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.
- An optional list of objects in pseudo-pack order (used with MIDX bitmaps).

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 (which is the default) to consume MIDX files. Setting it to false prevents Git from reading a MIDX file, even if one exists.
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.

Incremental multi-pack indexes

As repositories grow in size, it becomes more expensive to write a multi-pack index (MIDX) that includes all packfiles. To accommodate this, the "incremental multi-pack indexes" feature allows for combining a "chain" of multi-pack indexes.

Each individual component of the chain need only contain a small number of packfiles. Appending to the chain does not invalidate earlier parts of the chain, so repositories can control how much time is spent updating the MIDX chain by determining the number of packs in each layer of the MIDX chain.

Design state

At present, the incremental multi-pack indexes feature is missing two important components:

The ability to rewrite earlier portions of the MIDX chain (i.e., to "compact" some collection of adjacent MIDX layers into a single MIDX). At present the only supported way of shrinking a MIDX chain is to rewrite the entire chain from scratch without the --split flag.

There are no fundamental limitations that stand in the way of being able to implement this feature. It is omitted from the initial implementation in order to reduce the complexity, but will be added later.
Support for reachability bitmaps. The classic single MIDX implementation does support reachability bitmaps (see the section titled "multi-pack-index reverse indexes" in gitformat-pack(5) for more details).

As above, there are no fundamental limitations that stand in the way of extending the incremental MIDX format to support reachability bitmaps. The design below specifically takes this into account, and support for reachability bitmaps will be added in a future patch series. It is omitted from the current implementation for the same reason as above.

In brief, to support reachability bitmaps with the incremental MIDX feature, the concept of the pseudo-pack order is extended across each layer of the incremental MIDX chain to form a concatenated pseudo-pack order. This concatenation takes place in the same order as the chain itself (in other words, the concatenated pseudo-pack order for a chain {$H1, $H2, $H3} would be the pseudo-pack order for $H1, followed by the pseudo-pack order for $H2, followed by the pseudo-pack order for $H3).

The layout will then be extended so that each layer of the incremental MIDX chain can write a *.bitmap. The objects in each layer’s bitmap are offset by the number of objects in the previous layers of the chain.

File layout

Instead of storing a single multi-pack-index file (with an optional .rev and .bitmap extension) in $GIT_DIR/objects/pack, incremental MIDXs are stored in the following layout:

$GIT_DIR/objects/pack/multi-pack-index.d/
$GIT_DIR/objects/pack/multi-pack-index.d/multi-pack-index-chain
$GIT_DIR/objects/pack/multi-pack-index.d/multi-pack-index-$H1.midx
$GIT_DIR/objects/pack/multi-pack-index.d/multi-pack-index-$H2.midx
$GIT_DIR/objects/pack/multi-pack-index.d/multi-pack-index-$H3.midx

The multi-pack-index-chain file contains a list of the incremental MIDX files in the chain, in order. The above example shows a chain whose multi-pack-index-chain file would contain the following lines:

$H1
$H2
$H3

The multi-pack-index-$H1.midx file contains the first layer of the multi-pack-index chain. The multi-pack-index-$H2.midx file contains the second layer of the chain, and so on.

When both an incremental- and non-incremental MIDX are present, the non-incremental MIDX is always read first.

Object positions for incremental MIDXs

In the original multi-pack-index design, we refer to objects via their lexicographic position (by object IDs) within the repository’s singular multi-pack-index. In the incremental multi-pack-index design, we refer to objects via their index into a concatenated lexicographic ordering among each component in the MIDX chain.

If objects_nr() is a function that returns the number of objects in a given MIDX layer, then the index of an object at lexicographic position i within, say, $H3 is defined as:

objects_nr($H2) + objects_nr($H1) + i

(in the C implementation, this is often computed as i + m->num_objects_in_base).

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.
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 MIDX bitmaps could be updated independently of the MIDX.
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.