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![Joe Thornber](/assets/img/avatar_default.png)
Internally the mq policy maintains a promotion threshold variable. If the hit count of a block not in the cache goes above this threshold it gets promoted to the cache. This patch introduces three new tunables that allow you to tweak the promotion threshold by adding a small value. These adjustments depend on the io type: read_promote_adjustment: READ io, default 4 write_promote_adjustment: WRITE io, default 8 discard_promote_adjustment: READ/WRITE io to a discarded block, default 1 If you're trying to quickly warm a new cache device you may wish to reduce these to encourage promotion. Remember to switch them back to their defaults after the cache fills though. Signed-off-by: Joe Thornber <ejt@redhat.com> Signed-off-by: Mike Snitzer <snitzer@redhat.com>
91 lines
3.5 KiB
Text
91 lines
3.5 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 evicte
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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
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----------
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This policy is the default.
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The multiqueue policy has three sets of 16 queues: one set for entries
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waiting for the cache and another two for those in the cache (a set for
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clean entries and a set for dirty entries).
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Cache entries in the queues are aged based on logical time. Entry into
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the cache is based on variable thresholds and queue selection is based
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on hit count on entry. The policy aims to take different cache miss
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costs into account and to adjust to varying load patterns automatically.
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Message and constructor argument pairs are:
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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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The sequential threshold indicates the number of contiguous I/Os
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required before a stream is treated as sequential. The random threshold
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is the number of intervening non-contiguous I/Os that must be seen
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before the stream is treated as random again.
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The sequential and random thresholds default to 512 and 4 respectively.
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Large, sequential ios are probably better left on the origin device
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since spindles tend to have good bandwidth. The io_tracker counts
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contiguous I/Os to try to spot when the io is in one of these sequential
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modes.
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Internally the mq policy maintains a promotion threshold variable. If
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the hit count of a block not in the cache goes above this threshold it
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gets promoted to the cache. The read, write and discard promote adjustment
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tunables allow you to tweak the promotion threshold by adding a small
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value based on the io type. They default to 4, 8 and 1 respectively.
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If you're trying to quickly warm a new cache device you may wish to
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reduce these to encourage promotion. Remember to switch them back to
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their defaults after the cache fills though.
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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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