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 existingblob.

  • write to unused chunk(s) of existing blob

  • kv commit

• W: WAL overwrite: commit intent to overwrite, then overwriteasync. 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 sparselyutilized. 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 apiecemeal blob (that collects random, noncontiguous bits of data weneed 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 legacybluestore). No checksum.

  • kv commit

  • wal read/modify/write

Mapping

This very roughly maps the type of write onto what we do when weencounter a given blob. In practice it’s a bit more complicated since theremight be several blobs to consider (e.g., we might be able to W into one orP into another), but it should communicate a rough idea of strategy.