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