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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?) |
++--------------------------+--------+--------------+-------------+--------------+---------------+