266 lines
11 KiB
Plaintext
266 lines
11 KiB
Plaintext
Multi-Pack-Index (MIDX) Design Notes
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====================================
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The Git object directory contains a 'pack' directory containing
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packfiles (with suffix ".pack") and pack-indexes (with suffix
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".idx"). The pack-indexes provide a way to lookup objects and
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navigate to their offset within the pack, but these must come
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in pairs with the packfiles. This pairing depends on the file
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names, as the pack-index differs only in suffix with its pack-
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file. While the pack-indexes provide fast lookup per packfile,
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this performance degrades as the number of packfiles increases,
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because abbreviations need to inspect every packfile and we are
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more likely to have a miss on our most-recently-used packfile.
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For some large repositories, repacking into a single packfile
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is not feasible due to storage space or excessive repack times.
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The multi-pack-index (MIDX for short) stores a list of objects
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and their offsets into multiple packfiles. It contains:
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* A list of packfile names.
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* A sorted list of object IDs.
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* A list of metadata for the ith object ID including:
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** A value j referring to the jth packfile.
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** An offset within the jth packfile for the object.
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* If large offsets are required, we use another list of large
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offsets similar to version 2 pack-indexes.
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- An optional list of objects in pseudo-pack order (used with MIDX bitmaps).
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Thus, we can provide O(log N) lookup time for any number
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of packfiles.
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Design Details
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--------------
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- The MIDX is stored in a file named 'multi-pack-index' in the
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.git/objects/pack directory. This could be stored in the pack
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directory of an alternate. It refers only to packfiles in that
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same directory.
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- The core.multiPackIndex config setting must be on (which is the
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default) to consume MIDX files. Setting it to `false` prevents
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Git from reading a MIDX file, even if one exists.
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- The file format includes parameters for the object ID hash
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function, so a future change of hash algorithm does not require
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a change in format.
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- The MIDX keeps only one record per object ID. If an object appears
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in multiple packfiles, then the MIDX selects the copy in the
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preferred packfile, otherwise selecting from the most-recently
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modified packfile.
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- If there exist packfiles in the pack directory not registered in
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the MIDX, then those packfiles are loaded into the `packed_git`
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list and `packed_git_mru` cache.
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- The pack-indexes (.idx files) remain in the pack directory so we
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can delete the MIDX file, set core.midx to false, or downgrade
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without any loss of information.
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- The MIDX file format uses a chunk-based approach (similar to the
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commit-graph file) that allows optional data to be added.
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Incremental multi-pack indexes
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------------------------------
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As repositories grow in size, it becomes more expensive to write a
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multi-pack index (MIDX) that includes all packfiles. To accommodate
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this, the "incremental multi-pack indexes" feature allows for combining
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a "chain" of multi-pack indexes.
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Each individual component of the chain need only contain a small number
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of packfiles. Appending to the chain does not invalidate earlier parts
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of the chain, so repositories can control how much time is spent
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updating the MIDX chain by determining the number of packs in each layer
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of the MIDX chain.
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=== Design state
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At present, the incremental multi-pack indexes feature is missing two
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important components:
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- The ability to rewrite earlier portions of the MIDX chain (i.e., to
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"compact" some collection of adjacent MIDX layers into a single
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MIDX). At present the only supported way of shrinking a MIDX chain
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is to rewrite the entire chain from scratch without the `--split`
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flag.
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+
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There are no fundamental limitations that stand in the way of being able
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to implement this feature. It is omitted from the initial implementation
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in order to reduce the complexity, but will be added later.
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- Support for reachability bitmaps. The classic single MIDX
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implementation does support reachability bitmaps (see the section
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titled "multi-pack-index reverse indexes" in
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linkgit:gitformat-pack[5] for more details).
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+
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As above, there are no fundamental limitations that stand in the way of
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extending the incremental MIDX format to support reachability bitmaps.
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The design below specifically takes this into account, and support for
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reachability bitmaps will be added in a future patch series. It is
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omitted from the current implementation for the same reason as above.
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+
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In brief, to support reachability bitmaps with the incremental MIDX
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feature, the concept of the pseudo-pack order is extended across each
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layer of the incremental MIDX chain to form a concatenated pseudo-pack
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order. This concatenation takes place in the same order as the chain
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itself (in other words, the concatenated pseudo-pack order for a chain
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`{$H1, $H2, $H3}` would be the pseudo-pack order for `$H1`, followed by
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the pseudo-pack order for `$H2`, followed by the pseudo-pack order for
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`$H3`).
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+
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The layout will then be extended so that each layer of the incremental
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MIDX chain can write a `*.bitmap`. The objects in each layer's bitmap
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are offset by the number of objects in the previous layers of the chain.
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=== File layout
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Instead of storing a single `multi-pack-index` file (with an optional
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`.rev` and `.bitmap` extension) in `$GIT_DIR/objects/pack`, incremental
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MIDXs are stored in the following layout:
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----
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$GIT_DIR/objects/pack/multi-pack-index.d/
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$GIT_DIR/objects/pack/multi-pack-index.d/multi-pack-index-chain
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$GIT_DIR/objects/pack/multi-pack-index.d/multi-pack-index-$H1.midx
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$GIT_DIR/objects/pack/multi-pack-index.d/multi-pack-index-$H2.midx
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$GIT_DIR/objects/pack/multi-pack-index.d/multi-pack-index-$H3.midx
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----
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The `multi-pack-index-chain` file contains a list of the incremental
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MIDX files in the chain, in order. The above example shows a chain whose
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`multi-pack-index-chain` file would contain the following lines:
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----
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$H1
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$H2
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$H3
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----
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The `multi-pack-index-$H1.midx` file contains the first layer of the
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multi-pack-index chain. The `multi-pack-index-$H2.midx` file contains
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the second layer of the chain, and so on.
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When both an incremental- and non-incremental MIDX are present, the
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non-incremental MIDX is always read first.
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=== Object positions for incremental MIDXs
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In the original multi-pack-index design, we refer to objects via their
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lexicographic position (by object IDs) within the repository's singular
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multi-pack-index. In the incremental multi-pack-index design, we refer
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to objects via their index into a concatenated lexicographic ordering
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among each component in the MIDX chain.
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If `objects_nr()` is a function that returns the number of objects in a
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given MIDX layer, then the index of an object at lexicographic position
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`i` within, say, $H3 is defined as:
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----
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objects_nr($H2) + objects_nr($H1) + i
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----
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(in the C implementation, this is often computed as `i +
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m->num_objects_in_base`).
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=== Pseudo-pack order for incremental MIDXs
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The original implementation of multi-pack reachability bitmaps defined
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the pseudo-pack order in linkgit:gitformat-pack[5] (see the section
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titled "multi-pack-index reverse indexes") roughly as follows:
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____
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In short, a MIDX's pseudo-pack is the de-duplicated concatenation of
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objects in packs stored by the MIDX, laid out in pack order, and the
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packs arranged in MIDX order (with the preferred pack coming first).
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____
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In the incremental MIDX design, we extend this definition to include
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objects from multiple layers of the MIDX chain. The pseudo-pack order
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for incremental MIDXs is determined by concatenating the pseudo-pack
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ordering for each layer of the MIDX chain in order. Formally two objects
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`o1` and `o2` are compared as follows:
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1. If `o1` appears in an earlier layer of the MIDX chain than `o2`, then
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`o1` sorts ahead of `o2`.
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2. Otherwise, if `o1` and `o2` appear in the same MIDX layer, and that
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MIDX layer has no base, then if one of `pack(o1)` and `pack(o2)` is
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preferred and the other is not, then the preferred one sorts ahead of
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the non-preferred one. If there is a base layer (i.e. the MIDX layer
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is not the first layer in the chain), then if `pack(o1)` appears
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earlier in that MIDX layer's pack order, then `o1` sorts ahead of
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`o2`. Likewise if `pack(o2)` appears earlier, then the opposite is
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true.
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3. Otherwise, `o1` and `o2` appear in the same pack, and thus in the
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same MIDX layer. Sort `o1` and `o2` by their offset within their
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containing packfile.
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Note that the preferred pack is a property of the MIDX chain, not the
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individual layers themselves. Fundamentally we could introduce a
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per-layer preferred pack, but this is less relevant now that we can
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perform multi-pack reuse across the set of packs in a MIDX.
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=== Reachability bitmaps and incremental MIDXs
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Each layer of an incremental MIDX chain may have its objects (and the
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objects from any previous layer in the same MIDX chain) represented in
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its own `*.bitmap` file.
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The structure of a `*.bitmap` file belonging to an incremental MIDX
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chain is identical to that of a non-incremental MIDX bitmap, or a
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classic single-pack bitmap. Since objects are added to the end of the
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incremental MIDX's pseudo-pack order (see above), it is possible to
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extend a bitmap when appending to the end of a MIDX chain.
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(Note: it is possible likewise to compress a contiguous sequence of MIDX
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incremental layers, and their `*.bitmap` files into a single layer and
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`*.bitmap`, but this is not yet implemented.)
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The object positions used are global within the pseudo-pack order, so
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subsequent layers will have, for example, `m->num_objects_in_base`
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number of `0` bits in each of their four type bitmaps. This follows from
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the fact that we only write type bitmap entries for objects present in
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the layer immediately corresponding to the bitmap).
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Note also that only the bitmap pertaining to the most recent layer in an
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incremental MIDX chain is used to store reachability information about
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the interesting and uninteresting objects in a reachability query.
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Earlier bitmap layers are only used to look up commit and pseudo-merge
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bitmaps from that layer, as well as the type-level bitmaps for objects
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in that layer.
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To simplify the implementation, type-level bitmaps are iterated
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simultaneously, and their results are OR'd together to avoid recursively
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calling internal bitmap functions.
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Future Work
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-----------
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- If the multi-pack-index is extended to store a "stable object order"
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(a function Order(hash) = integer that is constant for a given hash,
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even as the multi-pack-index is updated) then MIDX bitmaps could be
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updated independently of the MIDX.
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- Packfiles can be marked as "special" using empty files that share
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the initial name but replace ".pack" with ".keep" or ".promisor".
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We can add an optional chunk of data to the multi-pack-index that
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records flags of information about the packfiles. This allows new
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states, such as 'repacked' or 'redeltified', that can help with
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pack maintenance in a multi-pack environment. It may also be
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helpful to organize packfiles by object type (commit, tree, blob,
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etc.) and use this metadata to help that maintenance.
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Related Links
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-------------
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[0] https://bugs.chromium.org/p/git/issues/detail?id=6
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Chromium work item for: Multi-Pack Index (MIDX)
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[1] https://lore.kernel.org/git/20180107181459.222909-1-dstolee@microsoft.com/
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An earlier RFC for the multi-pack-index feature
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[2] https://lore.kernel.org/git/alpine.DEB.2.20.1803091557510.23109@alexmv-linux/
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Git Merge 2018 Contributor's summit notes (includes discussion of MIDX)
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