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https://github.com/ceph/ceph
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73 lines
1.9 KiB
ReStructuredText
73 lines
1.9 KiB
ReStructuredText
Cache pool
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==========
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Purpose
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-------
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Use a pool of fast storage devices (probably SSDs) and use it as a
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cache for an existing larger pool.
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We should be able to create and add a cache pool to an existing pool
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of data, and later remove it, without disrupting service or migrating
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data around.
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Use cases
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---------
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Read-write pool, writeback
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~~~~~~~~~~~~~~~~~~~~~~~~~~
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We have an existing data pool and put a fast cache pool "in front" of it. Writes will
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go to the cache pool and immediately ack. We flush them back to the data pool based on
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some policy.
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Read-only pool, weak consistency
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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We have an existing data pool and add one or more read-only cache
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pools. We copy data to the cache pool(s) on read. Writes are
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forwarded to the original data pool. Stale data is expired from the
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cache pools based on some as-yet undetermined policy.
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This is likely only useful for specific applications with specific
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data access patterns. It may be a match for rgw, for example.
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Interface
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---------
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Set up a read/write cache pool foo-hot for pool foo::
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ceph osd tier add foo foo-hot
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ceph osd tier cache-mode foo-hot writeback
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Direct all traffic for foo to foo-hot::
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ceph osd tier set-overlay foo foo-hot
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Set the target size and enable the tiering agent for foo-hit::
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ceph osd pool set foo-hot hit_set_type bloom
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ceph osd pool set foo-hot hit_set_count 1
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ceph osd pool set foo-hot hit_set_period 3600 # 1 hour
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ceph osd pool set foo-hot target_max_bytes 1000000000000 # 1 TB
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Drain the cache in preparation for turning it off::
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ceph osd tier cache-mode foo-hot forward
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rados -p foo-hot cache-flush-evict-all
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When cache pool is finally empty, disable it::
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ceph osd tier remove-overlay foo
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ceph osd tier remove foo foo-hot
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Read-only pools with lazy consistency::
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ceph osd tier add foo foo-east
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ceph osd tier cache-mode foo-east readonly
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ceph osd tier add foo foo-west
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ceph osd tier cache-mode foo-west readonly
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