multi-pack-index: add design document
Signed-off-by: Derrick Stolee <dstolee@microsoft.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>maint
Derrick Stolee
Junio C Hamano
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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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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 pack.multiIndex config setting must be on to consume MIDX files.
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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 most-
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recently 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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Future Work
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-----------
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- Add a 'verify' subcommand to the 'git midx' builtin to verify the
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contents of the multi-pack-index file match the offsets listed in
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the corresponding pack-indexes.
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- The multi-pack-index allows many packfiles, especially in a context
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where repacking is expensive (such as a very large repo), or
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unexpected maintenance time is unacceptable (such as a high-demand
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build machine). However, the multi-pack-index needs to be rewritten
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in full every time. We can extend the format to be incremental, so
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writes are fast. By storing a small "tip" multi-pack-index that
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points to large "base" MIDX files, we can keep writes fast while
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still reducing the number of binary searches required for object
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lookups.
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- The reachability bitmap is currently paired directly with a single
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packfile, using the pack-order as the object order to hopefully
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compress the bitmaps well using run-length encoding. This could be
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extended to pair a reachability bitmap with a multi-pack-index. If
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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 a reachability bitmap
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could point to a multi-pack-index and be updated independently.
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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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- The partial clone feature records special "promisor" packs that
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may point to objects that are not stored locally, but available
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on request to a server. The multi-pack-index does not currently
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track these promisor packs.
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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://public-inbox.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://public-inbox.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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