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===================
BlueStore Internals
===================
Small write strategies
----------------------
* *U*: Uncompressed write of a complete, new blob.
- write to new blob
- kv commit
* *P*: Uncompressed partial write to unused region of an existing
blob.
- write to unused chunk(s) of existing blob
- kv commit
* *W*: WAL overwrite: commit intent to overwrite, then overwrite
async. Must be chunk_size = MAX(block_size, csum_block_size)
aligned.
- kv commit
- wal overwrite (chunk-aligned) of existing blob
* *N*: Uncompressed partial write to a new blob. Initially sparsely
utilized. Future writes will either be *P* or *W*.
- write into a new (sparse) blob
- kv commit
* *R+W*: Read partial chunk, then to WAL overwrite.
- read (out to chunk boundaries)
- kv commit
- wal overwrite (chunk-aligned) of existing blob
* *C*: Compress data, write to new blob.
- compress and write to new blob
- kv commit
Possible future modes
---------------------
* *F*: Fragment lextent space by writing small piece of data into a
piecemeal blob (that collects random, noncontiguous bits of data we
need to write).
- write to a piecemeal blob (min_alloc_size or larger, but we use just one block of it)
- kv commit
* *X*: WAL read/modify/write on a single block (like legacy
bluestore). No checksum.
- kv commit
- wal read/modify/write
Mapping
-------
This very roughly maps the type of write onto what we do when we
encounter a given blob. In practice it's a bit more complicated since there
might be several blobs to consider (e.g., we might be able to *W* into one or
*P* into another), but it should communicate a rough idea of strategy.
+--------------------------+--------+--------------+-------------+--------------+---------------+
| | raw | raw (cached) | csum (4 KB) | csum (16 KB) | comp (128 KB) |
+--------------------------+--------+--------------+-------------+--------------+---------------+
| 128+ KB (over)write | U | U | U | U | C |
+--------------------------+--------+--------------+-------------+--------------+---------------+
| 64 KB (over)write | U | U | U | U | U or C |
+--------------------------+--------+--------------+-------------+--------------+---------------+
| 4 KB overwrite | W | P | W | P | W | P | R+W | P | N (F?) |
+--------------------------+--------+--------------+-------------+--------------+---------------+
| 100 byte overwrite | R+W | P | W | P | R+W | P | R+W | P | N (F?) |
+--------------------------+--------+--------------+-------------+--------------+---------------+
| 100 byte append | R+W | P | W | P | R+W | P | R+W | P | N (F?) |
+--------------------------+--------+--------------+-------------+--------------+---------------+
+--------------------------+--------+--------------+-------------+--------------+---------------+
| 4 KB clone overwrite | P | N | P | N | P | N | P | N | N (F?) |
+--------------------------+--------+--------------+-------------+--------------+---------------+
| 100 byte clone overwrite | P | N | P | N | P | N | P | N | N (F?) |
+--------------------------+--------+--------------+-------------+--------------+---------------+
|
