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https://github.com/ceph/ceph
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201 lines
7.0 KiB
ReStructuredText
201 lines
7.0 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 slower and larger pool.
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Use a replicated pool as a front-end to service most I/O, and destage
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cold data to a separate erasure coded pool that does not currently (and
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cannot efficiently) handle the workload.
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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
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it. Writes will go to the cache pool and immediately ack. We flush
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them back to the data pool based on the defined 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 the defined 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-hot::
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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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ceph osd pool set foo-hot min_read_recency_for_promote 1
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ceph osd pool set foo-hot min_write_recency_for_promote 1
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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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Tiering agent
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-------------
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The tiering policy is defined as properties on the cache pool itself.
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HitSet metadata
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~~~~~~~~~~~~~~~
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First, the agent requires HitSet information to be tracked on the
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cache pool in order to determine which objects in the pool are being
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accessed. This is enabled with::
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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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The supported HitSet types include 'bloom' (a bloom filter, the
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default), 'explicit_hash', and 'explicit_object'. The latter two
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explicitly enumerate accessed objects and are less memory efficient.
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They are there primarily for debugging and to demonstrate pluggability
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for the infrastructure. For the bloom filter type, you can additionally
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define the false positive probability for the bloom filter (default is 0.05)::
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ceph osd pool set foo-hot hit_set_fpp 0.15
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The hit_set_count and hit_set_period define how much time each HitSet
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should cover, and how many such HitSets to store. Binning accesses
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over time allows Ceph to independently determine whether an object was
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accessed at least once and whether it was accessed more than once over
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some time period ("age" vs "temperature").
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The ``min_read_recency_for_promote`` defines how many HitSets to check for the
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existence of an object when handling a read operation. The checking result is
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used to decide whether to promote the object asynchronously. Its value should be
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between 0 and ``hit_set_count``. If it's set to 0, the object is always promoted.
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If it's set to 1, the current HitSet is checked. And if this object is in the
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current HitSet, it's promoted. Otherwise not. For the other values, the exact
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number of archive HitSets are checked. The object is promoted if the object is
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found in any of the most recent ``min_read_recency_for_promote`` HitSets.
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A similar parameter can be set for the write operation, which is
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``min_write_recency_for_promote``. ::
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ceph osd pool set {cachepool} min_read_recency_for_promote 1
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ceph osd pool set {cachepool} min_write_recency_for_promote 1
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Note that the longer the ``hit_set_period`` and the higher the
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``min_read_recency_for_promote``/``min_write_recency_for_promote`` the more RAM
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will be consumed by the ceph-osd process. In particular, when the agent is active
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to flush or evict cache objects, all hit_set_count HitSets are loaded into RAM.
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Cache mode
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~~~~~~~~~~
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The most important policy is the cache mode:
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ceph osd pool set foo-hot cache-mode writeback
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The supported modes are 'none', 'writeback', 'forward', and
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'readonly'. Most installations want 'writeback', which will write
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into the cache tier and only later flush updates back to the base
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tier. Similarly, any object that is read will be promoted into the
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cache tier.
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The 'forward' mode is intended for when the cache is being disabled
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and needs to be drained. No new objects will be promoted or written
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to the cache pool unless they are already present. A background
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operation can then do something like::
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rados -p foo-hot cache-try-flush-evict-all
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rados -p foo-hot cache-flush-evict-all
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to force all data to be flushed back to the base tier.
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The 'readonly' mode is intended for read-only workloads that do not
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require consistency to be enforced by the storage system. Writes will
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be forwarded to the base tier, but objects that are read will get
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promoted to the cache. No attempt is made by Ceph to ensure that the
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contents of the cache tier(s) are consistent in the presence of object
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updates.
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Cache sizing
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~~~~~~~~~~~~
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The agent performs two basic functions: flushing (writing 'dirty'
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cache objects back to the base tier) and evicting (removing cold and
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clean objects from the cache).
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The thresholds at which Ceph will flush or evict objects is specified
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relative to a 'target size' of the pool. For example::
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ceph osd pool set foo-hot cache_target_dirty_ratio .4
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ceph osd pool set foo-hot cache_target_dirty_high_ratio .6
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ceph osd pool set foo-hot cache_target_full_ratio .8
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will begin flushing dirty objects when 40% of the pool is dirty and begin
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evicting clean objects when we reach 80% of the target size.
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The target size can be specified either in terms of objects or bytes::
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ceph osd pool set foo-hot target_max_bytes 1000000000000 # 1 TB
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ceph osd pool set foo-hot target_max_objects 1000000 # 1 million objects
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Note that if both limits are specified, Ceph will begin flushing or
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evicting when either threshold is triggered.
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Other tunables
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~~~~~~~~~~~~~~
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You can specify a minimum object age before a recently updated object is
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flushed to the base tier::
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ceph osd pool set foo-hot cache_min_flush_age 600 # 10 minutes
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You can specify the minimum age of an object before it will be evicted from
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the cache tier::
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ceph osd pool set foo-hot cache_min_evict_age 1800 # 30 minutes
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