Mercurial > hg
annotate mercurial/help/internals/revlogs.txt @ 27631:c18292a6ff54
internals: document revlog format
It seems like a good idea to document the revlog format.
There is a lot more that could be added to this documentation.
But you have to start somewhere.
author | Gregory Szorc <gregory.szorc@gmail.com> |
---|---|
date | Wed, 30 Dec 2015 16:21:57 -0700 |
parents | |
children | b0b9f6b0a777 |
rev | line source |
---|---|
27631
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
1 Revisions Logs |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
2 ============== |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
3 |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
4 Revision logs - or *revlogs* - are an append only data structure for |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
5 storing discrete entries, or *revisions*. They are the primary storage |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
6 mechanism of repository data. |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
7 |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
8 Revlogs effectively model a directed acyclic graph (DAG). Each node |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
9 has edges to 1 or 2 *parent* nodes. Each node contains metadata and |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
10 the raw value for that node. |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
11 |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
12 Revlogs consist of entries which have metadata and revision data. |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
13 Metadata includes the hash of the revision's content, sizes, and |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
14 links to its *parent* entries. The collective metadata is referred |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
15 to as the *index* and the revision data is the *data*. |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
16 |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
17 Revision data is stored as a series of compressed deltas against previous |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
18 revisions. |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
19 |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
20 Revlogs are written in an append-only fashion. We never need to rewrite |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
21 a file to insert nor do we need to remove data. Rolling back in-progress |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
22 writes can be performed by truncating files. Read locks can be avoided |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
23 using simple techniques. This means that references to other data in |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
24 the same revlog *always* refer to a previous entry. |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
25 |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
26 Revlogs can be modeled as 0-indexed arrays. The first revision is |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
27 revision #0 and the second is revision #1. The revision -1 is typically |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
28 used to mean *does not exist* or *not defined*. |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
29 |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
30 File Format |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
31 ----------- |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
32 |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
33 A revlog begins with a 32-bit big endian integer holding version info |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
34 and feature flags. |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
35 |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
36 This integer is logically divided into 2 16-bit shorts. The least |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
37 significant half of the integer is the format/version short. The other |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
38 short holds feature flags that dictate behavior of the revlog. |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
39 |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
40 Only 1 bit of the format/version short is currently used. Remaining |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
41 bits are reserved for future use. |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
42 |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
43 The following values for the format/version short are defined: |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
44 |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
45 0 |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
46 The original revlog version. |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
47 1 |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
48 RevlogNG (*next generation*). It replaced version 0 when it was |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
49 implemented in 2006. |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
50 |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
51 The feature flags short consists of bit flags. Where 0 is the least |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
52 significant bit, the following bit offsets define flags: |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
53 |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
54 0 |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
55 Store revision data inline. |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
56 1 |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
57 Generaldelta encoding. |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
58 |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
59 2-15 |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
60 Reserved for future use. |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
61 |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
62 The following header values are common: |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
63 |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
64 00 00 00 01 |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
65 RevlogNG |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
66 00 01 00 01 |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
67 RevlogNG + inline |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
68 00 02 00 01 |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
69 RevlogNG + generaldelta |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
70 00 03 00 01 |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
71 RevlogNG + inline + generaldelta |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
72 |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
73 Following the 32-bit header is *index* data. Inlined revision data is possibly |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
74 located between index entries. More on this layout is described below. |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
75 |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
76 RevlogNG Format |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
77 --------------- |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
78 |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
79 RevlogNG (version 1) begins with an index describing the revisions in |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
80 the revlog. If the ``inline`` flag is set, revision data is stored inline, |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
81 or between index entries (as opposed to in a separate container). |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
82 |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
83 Each index entry is 64 bytes. The byte layout of each entry is as |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
84 follows, with byte 0 being the first byte (all data stored as big endian): |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
85 |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
86 0-5 (6 bytes) |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
87 Absolute offset of revision data from beginning of revlog. |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
88 6-7 (2 bytes) |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
89 Bit flags impacting revision behavior. |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
90 8-11 (4 bytes) |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
91 Compressed length of revision data / chunk as stored in revlog. |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
92 12-15 (4 bytes) |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
93 Uncompressed length of revision data / chunk. |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
94 16-19 (4 bytes) |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
95 Base or previous revision this revision's delta was produced against. |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
96 -1 means this revision holds full text (as opposed to a delta). |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
97 For generaldelta repos, this is the previous revision in the delta |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
98 chain. For non-generaldelta repos, this is the base or first |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
99 revision in the delta chain. |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
100 20-23 (4 bytes) |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
101 A revision this revision is *linked* to. This allows a revision in |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
102 one revlog to be forever associated with a revision in another |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
103 revlog. For example, a file's revlog may point to the changelog |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
104 revision that introduced it. |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
105 24-27 (4 bytes) |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
106 Revision of 1st parent. -1 indicates no parent. |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
107 28-31 (4 bytes) |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
108 Revision of 2nd parent. -1 indicates no 2nd parent. |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
109 32-63 (32 bytes) |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
110 Hash of revision's full text. Currently, SHA-1 is used and only |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
111 the first 20 bytes of this field are used. The rest of the bytes |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
112 are ignored and should be stored as \0. |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
113 |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
114 If inline revision data is being stored, the compressed revision data |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
115 (of length from bytes offset 8-11 from the index entry) immediately |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
116 follows the index entry. There is no header on the revision data. There |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
117 is no padding between it and the index entries before and after. |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
118 |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
119 If revision data is not inline, then raw revision data is stored in a |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
120 separate byte container. The offsets from bytes 0-5 and the compressed |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
121 length from bytes 8-11 define how to access this data. |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
122 |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
123 Delta Chains |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
124 ------------ |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
125 |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
126 Revision data is encoded as a chain of *chunks*. Each chain begins with |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
127 the compressed original full text for that revision. Each subsequent |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
128 *chunk* is a *delta* against the previous revision. We therefore call |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
129 these chains of chunks/deltas *delta chains*. |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
130 |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
131 The full text for a revision is reconstructed by loading the original |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
132 full text for the base revision of a *delta chain* and then applying |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
133 *deltas* until the target revision is reconstructed. |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
134 |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
135 *Delta chains* are limited in length so lookup time is bound. They are |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
136 limited to ~2x the length of the revision's data. The linear distance |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
137 between the base chunk and the final chunk is also limited so the |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
138 amount of read I/O to load all chunks in the delta chain is bound. |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
139 |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
140 Deltas and delta chains are either computed against the previous |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
141 revision in the revlog or another revision (almost certainly one of |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
142 the parents of the revision). Historically, deltas were computed against |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
143 the previous revision. The *generaldelta* revlog feature flag (enabled |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
144 by default in Mercurial 3.7) activates the mode where deltas are |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
145 computed against an arbitrary revision (almost certainly a parent revision). |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
146 |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
147 File Storage |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
148 ------------ |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
149 |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
150 Revlogs logically consist of an index (metadata of entries) and |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
151 revision data. This data may be stored together in a single file or in |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
152 separate files. The mechanism used is indicated by the ``inline`` feature |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
153 flag on the revlog. |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
154 |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
155 Mercurial's behavior is to use inline storage until a revlog reaches a |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
156 certain size, at which point it will be converted to non-inline. The |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
157 reason there is a size limit on inline storage is to establish an upper |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
158 bound on how much data must be read to load the index. It would be a waste |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
159 to read tens or hundreds of extra megabytes of data just to access the |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
160 index data. |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
161 |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
162 The actual layout of revlog files on disk is governed by the repository's |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
163 *store format*. Typically, a ``.i`` file represents the index revlog |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
164 (possibly containing inline data) and a ``.d`` file holds the revision data. |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
165 |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
166 Revision Entries |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
167 ---------------- |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
168 |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
169 Revision entries consist of an optional 1 byte header followed by an |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
170 encoding of the revision data. The headers are as follows: |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
171 |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
172 \0 (0x00) |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
173 Revision data is the entirety of the entry, including this header. |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
174 u (0x75) |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
175 Raw revision data follows. |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
176 x (0x78) |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
177 zlib (RFC 1950) data. |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
178 |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
179 The 0x78 value is actually the first byte of the zlib header (CMF byte). |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
180 |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
181 Hash Computation |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
182 ---------------- |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
183 |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
184 The hash of the revision is stored in the index and is used both as a primary |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
185 key and for data integrity verification. |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
186 |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
187 Currently, SHA-1 is the only supported hashing algorithm. To obtain the SHA-1 |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
188 hash of a revision: |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
189 |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
190 1. Hash the parent nodes |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
191 2. Hash the fulltext of the revision |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
192 |
c18292a6ff54
internals: document revlog format
Gregory Szorc <gregory.szorc@gmail.com>
parents:
diff
changeset
|
193 The 20 byte node ids of the parents are fed into the hasher in ascending order. |