252     corresponding packfile.
 253
 254     20-byte SHA-1-checksum of all of the above.
 255
 256 == multi-pack-index (MIDX) files have the following format:
 257
 258 The multi-pack-index files refer to multiple pack-files and loose objects.
 259
 260 In order to allow extensions that add extra data to the MIDX, we organize
 261 the body into "chunks" and provide a lookup table at the beginning of the
 262 body. The header includes certain length values, such as the number of packs,
 263 the number of base MIDX files, hash lengths and types.
 264
 265 All 4-byte numbers are in network order.
 266
 267 HEADER:
 268
 269         4-byte signature:
 270             The signature is: {'M', 'I', 'D', 'X'}
 271
 272         1-byte version number:
 273             Git only writes or recognizes version 1.
 274
 275         1-byte Object Id Version
 276             Git only writes or recognizes version 1 (SHA1).
 277
 278         1-byte number of "chunks"
 279
 280         1-byte number of base multi-pack-index files:
 281             This value is currently always zero.
 282
 283         4-byte number of pack files
 284
 285 CHUNK LOOKUP:
 286
 287         (C + 1) * 12 bytes providing the chunk offsets:
 288             First 4 bytes describe chunk id. Value 0 is a terminating label.
 289             Other 8 bytes provide offset in current file for chunk to start.
 290             (Chunks are provided in file-order, so you can infer the length
 291             using the next chunk position if necessary.)
 292
 293         The remaining data in the body is described one chunk at a time, and
 294         these chunks may be given in any order. Chunks are required unless
 295         otherwise specified.
 296
 297 CHUNK DATA:
 298
 299         Packfile Names (ID: {'P', 'N', 'A', 'M'})
 300             Stores the packfile names as concatenated, null-terminated strings.
 301             Packfiles must be listed in lexicographic order for fast lookups by
 302             name. This is the only chunk not guaranteed to be a multiple of four
 303             bytes in length, so should be the last chunk for alignment reasons.
 304
 305         OID Fanout (ID: {'O', 'I', 'D', 'F'})
 306             The ith entry, F[i], stores the number of OIDs with first
 307             byte at most i. Thus F[255] stores the total
 308             number of objects.
 309
 310         OID Lookup (ID: {'O', 'I', 'D', 'L'})
 311             The OIDs for all objects in the MIDX are stored in lexicographic
 312             order in this chunk.
 313
 314         Object Offsets (ID: {'O', 'O', 'F', 'F'})
 315             Stores two 4-byte values for every object.
 316             1: The pack-int-id for the pack storing this object.
 317             2: The offset within the pack.
 318                 If all offsets are less than 2^32, then the large offset chunk
 319                 will not exist and offsets are stored as in IDX v1.
 320                 If there is at least one offset value larger than 2^32-1, then
 321                 the large offset chunk must exist, and offsets larger than
 322                 2^31-1 must be stored in it instead. If the large offset chunk
 323                 exists and the 31st bit is on, then removing that bit reveals
 324                 the row in the large offsets containing the 8-byte offset of
 325                 this object.
 326
 327         [Optional] Object Large Offsets (ID: {'L', 'O', 'F', 'F'})
 328             8-byte offsets into large packfiles.
 329
 330 TRAILER:
 331
 332         20-byte SHA1-checksum of the above contents.