Documentation/technical/pack-format.txt

   1 Git pack format
   2 ===============
   3
   4 == Checksums and object IDs
   5
   6 In a repository using the traditional SHA-1, pack checksums, index checksums,
   7 and object IDs (object names) mentioned below are all computed using SHA-1.
   8 Similarly, in SHA-256 repositories, these values are computed using SHA-256.
   9
  10 == pack-*.pack files have the following format:
  11
  12    - A header appears at the beginning and consists of the following:
  13
  14      4-byte signature:
  15          The signature is: {'P', 'A', 'C', 'K'}
  16
  17      4-byte version number (network byte order):
  18          Git currently accepts version number 2 or 3 but
  19          generates version 2 only.
  20
  21      4-byte number of objects contained in the pack (network byte order)
  22
  23      Observation: we cannot have more than 4G versions ;-) and
  24      more than 4G objects in a pack.
  25
  26    - The header is followed by number of object entries, each of
  27      which looks like this:
  28
  29      (undeltified representation)
  30      n-byte type and length (3-bit type, (n-1)*7+4-bit length)
  31      compressed data
  32
  33      (deltified representation)
  34      n-byte type and length (3-bit type, (n-1)*7+4-bit length)
  35      base object name if OBJ_REF_DELTA or a negative relative
  36          offset from the delta object's position in the pack if this
  37          is an OBJ_OFS_DELTA object
  38      compressed delta data
  39
  40      Observation: length of each object is encoded in a variable
  41      length format and is not constrained to 32-bit or anything.
  42
  43   - The trailer records a pack checksum of all of the above.
  44
  45 === Object types
  46
  47 Valid object types are:
  48
  49 - OBJ_COMMIT (1)
  50 - OBJ_TREE (2)
  51 - OBJ_BLOB (3)
  52 - OBJ_TAG (4)
  53 - OBJ_OFS_DELTA (6)
  54 - OBJ_REF_DELTA (7)
  55
  56 Type 5 is reserved for future expansion. Type 0 is invalid.
  57
  58 === Size encoding
  59
  60 This document uses the following "size encoding" of non-negative
  61 integers: From each byte, the seven least significant bits are
  62 used to form the resulting integer. As long as the most significant
  63 bit is 1, this process continues; the byte with MSB 0 provides the
  64 last seven bits.  The seven-bit chunks are concatenated. Later
  65 values are more significant.
  66
  67 This size encoding should not be confused with the "offset encoding",
  68 which is also used in this document.
  69
  70 === Deltified representation
  71
  72 Conceptually there are only four object types: commit, tree, tag and
  73 blob. However to save space, an object could be stored as a "delta" of
  74 another "base" object. These representations are assigned new types
  75 ofs-delta and ref-delta, which is only valid in a pack file.
  76
  77 Both ofs-delta and ref-delta store the "delta" to be applied to
  78 another object (called 'base object') to reconstruct the object. The
  79 difference between them is, ref-delta directly encodes base object
  80 name. If the base object is in the same pack, ofs-delta encodes
  81 the offset of the base object in the pack instead.
  82
  83 The base object could also be deltified if it's in the same pack.
  84 Ref-delta can also refer to an object outside the pack (i.e. the
  85 so-called "thin pack"). When stored on disk however, the pack should
  86 be self contained to avoid cyclic dependency.
  87
  88 The delta data starts with the size of the base object and the
  89 size of the object to be reconstructed. These sizes are
  90 encoded using the size encoding from above.  The remainder of
  91 the delta data is a sequence of instructions to reconstruct the object
  92 from the base object. If the base object is deltified, it must be
  93 converted to canonical form first. Each instruction appends more and
  94 more data to the target object until it's complete. There are two
  95 supported instructions so far: one for copy a byte range from the
  96 source object and one for inserting new data embedded in the
  97 instruction itself.
  98
  99 Each instruction has variable length. Instruction type is determined
 100 by the seventh bit of the first octet. The following diagrams follow
 101 the convention in RFC 1951 (Deflate compressed data format).
 102
 103 ==== Instruction to copy from base object
 104
 105   +----------+---------+---------+---------+---------+-------+-------+-------+
 106   | 1xxxxxxx | offset1 | offset2 | offset3 | offset4 | size1 | size2 | size3 |
 107   +----------+---------+---------+---------+---------+-------+-------+-------+
 108
 109 This is the instruction format to copy a byte range from the source
 110 object. It encodes the offset to copy from and the number of bytes to
 111 copy. Offset and size are in little-endian order.
 112
 113 All offset and size bytes are optional. This is to reduce the
 114 instruction size when encoding small offsets or sizes. The first seven
 115 bits in the first octet determines which of the next seven octets is
 116 present. If bit zero is set, offset1 is present. If bit one is set
 117 offset2 is present and so on.
 118
 119 Note that a more compact instruction does not change offset and size
 120 encoding. For example, if only offset2 is omitted like below, offset3
 121 still contains bits 16-23. It does not become offset2 and contains
 122 bits 8-15 even if it's right next to offset1.
 123
 124   +----------+---------+---------+
 125   | 10000101 | offset1 | offset3 |
 126   +----------+---------+---------+
 127
 128 In its most compact form, this instruction only takes up one byte
 129 (0x80) with both offset and size omitted, which will have default
 130 values zero. There is another exception: size zero is automatically
 131 converted to 0x10000.
 132
 133 ==== Instruction to add new data
 134
 135   +----------+============+
 136   | 0xxxxxxx |    data    |
 137   +----------+============+
 138
 139 This is the instruction to construct target object without the base
 140 object. The following data is appended to the target object. The first
 141 seven bits of the first octet determines the size of data in
 142 bytes. The size must be non-zero.
 143
 144 ==== Reserved instruction
 145
 146   +----------+============
 147   | 00000000 |
 148   +----------+============
 149
 150 This is the instruction reserved for future expansion.
 151
 152 == Original (version 1) pack-*.idx files have the following format:
 153
 154   - The header consists of 256 4-byte network byte order
 155     integers.  N-th entry of this table records the number of
 156     objects in the corresponding pack, the first byte of whose
 157     object name is less than or equal to N.  This is called the
 158     'first-level fan-out' table.
 159
 160   - The header is followed by sorted 24-byte entries, one entry
 161     per object in the pack.  Each entry is:
 162
 163     4-byte network byte order integer, recording where the
 164     object is stored in the packfile as the offset from the
 165     beginning.
 166
 167     one object name of the appropriate size.
 168
 169   - The file is concluded with a trailer:
 170
 171     A copy of the pack checksum at the end of the corresponding
 172     packfile.
 173
 174     Index checksum of all of the above.
 175
 176 Pack Idx file:
 177
 178         --  +--------------------------------+
 179 fanout      | fanout[0] = 2 (for example)    |-.
 180 table       +--------------------------------+ |
 181             | fanout[1]                      | |
 182             +--------------------------------+ |
 183             | fanout[2]                      | |
 184             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
 185             | fanout[255] = total objects    |---.
 186         --  +--------------------------------+ | |
 187 main        | offset                         | | |
 188 index       | object name 00XXXXXXXXXXXXXXXX | | |
 189 table       +--------------------------------+ | |
 190             | offset                         | | |
 191             | object name 00XXXXXXXXXXXXXXXX | | |
 192             +--------------------------------+<+ |
 193           .-| offset                         |   |
 194           | | object name 01XXXXXXXXXXXXXXXX |   |
 195           | +--------------------------------+   |
 196           | | offset                         |   |
 197           | | object name 01XXXXXXXXXXXXXXXX |   |
 198           | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~   |
 199           | | offset                         |   |
 200           | | object name FFXXXXXXXXXXXXXXXX |   |
 201         --| +--------------------------------+<--+
 202 trailer   | | packfile checksum              |
 203           | +--------------------------------+
 204           | | idxfile checksum               |
 205           | +--------------------------------+
 206           .-------.
 207                   |
 208 Pack file entry: <+
 209
 210      packed object header:
 211         1-byte size extension bit (MSB)
 212                type (next 3 bit)
 213                size0 (lower 4-bit)
 214         n-byte sizeN (as long as MSB is set, each 7-bit)
 215                 size0..sizeN form 4+7+7+..+7 bit integer, size0
 216                 is the least significant part, and sizeN is the
 217                 most significant part.
 218      packed object data:
 219         If it is not DELTA, then deflated bytes (the size above
 220                 is the size before compression).
 221         If it is REF_DELTA, then
 222           base object name (the size above is the
 223                 size of the delta data that follows).
 224           delta data, deflated.
 225         If it is OFS_DELTA, then
 226           n-byte offset (see below) interpreted as a negative
 227                 offset from the type-byte of the header of the
 228                 ofs-delta entry (the size above is the size of
 229                 the delta data that follows).
 230           delta data, deflated.
 231
 232      offset encoding:
 233           n bytes with MSB set in all but the last one.
 234           The offset is then the number constructed by
 235           concatenating the lower 7 bit of each byte, and
 236           for n >= 2 adding 2^7 + 2^14 + ... + 2^(7*(n-1))
 237           to the result.
 238
 239
 240
 241 == Version 2 pack-*.idx files support packs larger than 4 GiB, and
 242    have some other reorganizations.  They have the format:
 243
 244   - A 4-byte magic number '\377tOc' which is an unreasonable
 245     fanout[0] value.
 246
 247   - A 4-byte version number (= 2)
 248
 249   - A 256-entry fan-out table just like v1.
 250
 251   - A table of sorted object names.  These are packed together
 252     without offset values to reduce the cache footprint of the
 253     binary search for a specific object name.
 254
 255   - A table of 4-byte CRC32 values of the packed object data.
 256     This is new in v2 so compressed data can be copied directly
 257     from pack to pack during repacking without undetected
 258     data corruption.
 259
 260   - A table of 4-byte offset values (in network byte order).
 261     These are usually 31-bit pack file offsets, but large
 262     offsets are encoded as an index into the next table with
 263     the msbit set.
 264
 265   - A table of 8-byte offset entries (empty for pack files less
 266     than 2 GiB).  Pack files are organized with heavily used
 267     objects toward the front, so most object references should
 268     not need to refer to this table.
 269
 270   - The same trailer as a v1 pack file:
 271
 272     A copy of the pack checksum at the end of
 273     corresponding packfile.
 274
 275     Index checksum of all of the above.
 276
 277 == multi-pack-index (MIDX) files have the following format:
 278
 279 The multi-pack-index files refer to multiple pack-files and loose objects.
 280
 281 In order to allow extensions that add extra data to the MIDX, we organize
 282 the body into "chunks" and provide a lookup table at the beginning of the
 283 body. The header includes certain length values, such as the number of packs,
 284 the number of base MIDX files, hash lengths and types.
 285
 286 All 4-byte numbers are in network order.
 287
 288 HEADER:
 289
 290         4-byte signature:
 291             The signature is: {'M', 'I', 'D', 'X'}
 292
 293         1-byte version number:
 294             Git only writes or recognizes version 1.
 295
 296         1-byte Object Id Version
 297             We infer the length of object IDs (OIDs) from this value:
 298                 1 => SHA-1
 299                 2 => SHA-256
 300             If the hash type does not match the repository's hash algorithm,
 301             the multi-pack-index file should be ignored with a warning
 302             presented to the user.
 303
 304         1-byte number of "chunks"
 305
 306         1-byte number of base multi-pack-index files:
 307             This value is currently always zero.
 308
 309         4-byte number of pack files
 310
 311 CHUNK LOOKUP:
 312
 313         (C + 1) * 12 bytes providing the chunk offsets:
 314             First 4 bytes describe chunk id. Value 0 is a terminating label.
 315             Other 8 bytes provide offset in current file for chunk to start.
 316             (Chunks are provided in file-order, so you can infer the length
 317             using the next chunk position if necessary.)
 318
 319         The remaining data in the body is described one chunk at a time, and
 320         these chunks may be given in any order. Chunks are required unless
 321         otherwise specified.
 322
 323 CHUNK DATA:
 324
 325         Packfile Names (ID: {'P', 'N', 'A', 'M'})
 326             Stores the packfile names as concatenated, null-terminated strings.
 327             Packfiles must be listed in lexicographic order for fast lookups by
 328             name. This is the only chunk not guaranteed to be a multiple of four
 329             bytes in length, so should be the last chunk for alignment reasons.
 330
 331         OID Fanout (ID: {'O', 'I', 'D', 'F'})
 332             The ith entry, F[i], stores the number of OIDs with first
 333             byte at most i. Thus F[255] stores the total
 334             number of objects.
 335
 336         OID Lookup (ID: {'O', 'I', 'D', 'L'})
 337             The OIDs for all objects in the MIDX are stored in lexicographic
 338             order in this chunk.
 339
 340         Object Offsets (ID: {'O', 'O', 'F', 'F'})
 341             Stores two 4-byte values for every object.
 342             1: The pack-int-id for the pack storing this object.
 343             2: The offset within the pack.
 344                 If all offsets are less than 2^32, then the large offset chunk
 345                 will not exist and offsets are stored as in IDX v1.
 346                 If there is at least one offset value larger than 2^32-1, then
 347                 the large offset chunk must exist, and offsets larger than
 348                 2^31-1 must be stored in it instead. If the large offset chunk
 349                 exists and the 31st bit is on, then removing that bit reveals
 350                 the row in the large offsets containing the 8-byte offset of
 351                 this object.
 352
 353         [Optional] Object Large Offsets (ID: {'L', 'O', 'F', 'F'})
 354             8-byte offsets into large packfiles.
 355
 356 TRAILER:
 357
 358         Index checksum of the above contents.