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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?)