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Differential D8953

documentation: add `zstd` compression to the internal `revlogs` documentation
ClosedPublic

Authored by acezar on Aug 26 2020, 9:23 AM.

Details

Reviewers
marmoute
Alphare
pulkit
Group Reviewers
hg-reviewers
Commits
rHG684083d104f9: documentation: add `zstd` compression to the internal `revlogs` documentation

Diff Detail

Repository
rHG Mercurial
Lint
Automatic diff as part of commit; lint not applicable.
Unit
Automatic diff as part of commit; unit tests not applicable.

Event Timeline

acezar created this revision.Aug 26 2020, 9:23 AM
acezar retitled this revision from documentation: add zstd compression to the insternal revlogs documentation to documentation: add `zstd` compression to the internal `revlogs` documentation.Aug 26 2020, 9:44 AM
marmoute accepted this revision.Aug 26 2020, 3:19 PM
Alphare accepted this revision.Aug 28 2020, 4:18 AM
pulkit requested changes to this revision.Sep 2 2020, 7:58 AM
This revision now requires changes to proceed.Sep 2 2020, 7:58 AM
pulkit accepted this revision.Sep 2 2020, 7:58 AM
This revision is now accepted and ready to land.Sep 2 2020, 7:58 AM

Revision Contents
Changeset List

Diff 22517

mercurial/helptext/internals/revlogs.txt

(possibly containing inline data) and a ``.d`` file holds the revision data. (possibly containing inline data) and a ``.d`` file holds the revision data.
Revision Entries Revision Entries
================ ================
Revision entries consist of an optional 1 byte header followed by an Revision entries consist of an optional 1 byte header followed by an
encoding of the revision data. The headers are as follows: encoding of the revision data. The headers are as follows:
\0 (0x00) \0 (0x00)
Revision data is the entirety of the entry, including this header. Revision data is the entirety of the entry, including this header.
( (0x28)
zstd https://github.com/facebook/zstd
u (0x75) u (0x75)
Raw revision data follows. Raw revision data follows.
x (0x78) x (0x78)
zlib (RFC 1950) data. zlib (RFC 1950) data.
The 0x78 value is actually the first byte of the zlib header (CMF byte). The 0x78 value is actually the first byte of the zlib header (CMF byte).
Hash Computation Hash Computation
================ ================
The hash of the revision is stored in the index and is used both as a primary The hash of the revision is stored in the index and is used both as a primary
key and for data integrity verification. key and for data integrity verification.
Currently, SHA-1 is the only supported hashing algorithm. To obtain the SHA-1 Currently, SHA-1 is the only supported hashing algorithm. To obtain the SHA-1
hash of a revision: hash of a revision:
1. Hash the parent nodes 1. Hash the parent nodes
2. Hash the fulltext of the revision 2. Hash the fulltext of the revision
The 20 byte node ids of the parents are fed into the hasher in ascending order. The 20 byte node ids of the parents are fed into the hasher in ascending order.