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- rHGbfd65b5e070b: help: clarify overlap of revlog header and first revlog entry
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This is an archive of the discontinued Mercurial Phabricator instance.
Differential D6449
help: clarify overlap of revlog header and first revlog entryClosedPublic
help: clarify overlap of revlog header and first revlog entry
ClosedPublic
Authored by ngoldbaum on May 28 2019, 2:39 PM.
Revision Contents
Revision Contents
| Path | Packages | |||
|---|---|---|---|---|
| M | mercurial/help/internals/revlogs.txt (20 lines) |
| Commit | Parents | Author | Summary | Date |
|---|---|---|---|---|
| 26a854f50ea7 | e2e507573c7c | Nathan Goldbaum | May 28 2019, 2:39 PM |
Diff 15277
Diff 15277
mercurial/help/internals/revlogs.txt
| Revlogs can be modeled as 0-indexed arrays. The first revision is | Revlogs can be modeled as 0-indexed arrays. The first revision is | ||||
| revision #0 and the second is revision #1. The revision -1 is typically | revision #0 and the second is revision #1. The revision -1 is typically | ||||
| used to mean *does not exist* or *not defined*. | used to mean *does not exist* or *not defined*. | ||||
| File Format | File Format | ||||
| =========== | =========== | ||||
| A revlog begins with a 32-bit big endian integer holding version info | A revlog begins with a 32-bit big endian integer holding version info | ||||
| and feature flags. This integer is shared with the first revision | and feature flags. This integer overlaps with the first four bytes of | ||||
| entry. | the first revision entry. | ||||
| This integer is logically divided into 2 16-bit shorts. The least | This integer is logically divided into 2 16-bit shorts. The least | ||||
| significant half of the integer is the format/version short. The other | significant half of the integer is the format/version short. The other | ||||
| short holds feature flags that dictate behavior of the revlog. | short holds feature flags that dictate behavior of the revlog. | ||||
| The following values for the format/version short are defined: | The following values for the format/version short are defined: | ||||
| 0 | 0 | ||||
| v1 | v1 | ||||
| 00 01 00 01 | 00 01 00 01 | ||||
| v1 + inline | v1 + inline | ||||
| 00 02 00 01 | 00 02 00 01 | ||||
| v1 + generaldelta | v1 + generaldelta | ||||
| 00 03 00 01 | 00 03 00 01 | ||||
| v1 + inline + generaldelta | v1 + inline + generaldelta | ||||
| Following the 32-bit header is the remainder of the first index entry. | Following the 32-bit header is the remaining 60 bytes of the first index | ||||
| Following that are remaining *index* data. Inlined revision data is | entry. Following that are additional *index* entries. Inlined revision | ||||
| possibly located between index entries. More on this layout is described | data is possibly located between index entries. More on the this inlined | ||||
| below. | layout is described below. | ||||
| Version 1 Format | Version 1 Format | ||||
| ================ | ================ | ||||
| Version 1 (RevlogNG) begins with an index describing the revisions in | Version 1 (RevlogNG) begins with an index describing the revisions in | ||||
| the revlog. If the ``inline`` flag is set, revision data is stored inline, | the revlog. If the ``inline`` flag is set, revision data is stored inline, | ||||
| or between index entries (as opposed to in a separate container). | or between index entries (as opposed to in a separate container). | ||||
| (of length from bytes offset 8-11 from the index entry) immediately | (of length from bytes offset 8-11 from the index entry) immediately | ||||
| follows the index entry. There is no header on the revision data. There | follows the index entry. There is no header on the revision data. There | ||||
| is no padding between it and the index entries before and after. | is no padding between it and the index entries before and after. | ||||
| If revision data is not inline, then raw revision data is stored in a | If revision data is not inline, then raw revision data is stored in a | ||||
| separate byte container. The offsets from bytes 0-5 and the compressed | separate byte container. The offsets from bytes 0-5 and the compressed | ||||
| length from bytes 8-11 define how to access this data. | length from bytes 8-11 define how to access this data. | ||||
| The first 4 bytes of the revlog are shared between the revlog header | The 6 byte absolute offset field from the first revlog entry overlaps | ||||
| and the 6 byte absolute offset field from the first revlog entry. | with the revlog header. That is, the first 6 bytes of the first revlog | ||||
| entry can be split into four bytes containing the header for the revlog | |||||
| file and an additional two bytes containing the offset for the first | |||||
| entry. Since this is the offset from the beginning of the file for the | |||||
| first revision entry, the two bytes will always be set to zero. | |||||
| Version 2 Format | Version 2 Format | ||||
| ================ | ================ | ||||
| (In development. Format not finalized or stable.) | (In development. Format not finalized or stable.) | ||||
| Version 2 is identical to version 2 with the following differences. | Version 2 is identical to version 2 with the following differences. | ||||