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revision storage mechanism

Revisions Logs

Revision logs - or *revlogs* - are an append only data structure for storing discrete entries, or *revisions*. They are the primary storage mechanism of repository data.

Revlogs effectively model a directed acyclic graph (DAG). Each node has edges to 1 or 2 *parent* nodes. Each node contains metadata and the raw value for that node.

Revlogs consist of entries which have metadata and revision data. Metadata includes the hash of the revision's content, sizes, and links to its *parent* entries. The collective metadata is referred to as the *index* and the revision data is the *data*.

Revision data is stored as a series of compressed deltas against previous revisions.

Revlogs are written in an append-only fashion. We never need to rewrite a file to insert nor do we need to remove data. Rolling back in-progress writes can be performed by truncating files. Read locks can be avoided using simple techniques. This means that references to other data in the same revlog *always* refer to a previous entry.

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 used to mean *does not exist* or *not defined*.

File Format

A revlog begins with a 32-bit big endian integer holding version info and feature flags.

This integer is logically divided into 2 16-bit shorts. The least significant half of the integer is the format/version short. The other short holds feature flags that dictate behavior of the revlog.

Only 1 bit of the format/version short is currently used. Remaining bits are reserved for future use.

The following values for the format/version short are defined:

The original revlog version.
RevlogNG (*next generation*). It replaced version 0 when it was implemented in 2006.

The feature flags short consists of bit flags. Where 0 is the least significant bit, the following bit offsets define flags:

Store revision data inline.
Generaldelta encoding.
Reserved for future use.

The following header values are common:

00 00 00 01
00 01 00 01
RevlogNG + inline
00 02 00 01
RevlogNG + generaldelta
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.

RevlogNG Format

RevlogNG (version 1) begins with an index describing the revisions in the revlog. If the "inline" flag is set, revision data is stored inline, 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):

0-5 (6 bytes)
Absolute offset of revision data from beginning of revlog.
6-7 (2 bytes)
Bit flags impacting revision behavior.
8-11 (4 bytes)
Compressed length of revision data / chunk as stored in revlog.
12-15 (4 bytes)
Uncompressed length of revision data / chunk.
16-19 (4 bytes)
Base or previous revision this revision's delta was produced against. -1 means this revision holds full text (as opposed to a delta). For generaldelta repos, this is the previous revision in the delta chain. For non-generaldelta repos, this is the base or first revision in the delta chain.
20-23 (4 bytes)
A revision this revision is *linked* to. This allows a revision in one revlog to be forever associated with a revision in another revlog. For example, a file's revlog may point to the changelog revision that introduced it.
24-27 (4 bytes)
Revision of 1st parent. -1 indicates no parent.
28-31 (4 bytes)
Revision of 2nd parent. -1 indicates no 2nd parent.
32-63 (32 bytes)
Hash of revision's full text. Currently, SHA-1 is used and only the first 20 bytes of this field are used. The rest of the bytes are ignored and should be stored as \0.

If inline revision data is being stored, the compressed revision data (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 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 separate byte container. The offsets from bytes 0-5 and the compressed length from bytes 8-11 define how to access this data.

Delta Chains

Revision data is encoded as a chain of *chunks*. Each chain begins with the compressed original full text for that revision. Each subsequent *chunk* is a *delta* against the previous revision. We therefore call these chains of chunks/deltas *delta chains*.

The full text for a revision is reconstructed by loading the original full text for the base revision of a *delta chain* and then applying *deltas* until the target revision is reconstructed.

*Delta chains* are limited in length so lookup time is bound. They are limited to ~2x the length of the revision's data. The linear distance between the base chunk and the final chunk is also limited so the amount of read I/O to load all chunks in the delta chain is bound.

Deltas and delta chains are either computed against the previous revision in the revlog or another revision (almost certainly one of the parents of the revision). Historically, deltas were computed against the previous revision. The *generaldelta* revlog feature flag (enabled by default in Mercurial 3.7) activates the mode where deltas are computed against an arbitrary revision (almost certainly a parent revision).

File Storage

Revlogs logically consist of an index (metadata of entries) and revision data. This data may be stored together in a single file or in separate files. The mechanism used is indicated by the "inline" feature flag on the revlog.

Mercurial's behavior is to use inline storage until a revlog reaches a certain size, at which point it will be converted to non-inline. The reason there is a size limit on inline storage is to establish an upper bound on how much data must be read to load the index. It would be a waste to read tens or hundreds of extra megabytes of data just to access the index data.

The actual layout of revlog files on disk is governed by the repository's *store format*. Typically, a ".i" file represents the index revlog (possibly containing inline data) and a ".d" file holds the revision data.

Revision Entries

Revision entries consist of an optional 1 byte header followed by an encoding of the revision data. The headers are as follows:

\0 (0x00)
Revision data is the entirety of the entry, including this header.
u (0x75)
Raw revision data follows.
x (0x78)
zlib (RFC 1950) data.

The 0x78 value is actually the first byte of the zlib header (CMF byte).

Hash Computation

The hash of the revision is stored in the index and is used both as a primary key and for data integrity verification.

Currently, SHA-1 is the only supported hashing algorithm. To obtain the SHA-1 hash of a revision:

  1. Hash the parent nodes
  2. Hash the fulltext of the revision

The 20 byte node ids of the parents are fed into the hasher in ascending order.