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1 | Multi-Pack-Index (MIDX) Design Notes |
2 | ==================================== | |
3 | ||
4 | The Git object directory contains a 'pack' directory containing | |
5 | packfiles (with suffix ".pack") and pack-indexes (with suffix | |
6 | ".idx"). The pack-indexes provide a way to lookup objects and | |
7 | navigate to their offset within the pack, but these must come | |
8 | in pairs with the packfiles. This pairing depends on the file | |
9 | names, as the pack-index differs only in suffix with its pack- | |
10 | file. While the pack-indexes provide fast lookup per packfile, | |
11 | this performance degrades as the number of packfiles increases, | |
12 | because abbreviations need to inspect every packfile and we are | |
13 | more likely to have a miss on our most-recently-used packfile. | |
14 | For some large repositories, repacking into a single packfile | |
15 | is not feasible due to storage space or excessive repack times. | |
16 | ||
17 | The multi-pack-index (MIDX for short) stores a list of objects | |
18 | and their offsets into multiple packfiles. It contains: | |
19 | ||
20 | - A list of packfile names. | |
21 | - A sorted list of object IDs. | |
22 | - A list of metadata for the ith object ID including: | |
23 | - A value j referring to the jth packfile. | |
24 | - An offset within the jth packfile for the object. | |
25 | - If large offsets are required, we use another list of large | |
26 | offsets similar to version 2 pack-indexes. | |
27 | ||
28 | Thus, we can provide O(log N) lookup time for any number | |
29 | of packfiles. | |
30 | ||
31 | Design Details | |
32 | -------------- | |
33 | ||
34 | - The MIDX is stored in a file named 'multi-pack-index' in the | |
35 | .git/objects/pack directory. This could be stored in the pack | |
36 | directory of an alternate. It refers only to packfiles in that | |
37 | same directory. | |
38 | ||
39 | - The pack.multiIndex config setting must be on to consume MIDX files. | |
40 | ||
41 | - The file format includes parameters for the object ID hash | |
42 | function, so a future change of hash algorithm does not require | |
43 | a change in format. | |
44 | ||
45 | - The MIDX keeps only one record per object ID. If an object appears | |
46 | in multiple packfiles, then the MIDX selects the copy in the most- | |
47 | recently modified packfile. | |
48 | ||
49 | - If there exist packfiles in the pack directory not registered in | |
50 | the MIDX, then those packfiles are loaded into the `packed_git` | |
51 | list and `packed_git_mru` cache. | |
52 | ||
53 | - The pack-indexes (.idx files) remain in the pack directory so we | |
54 | can delete the MIDX file, set core.midx to false, or downgrade | |
55 | without any loss of information. | |
56 | ||
57 | - The MIDX file format uses a chunk-based approach (similar to the | |
58 | commit-graph file) that allows optional data to be added. | |
59 | ||
60 | Future Work | |
61 | ----------- | |
62 | ||
63 | - Add a 'verify' subcommand to the 'git midx' builtin to verify the | |
64 | contents of the multi-pack-index file match the offsets listed in | |
65 | the corresponding pack-indexes. | |
66 | ||
67 | - The multi-pack-index allows many packfiles, especially in a context | |
68 | where repacking is expensive (such as a very large repo), or | |
69 | unexpected maintenance time is unacceptable (such as a high-demand | |
70 | build machine). However, the multi-pack-index needs to be rewritten | |
71 | in full every time. We can extend the format to be incremental, so | |
72 | writes are fast. By storing a small "tip" multi-pack-index that | |
73 | points to large "base" MIDX files, we can keep writes fast while | |
74 | still reducing the number of binary searches required for object | |
75 | lookups. | |
76 | ||
77 | - The reachability bitmap is currently paired directly with a single | |
78 | packfile, using the pack-order as the object order to hopefully | |
79 | compress the bitmaps well using run-length encoding. This could be | |
80 | extended to pair a reachability bitmap with a multi-pack-index. If | |
81 | the multi-pack-index is extended to store a "stable object order" | |
82 | (a function Order(hash) = integer that is constant for a given hash, | |
83 | even as the multi-pack-index is updated) then a reachability bitmap | |
84 | could point to a multi-pack-index and be updated independently. | |
85 | ||
86 | - Packfiles can be marked as "special" using empty files that share | |
87 | the initial name but replace ".pack" with ".keep" or ".promisor". | |
88 | We can add an optional chunk of data to the multi-pack-index that | |
89 | records flags of information about the packfiles. This allows new | |
90 | states, such as 'repacked' or 'redeltified', that can help with | |
91 | pack maintenance in a multi-pack environment. It may also be | |
92 | helpful to organize packfiles by object type (commit, tree, blob, | |
93 | etc.) and use this metadata to help that maintenance. | |
94 | ||
95 | - The partial clone feature records special "promisor" packs that | |
96 | may point to objects that are not stored locally, but available | |
97 | on request to a server. The multi-pack-index does not currently | |
98 | track these promisor packs. | |
99 | ||
100 | Related Links | |
101 | ------------- | |
102 | [0] https://bugs.chromium.org/p/git/issues/detail?id=6 | |
103 | Chromium work item for: Multi-Pack Index (MIDX) | |
104 | ||
105 | [1] https://public-inbox.org/git/20180107181459.222909-1-dstolee@microsoft.com/ | |
106 | An earlier RFC for the multi-pack-index feature | |
107 | ||
108 | [2] https://public-inbox.org/git/alpine.DEB.2.20.1803091557510.23109@alexmv-linux/ | |
109 | Git Merge 2018 Contributor's summit notes (includes discussion of MIDX) |