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Diffstat (limited to 'Documentation/technical/multi-pack-index.txt')
| -rw-r--r-- | Documentation/technical/multi-pack-index.txt | 45 |
1 files changed, 18 insertions, 27 deletions
diff --git a/Documentation/technical/multi-pack-index.txt b/Documentation/technical/multi-pack-index.txt index d7e5763..f2221d2 100644 --- a/Documentation/technical/multi-pack-index.txt +++ b/Documentation/technical/multi-pack-index.txt @@ -17,13 +17,14 @@ 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 +* 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. @@ -36,15 +37,18 @@ Design Details directory of an alternate. It refers only to packfiles in that same directory. -- The pack.multiIndex config setting must be on to consume MIDX files. +- 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 most- - recently modified packfile. + 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` @@ -60,10 +64,6 @@ Design Details Future Work ----------- -- Add a 'verify' subcommand to the 'git midx' builtin to verify the - contents of the multi-pack-index file match the offsets listed in - the corresponding pack-indexes. - - 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 @@ -74,14 +74,10 @@ Future Work 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" +- 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. + 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". @@ -92,18 +88,13 @@ Future Work 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://public-inbox.org/git/20180107181459.222909-1-dstolee@microsoft.com/ +[1] https://lore.kernel.org/git/20180107181459.222909-1-dstolee@microsoft.com/ An earlier RFC for the multi-pack-index feature -[2] https://public-inbox.org/git/alpine.DEB.2.20.1803091557510.23109@alexmv-linux/ +[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) |