technical/multi-pack-index.txt

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 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 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://lore.kernel.org/git/20180107181459.222909-1-dstolee@microsoft.com/
 An earlier RFC for the multi-pack-index feature

[2] https://lore.kernel.org/git/alpine.DEB.2.20.1803091557510.23109@alexmv-linux/
 Git Merge 2018 Contributor's summit notes (includes discussion of MIDX)