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diff --git a/third_party/git/Documentation/technical/multi-pack-index.txt b/third_party/git/Documentation/technical/multi-pack-index.txt deleted file mode 100644 index d7e57639f7..0000000000 --- a/third_party/git/Documentation/technical/multi-pack-index.txt +++ /dev/null @@ -1,109 +0,0 @@ -Multi-Pack-Index (MIDX) Design Notes -==================================== - -The Git object directory contains a 'pack' directory containing -packfiles (with suffix ".pack") and pack-indexes (with suffix -".idx"). The pack-indexes provide a way to lookup objects and -navigate to their offset within the pack, but these must come -in pairs with the packfiles. This pairing depends on the file -names, as the pack-index differs only in suffix with its pack- -file. While the pack-indexes provide fast lookup per packfile, -this performance degrades as the number of packfiles increases, -because abbreviations need to inspect every packfile and we are -more likely to have a miss on our most-recently-used packfile. -For some large repositories, repacking into a single packfile -is not feasible due to storage space or excessive repack times. - -The multi-pack-index (MIDX for short) stores a list of objects -and their offsets into multiple packfiles. It contains: - -- A list of packfile names. -- A sorted list of object IDs. -- A list of metadata for the ith object ID including: - - A value j referring to the jth packfile. - - An offset within the jth packfile for the object. -- If large offsets are required, we use another list of large - offsets similar to version 2 pack-indexes. - -Thus, we can provide O(log N) lookup time for any number -of packfiles. - -Design Details --------------- - -- The MIDX is stored in a file named 'multi-pack-index' in the - .git/objects/pack directory. This could be stored in the pack - directory of an alternate. It refers only to packfiles in that - same directory. - -- The pack.multiIndex config setting must be on to consume MIDX files. - -- 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. - -- The MIDX keeps only one record per object ID. If an object appears - in multiple packfiles, then the MIDX selects the copy in the most- - recently modified packfile. - -- If there exist packfiles in the pack directory not registered in - the MIDX, then those packfiles are loaded into the `packed_git` - list and `packed_git_mru` cache. - -- The pack-indexes (.idx files) remain in the pack directory so we - can delete the MIDX file, set core.midx to false, or downgrade - without any loss of information. - -- The MIDX file format uses a chunk-based approach (similar to the - commit-graph file) that allows optional data to be added. - -Future Work ------------ - -- Add a 'verify' subcommand to the 'git midx' builtin to verify the - contents of the multi-pack-index file match the offsets listed in - the corresponding pack-indexes. - -- The multi-pack-index allows many packfiles, especially in a context - where repacking is expensive (such as a very large repo), or - unexpected maintenance time is unacceptable (such as a high-demand - build machine). However, the multi-pack-index needs to be rewritten - in full every time. We can extend the format to be incremental, so - writes are fast. By storing a small "tip" multi-pack-index that - points to large "base" MIDX files, we can keep writes fast while - still reducing the number of binary searches required for object - lookups. - -- The reachability bitmap is currently paired directly with a single - packfile, using the pack-order as the object order to hopefully - compress the bitmaps well using run-length encoding. This could be - extended to pair a reachability bitmap with a multi-pack-index. If - the multi-pack-index is extended to store a "stable object order" - (a function Order(hash) = integer that is constant for a given hash, - even as the multi-pack-index is updated) then a reachability bitmap - could point to a multi-pack-index and be updated independently. - -- Packfiles can be marked as "special" using empty files that share - the initial name but replace ".pack" with ".keep" or ".promisor". - We can add an optional chunk of data to the multi-pack-index that - records flags of information about the packfiles. This allows new - states, such as 'repacked' or 'redeltified', that can help with - pack maintenance in a multi-pack environment. It may also be - helpful to organize packfiles by object type (commit, tree, blob, - etc.) and use this metadata to help that maintenance. - -- The partial clone feature records special "promisor" packs that - may point to objects that are not stored locally, but available - on request to a server. The multi-pack-index does not currently - track these promisor packs. - -Related Links -------------- -[0] https://bugs.chromium.org/p/git/issues/detail?id=6 - Chromium work item for: Multi-Pack Index (MIDX) - -[1] https://public-inbox.org/git/20180107181459.222909-1-dstolee@microsoft.com/ - An earlier RFC for the multi-pack-index feature - -[2] https://public-inbox.org/git/alpine.DEB.2.20.1803091557510.23109@alexmv-linux/ - Git Merge 2018 Contributor's summit notes (includes discussion of MIDX) |