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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 | ||
ad506e67 TL |
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 | |
ceab693d | 26 | offsets similar to version 2 pack-indexes. |
95e8383b | 27 | - An optional list of objects in pseudo-pack order (used with MIDX bitmaps). |
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28 | |
29 | Thus, we can provide O(log N) lookup time for any number | |
30 | of packfiles. | |
31 | ||
32 | Design Details | |
33 | -------------- | |
34 | ||
35 | - The MIDX is stored in a file named 'multi-pack-index' in the | |
36 | .git/objects/pack directory. This could be stored in the pack | |
37 | directory of an alternate. It refers only to packfiles in that | |
38 | same directory. | |
39 | ||
0d0d8d8a EW |
40 | - The core.multiPackIndex config setting must be on (which is the |
41 | default) to consume MIDX files. Setting it to `false` prevents | |
42 | Git from reading a MIDX file, even if one exists. | |
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43 | |
44 | - The file format includes parameters for the object ID hash | |
45 | function, so a future change of hash algorithm does not require | |
46 | a change in format. | |
47 | ||
48 | - The MIDX keeps only one record per object ID. If an object appears | |
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49 | in multiple packfiles, then the MIDX selects the copy in the |
50 | preferred packfile, otherwise selecting from the most-recently | |
51 | modified packfile. | |
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52 | |
53 | - If there exist packfiles in the pack directory not registered in | |
54 | the MIDX, then those packfiles are loaded into the `packed_git` | |
55 | list and `packed_git_mru` cache. | |
56 | ||
57 | - The pack-indexes (.idx files) remain in the pack directory so we | |
58 | can delete the MIDX file, set core.midx to false, or downgrade | |
59 | without any loss of information. | |
60 | ||
61 | - The MIDX file format uses a chunk-based approach (similar to the | |
62 | commit-graph file) that allows optional data to be added. | |
63 | ||
64 | Future Work | |
65 | ----------- | |
66 | ||
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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 | ||
917a54c0 | 77 | - If the multi-pack-index is extended to store a "stable object order" |
ceab693d | 78 | (a function Order(hash) = integer that is constant for a given hash, |
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79 | even as the multi-pack-index is updated) then MIDX bitmaps could be |
80 | updated independently of the MIDX. | |
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81 | |
82 | - Packfiles can be marked as "special" using empty files that share | |
83 | the initial name but replace ".pack" with ".keep" or ".promisor". | |
84 | We can add an optional chunk of data to the multi-pack-index that | |
85 | records flags of information about the packfiles. This allows new | |
86 | states, such as 'repacked' or 'redeltified', that can help with | |
87 | pack maintenance in a multi-pack environment. It may also be | |
88 | helpful to organize packfiles by object type (commit, tree, blob, | |
89 | etc.) and use this metadata to help that maintenance. | |
90 | ||
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91 | Related Links |
92 | ------------- | |
93 | [0] https://bugs.chromium.org/p/git/issues/detail?id=6 | |
94 | Chromium work item for: Multi-Pack Index (MIDX) | |
95 | ||
3eae30e4 | 96 | [1] https://lore.kernel.org/git/20180107181459.222909-1-dstolee@microsoft.com/ |
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97 | An earlier RFC for the multi-pack-index feature |
98 | ||
3eae30e4 | 99 | [2] https://lore.kernel.org/git/alpine.DEB.2.20.1803091557510.23109@alexmv-linux/ |
ceab693d | 100 | Git Merge 2018 Contributor's summit notes (includes discussion of MIDX) |