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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.
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 core.multiPackIndex 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 preferred packfile, otherwise selecting from 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
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.
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.
 https://bugs.chromium.org/p/git/issues/detail?id=6 Chromium work item for: Multi-Pack Index (MIDX)
 https://firstname.lastname@example.org/ An earlier RFC for the multi-pack-index feature
 https://lore.kernel.org/git/alpine.DEB.2.20.1803091557510.23109@alexmv-linux/ Git Merge 2018 Contributor’s summit notes (includes discussion of MIDX)