Documentation/technical/multi-pack-index.txt - jrn/git - Git at Google

 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
     linkgit: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.

 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)
	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
	linkgit: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.

	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)