121 lines
4.5 KiB
Text
121 lines
4.5 KiB
Text
Guidance for writing policies
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=============================
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Try to keep transactionality out of it. The core is careful to
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avoid asking about anything that is migrating. This is a pain, but
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makes it easier to write the policies.
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Mappings are loaded into the policy at construction time.
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Every bio that is mapped by the target is referred to the policy.
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The policy can return a simple HIT or MISS or issue a migration.
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Currently there's no way for the policy to issue background work,
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e.g. to start writing back dirty blocks that are going to be evicted
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soon.
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Because we map bios, rather than requests it's easy for the policy
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to get fooled by many small bios. For this reason the core target
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issues periodic ticks to the policy. It's suggested that the policy
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doesn't update states (eg, hit counts) for a block more than once
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for each tick. The core ticks by watching bios complete, and so
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trying to see when the io scheduler has let the ios run.
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Overview of supplied cache replacement policies
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===============================================
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multiqueue (mq)
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---------------
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This policy is now an alias for smq (see below).
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The following tunables are accepted, but have no effect:
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'sequential_threshold <#nr_sequential_ios>'
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'random_threshold <#nr_random_ios>'
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'read_promote_adjustment <value>'
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'write_promote_adjustment <value>'
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'discard_promote_adjustment <value>'
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Stochastic multiqueue (smq)
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---------------------------
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This policy is the default.
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The stochastic multi-queue (smq) policy addresses some of the problems
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with the multiqueue (mq) policy.
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The smq policy (vs mq) offers the promise of less memory utilization,
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improved performance and increased adaptability in the face of changing
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workloads. smq also does not have any cumbersome tuning knobs.
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Users may switch from "mq" to "smq" simply by appropriately reloading a
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DM table that is using the cache target. Doing so will cause all of the
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mq policy's hints to be dropped. Also, performance of the cache may
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degrade slightly until smq recalculates the origin device's hotspots
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that should be cached.
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Memory usage:
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The mq policy used a lot of memory; 88 bytes per cache block on a 64
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bit machine.
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smq uses 28bit indexes to implement it's data structures rather than
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pointers. It avoids storing an explicit hit count for each block. It
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has a 'hotspot' queue, rather than a pre-cache, which uses a quarter of
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the entries (each hotspot block covers a larger area than a single
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cache block).
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All this means smq uses ~25bytes per cache block. Still a lot of
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memory, but a substantial improvement nontheless.
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Level balancing:
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mq placed entries in different levels of the multiqueue structures
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based on their hit count (~ln(hit count)). This meant the bottom
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levels generally had the most entries, and the top ones had very
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few. Having unbalanced levels like this reduced the efficacy of the
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multiqueue.
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smq does not maintain a hit count, instead it swaps hit entries with
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the least recently used entry from the level above. The overall
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ordering being a side effect of this stochastic process. With this
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scheme we can decide how many entries occupy each multiqueue level,
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resulting in better promotion/demotion decisions.
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Adaptability:
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The mq policy maintained a hit count for each cache block. For a
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different block to get promoted to the cache it's hit count has to
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exceed the lowest currently in the cache. This meant it could take a
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long time for the cache to adapt between varying IO patterns.
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smq doesn't maintain hit counts, so a lot of this problem just goes
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away. In addition it tracks performance of the hotspot queue, which
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is used to decide which blocks to promote. If the hotspot queue is
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performing badly then it starts moving entries more quickly between
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levels. This lets it adapt to new IO patterns very quickly.
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Performance:
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Testing smq shows substantially better performance than mq.
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cleaner
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-------
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The cleaner writes back all dirty blocks in a cache to decommission it.
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Examples
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========
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The syntax for a table is:
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cache <metadata dev> <cache dev> <origin dev> <block size>
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<#feature_args> [<feature arg>]*
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<policy> <#policy_args> [<policy arg>]*
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The syntax to send a message using the dmsetup command is:
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dmsetup message <mapped device> 0 sequential_threshold 1024
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dmsetup message <mapped device> 0 random_threshold 8
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Using dmsetup:
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dmsetup create blah --table "0 268435456 cache /dev/sdb /dev/sdc \
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/dev/sdd 512 0 mq 4 sequential_threshold 1024 random_threshold 8"
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creates a 128GB large mapped device named 'blah' with the
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sequential threshold set to 1024 and the random_threshold set to 8.
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