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docs: updated for repository version 2, PUT2 tag

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Thomas Waldmann 2022-03-31 22:09:05 +02:00
parent 52f75d7722
commit 85d4b98a16
1 changed files with 18 additions and 9 deletions
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27
docs/internals/data-structures.rst
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@ -59,7 +59,7 @@ Each repository has a ``config`` file which is a ``INI``-style file
and looks like this::
[repository]
version = 1
version = 2
segments_per_dir = 1000
max_segment_size = 524288000
id = 57d6c1d52ce76a836b532b0e42e677dec6af9fca3673db511279358828a21ed6
@ -94,18 +94,27 @@ this value in a non-empty repository, you may also need to relocate the segment
files manually.
A segment starts with a magic number (``BORG_SEG`` as an eight byte ASCII string),
followed by a number of log entries. Each log entry consists of: (in this order)
followed by a number of log entries. Each log entry consists of (in this order):
* First, unsigned 32-bit number, the CRC32 of the entire entry (for a PUT including the DATA) excluding the CRC32 field
* Second, unsigned 32-bit size of the entry (including the whole header)
* Third, unsigned 8-bit entry tag: PUT(1), DELETE(2) or COMMIT(3)
* Fourth, on PUT or DELETE, 32 byte key
* Fifth, PUT only, (size - 41) bytes of data (length = size - sizeof(CRC32) - sizeof(size) - sizeof(entry tag) - sizeof(key))
* crc32 checksum (uint32):
- for PUT2: CRC32(size + tag + key + digest)
- for PUT: CRC32(size + tag + key + data)
- for DELETE: CRC32(size + tag + key)
- for COMMIT: CRC32(size + tag)
* size (uint32) of the entry (including the whole header)
* tag (uint8): PUT(0), DELETE(1), COMMIT(2) or PUT2(3)
* key (256 bit) - only for PUT/PUT2/DELETE
* data (size - 41 bytes) - only for PUT
* xxh64 digest (64 bit) = XXH64(size + tag + key + data) - only for PUT2
* data (size - 41 - 8 bytes) - only for PUT2
PUT2 is new since repository version 2. For new log entries PUT2 is used.
PUT is still supported to read version 1 repositories, but not generated any more.
If we talk about ``PUT`` in general, it shall usually mean PUT2 for repository
version 2+.
Those files are strictly append-only and modified only once.
Tag is either ``PUT``, ``DELETE``, or ``COMMIT``.
When an object is written to the repository a ``PUT`` entry is written
to the file containing the object id and data. If an object is deleted
a ``DELETE`` entry is appended with the object id.