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
  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 `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
-----------

- 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)