gitformat-commit-graph(5) — Linux manual page



NAME         top

       gitformat-commit-graph - Git commit-graph format

SYNOPSIS         top


DESCRIPTION         top

       The Git commit-graph stores a list of commit OIDs and some
       associated metadata, including:

       •   The generation number of the commit.

       •   The root tree OID.

       •   The commit date.

       •   The parents of the commit, stored using positional references
           within the graph file.

       •   The Bloom filter of the commit carrying the paths that were
           changed between the commit and its first parent, if

       These positional references are stored as unsigned 32-bit
       integers corresponding to the array position within the list of
       commit OIDs. Due to some special constants we use to track
       parents, we can store at most (1 << 30) + (1 << 29) + (1 << 28) -
       1 (around 1.8 billion) commits.


       In order to allow extensions that add extra data to the graph, we
       organize the body into "chunks" and provide a binary lookup table
       at the beginning of the body. The header includes certain values,
       such as number of chunks and hash type.

       All multi-byte numbers are in network byte order.

           4-byte signature:
               The signature is: {'C', 'G', 'P', 'H'}

           1-byte version number:
               Currently, the only valid version is 1.

           1-byte Hash Version
               We infer the hash length (H) from this value:
                 1 => SHA-1
                 2 => SHA-256
               If the hash type does not match the repository's hash algorithm, the
               commit-graph file should be ignored with a warning presented to the

           1-byte number (C) of "chunks"

           1-byte number (B) of base commit-graphs
               We infer the length (H*B) of the Base Graphs chunk
               from this value.

           (C + 1) * 12 bytes listing the table of contents for the chunks:
               First 4 bytes describe the chunk id. Value 0 is a terminating label.
               Other 8 bytes provide the byte-offset in current file for chunk to
               start. (Chunks are ordered contiguously in the file, so you can infer
               the length using the next chunk position if necessary.) Each chunk
               ID appears at most once.

           The CHUNK LOOKUP matches the table of contents from
           the chunk-based file format, see linkgit:gitformat-chunk[5]

           The remaining data in the body is described one chunk at a time, and
           these chunks may be given in any order. Chunks are required unless
           otherwise specified.

       OID Fanout (ID: {O, I, D, F}) (256 * 4 bytes)

               The ith entry, F[i], stores the number of OIDs with first
               byte at most i. Thus F[255] stores the total
               number of commits (N).

       OID Lookup (ID: {O, I, D, L}) (N * H bytes)

               The OIDs for all commits in the graph, sorted in ascending order.

       Commit Data (ID: {C, D, A, T }) (N * (H + 16) bytes)

           •   The first H bytes are for the OID of the root tree.

           •   The next 8 bytes are for the positions of the first two
               parents of the ith commit. Stores value 0x70000000 if no
               parent in that position. If there are more than two
               parents, the second value has its most-significant bit on
               and the other bits store an array position into the Extra
               Edge List chunk.

           •   The next 8 bytes store the topological level (generation
               number v1) of the commit and the commit time in seconds
               since EPOCH. The generation number uses the higher 30
               bits of the first 4 bytes, while the commit time uses the
               32 bits of the second 4 bytes, along with the lowest 2
               bits of the lowest byte, storing the 33rd and 34th bit of
               the commit time.

       Generation Data (ID: {G, D, A, 2 }) (N * 4 bytes) [Optional]

           •   This list of 4-byte values store corrected commit date
               offsets for the commits, arranged in the same order as
               commit data chunk.

           •   If the corrected commit date offset cannot be stored
               within 31 bits, the value has its most-significant bit on
               and the other bits store the position of corrected commit
               date into the Generation Data Overflow chunk.

           •   Generation Data chunk is present only when commit-graph
               file is written by compatible versions of Git and in case
               of split commit-graph chains, the topmost layer also has
               Generation Data chunk.

       Generation Data Overflow (ID: {G, D, O, 2 }) [Optional]

           •   This list of 8-byte values stores the corrected commit
               date offsets for commits with corrected commit date
               offsets that cannot be stored within 31 bits.

           •   Generation Data Overflow chunk is present only when
               Generation Data chunk is present and atleast one
               corrected commit date offset cannot be stored within 31

       Extra Edge List (ID: {E, D, G, E}) [Optional]

               This list of 4-byte values store the second through nth parents for
               all octopus merges. The second parent value in the commit data stores
               an array position within this list along with the most-significant bit
               on. Starting at that array position, iterate through this list of commit
               positions for the parents until reaching a value with the most-significant
               bit on. The other bits correspond to the position of the last parent.

       Bloom Filter Index (ID: {B, I, D, X}) (N * 4 bytes) [Optional]

           •   The ith entry, BIDX[i], stores the number of bytes in all
               Bloom filters from commit 0 to commit i (inclusive) in
               lexicographic order. The Bloom filter for the i-th commit
               spans from BIDX[i-1] to BIDX[i] (plus header length),
               where BIDX[-1] is 0.

           •   The BIDX chunk is ignored if the BDAT chunk is not

       Bloom Filter Data (ID: {B, D, A, T}) [Optional]

           •   It starts with header consisting of three unsigned 32-bit

               •   Version of the hash algorithm being used. We
                   currently only support value 1 which corresponds to
                   the 32-bit version of the murmur3 hash implemented
                   exactly as described in
                   and the double hashing technique using seed values
                   0x293ae76f and 0x7e646e2 as described in
                   Filters in Probabilistic Verification"

               •   The number of times a path is hashed and hence the
                   number of bit positions that cumulatively determine
                   whether a file is present in the commit.

               •   The minimum number of bits b per entry in the Bloom
                   filter. If the filter contains n entries, then the
                   filter size is the minimum number of 64-bit words
                   that contain n*b bits.

           •   The rest of the chunk is the concatenation of all the
               computed Bloom filters for the commits in lexicographic

           •   Note: Commits with no changes or more than 512 changes
               have Bloom filters of length one, with either all bits
               set to zero or one respectively.

           •   The BDAT chunk is present if and only if BIDX is present.

       Base Graphs List (ID: {B, A, S, E}) [Optional]

               This list of H-byte hashes describe a set of B commit-graph files that
               form a commit-graph chain. The graph position for the ith commit in this
               file's OID Lookup chunk is equal to i plus the number of commits in all
               base graphs.  If B is non-zero, this chunk must exist.

           H-byte HASH-checksum of all of the above.


       The Generation Data (GDA2) and Generation Data Overflow (GDO2)
       chunks have the number 2 in their chunk IDs because a previous
       version of Git wrote possibly erroneous data in these chunks with
       the IDs "GDAT" and "GDOV". By changing the IDs, newer versions of
       Git will silently ignore those older chunks and write the new
       information without trusting the incorrect data.

GIT         top

       Part of the git(1) suite

COLOPHON         top

       This page is part of the git (Git distributed version control
       system) project.  Information about the project can be found at
       ⟨⟩.  If you have a bug report for this manual
       page, see ⟨⟩.  This page was obtained
       from the project's upstream Git repository
       ⟨⟩ on 2024-06-14.  (At that time,
       the date of the most recent commit that was found in the
       repository was 2024-06-12.)  If you discover any rendering
       problems in this HTML version of the page, or you believe there
       is a better or more up-to-date source for the page, or you have
       corrections or improvements to the information in this COLOPHON
       (which is not part of the original manual page), send a mail to

Git         2024-06-12      GITFORMAT-COMMIT-GRAPH(5)

Pages that refer to this page: git(1)git-commit-graph(1)gitformat-chunk(5)