Subject: [PATCH v5 01/16] commit-graph: document commit-graph chains

From: "Derrick Stolee via GitGitGadget" <[email-masked]>
Date: Fri, 07 Jun 2019 11:38:21 -0700 (PDT)

From: Derrick Stolee <[email-masked]>

Add a basic description of commit-graph chains. More details about the
feature will be added as we add functionality. This introduction gives a
high-level overview to the goals of the feature and the basic layout of
commit-graph chains.

Signed-off-by: Derrick Stolee <[email-masked]>
---
 Documentation/technical/commit-graph.txt | 59 ++++++++++++++++++++++++
 1 file changed, 59 insertions(+)

diff --git a/Documentation/technical/commit-graph.txt b/Documentation/technical/commit-graph.txt
index fb53341d5e..1dca3bd8fe 100644
--- a/Documentation/technical/commit-graph.txt
+++ b/Documentation/technical/commit-graph.txt
@@ -127,6 +127,65 @@ Design Details
   helpful for these clones, anyway. The commit-graph will not be read or
   written when shallow commits are present.
 
+Commit Graphs Chains
+--------------------
+
+Typically, repos grow with near-constant velocity (commits per day). Over time,
+the number of commits added by a fetch operation is much smaller than the
+number of commits in the full history. By creating a "chain" of commit-graphs,
+we enable fast writes of new commit data without rewriting the entire commit
+history -- at least, most of the time.
+
+## File Layout
+
+A commit-graph chain uses multiple files, and we use a fixed naming convention
+to organize these files. Each commit-graph file has a name
+`$OBJDIR/info/commit-graphs/graph-{hash}.graph` where `{hash}` is the hex-
+valued hash stored in the footer of that file (which is a hash of the file's
+contents before that hash). For a chain of commit-graph files, a plain-text
+file at `$OBJDIR/info/commit-graphs/commit-graph-chain` contains the
+hashes for the files in order from "lowest" to "highest".
+
+For example, if the `commit-graph-chain` file contains the lines
+
+```
+	{hash0}
+	{hash1}
+	{hash2}
+```
+
+then the commit-graph chain looks like the following diagram:
+
+ +-----------------------+
+ |  graph-{hash2}.graph  |
+ +-----------------------+
+	  |
+ +-----------------------+
+ |                       |
+ |  graph-{hash1}.graph  |
+ |                       |
+ +-----------------------+
+	  |
+ +-----------------------+
+ |                       |
+ |                       |
+ |                       |
+ |  graph-{hash0}.graph  |
+ |                       |
+ |                       |
+ |                       |
+ +-----------------------+
+
+Let X0 be the number of commits in `graph-{hash0}.graph`, X1 be the number of
+commits in `graph-{hash1}.graph`, and X2 be the number of commits in
+`graph-{hash2}.graph`. If a commit appears in position i in `graph-{hash2}.graph`,
+then we interpret this as being the commit in position (X0 + X1 + i), and that
+will be used as its "graph position". The commits in `graph-{hash2}.graph` use these
+positions to refer to their parents, which may be in `graph-{hash1}.graph` or
+`graph-{hash0}.graph`. We can navigate to an arbitrary commit in position j by checking
+its containment in the intervals [0, X0), [X0, X0 + X1), [X0 + X1, X0 + X1 +
+X2).
+
 Related Links
 -------------
 [0] https://bugs.chromium.org/p/git/issues/detail?id=8
-- 
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