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help: document sharing of revlog header with revision 0

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The previous docs were incorrect about there being a discrete header
on revlogs.
This commit is contained in:
Gregory Szorc 2016-03-19 15:17:33 -07:00
parent 7347261bf4
commit db9b5063c1
1 changed files with 12 additions and 4 deletions
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14
mercurial/help/internals/revlogs.txt
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@ -31,7 +31,8 @@ File Format
-----------
A revlog begins with a 32-bit big endian integer holding version info
and feature flags.
and feature flags. This integer is shared with the first revision
entry.
This integer is logically divided into 2 16-bit shorts. The least
significant half of the integer is the format/version short. The other
@ -70,8 +71,10 @@ The following header values are common:
00 03 00 01
RevlogNG + inline + generaldelta
Following the 32-bit header is *index* data. Inlined revision data is possibly
located between index entries. More on this layout is described below.
Following the 32-bit header is the remainder of the first index entry.
Following that are remaining *index* data. Inlined revision data is
possibly located between index entries. More on this layout is described
below.
RevlogNG Format
---------------
@ -83,6 +86,8 @@ or between index entries (as opposed to in a separate container).
Each index entry is 64 bytes. The byte layout of each entry is as
follows, with byte 0 being the first byte (all data stored as big endian):
0-3 (4 bytes) (rev 0 only)
Revlog header
0-5 (6 bytes)
Absolute offset of revision data from beginning of revlog.
6-7 (2 bytes)
@ -120,6 +125,9 @@ 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
length from bytes 8-11 define how to access this data.
The first 4 bytes of the revlog are shared between the revlog header
and the 6 byte absolute offset field from the first revlog entry.
Delta Chains
------------