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Diffstat (limited to 'Documentation/technical/commit-graph.txt')
| -rw-r--r-- | Documentation/technical/commit-graph.txt | 39 |
1 files changed, 17 insertions, 22 deletions
diff --git a/Documentation/technical/commit-graph.txt b/Documentation/technical/commit-graph.txt index c664acbd76..fb53341d5e 100644 --- a/Documentation/technical/commit-graph.txt +++ b/Documentation/technical/commit-graph.txt @@ -15,13 +15,13 @@ There are two main costs here: 1. Decompressing and parsing commits. 2. Walking the entire graph to satisfy topological order constraints. -The commit graph file is a supplemental data structure that accelerates +The commit-graph file is a supplemental data structure that accelerates commit graph walks. If a user downgrades or disables the 'core.commitGraph' config setting, then the existing ODB is sufficient. The file is stored as "commit-graph" either in the .git/objects/info directory or in the info directory of an alternate. -The commit graph file stores the commit graph structure along with some +The commit-graph file stores the commit graph structure along with some extra metadata to speed up graph walks. By listing commit OIDs in lexi- cographic order, we can identify an integer position for each commit and refer to the parents of a commit using those integer positions. We use @@ -103,7 +103,7 @@ that of a parent. Design Details -------------- -- The commit graph file is stored in a file named 'commit-graph' in the +- The commit-graph file is stored in a file named 'commit-graph' in the .git/objects/info directory. This could be stored in the info directory of an alternate. @@ -112,25 +112,20 @@ Design Details - The file format includes parameters for the object ID hash function, so a future change of hash algorithm does not require a change in format. -Future Work ------------ - -- The commit graph feature currently does not honor commit grafts. This can - be remedied by duplicating or refactoring the current graft logic. - -- After computing and storing generation numbers, we must make graph - walks aware of generation numbers to gain the performance benefits they - enable. This will mostly be accomplished by swapping a commit-date-ordered - priority queue with one ordered by generation number. The following - operations are important candidates: - - - 'log --topo-order' - - 'tag --merged' - -- A server could provide a commit graph file as part of the network protocol - to avoid extra calculations by clients. This feature is only of benefit if - the user is willing to trust the file, because verifying the file is correct - is as hard as computing it from scratch. +- Commit grafts and replace objects can change the shape of the commit + history. The latter can also be enabled/disabled on the fly using + `--no-replace-objects`. This leads to difficultly storing both possible + interpretations of a commit id, especially when computing generation + numbers. The commit-graph will not be read or written when + replace-objects or grafts are present. + +- Shallow clones create grafts of commits by dropping their parents. This + leads the commit-graph to think those commits have generation number 1. + If and when those commits are made unshallow, those generation numbers + become invalid. Since shallow clones are intended to restrict the commit + history to a very small set of commits, the commit-graph feature is less + helpful for these clones, anyway. The commit-graph will not be read or + written when shallow commits are present. Related Links ------------- |