Add the rt linux 4.1.3-rt3 as base

[kvmfornfv.git] / kernel / Documentation / device-mapper / cache-policies.txt

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+Guidance for writing policies
+=============================
+
+Try to keep transactionality out of it.  The core is careful to
+avoid asking about anything that is migrating.  This is a pain, but
+makes it easier to write the policies.
+
+Mappings are loaded into the policy at construction time.
+
+Every bio that is mapped by the target is referred to the policy.
+The policy can return a simple HIT or MISS or issue a migration.
+
+Currently there's no way for the policy to issue background work,
+e.g. to start writing back dirty blocks that are going to be evicte
+soon.
+
+Because we map bios, rather than requests it's easy for the policy
+to get fooled by many small bios.  For this reason the core target
+issues periodic ticks to the policy.  It's suggested that the policy
+doesn't update states (eg, hit counts) for a block more than once
+for each tick.  The core ticks by watching bios complete, and so
+trying to see when the io scheduler has let the ios run.
+
+
+Overview of supplied cache replacement policies
+===============================================
+
+multiqueue
+----------
+
+This policy is the default.
+
+The multiqueue policy has three sets of 16 queues: one set for entries
+waiting for the cache and another two for those in the cache (a set for
+clean entries and a set for dirty entries).
+
+Cache entries in the queues are aged based on logical time. Entry into
+the cache is based on variable thresholds and queue selection is based
+on hit count on entry. The policy aims to take different cache miss
+costs into account and to adjust to varying load patterns automatically.
+
+Message and constructor argument pairs are:
+       'sequential_threshold <#nr_sequential_ios>'
+       'random_threshold <#nr_random_ios>'
+       'read_promote_adjustment <value>'
+       'write_promote_adjustment <value>'
+       'discard_promote_adjustment <value>'
+
+The sequential threshold indicates the number of contiguous I/Os
+required before a stream is treated as sequential.  Once a stream is
+considered sequential it will bypass the cache.  The random threshold
+is the number of intervening non-contiguous I/Os that must be seen
+before the stream is treated as random again.
+
+The sequential and random thresholds default to 512 and 4 respectively.
+
+Large, sequential I/Os are probably better left on the origin device
+since spindles tend to have good sequential I/O bandwidth.  The
+io_tracker counts contiguous I/Os to try to spot when the I/O is in one
+of these sequential modes.  But there are use-cases for wanting to
+promote sequential blocks to the cache (e.g. fast application startup).
+If sequential threshold is set to 0 the sequential I/O detection is
+disabled and sequential I/O will no longer implicitly bypass the cache.
+Setting the random threshold to 0 does _not_ disable the random I/O
+stream detection.
+
+Internally the mq policy determines a promotion threshold.  If the hit
+count of a block not in the cache goes above this threshold it gets
+promoted to the cache.  The read, write and discard promote adjustment
+tunables allow you to tweak the promotion threshold by adding a small
+value based on the io type.  They default to 4, 8 and 1 respectively.
+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.
+
+cleaner
+-------
+
+The cleaner writes back all dirty blocks in a cache to decommission it.
+
+Examples
+========
+
+The syntax for a table is:
+       cache <metadata dev> <cache dev> <origin dev> <block size>
+       <#feature_args> [<feature arg>]*
+       <policy> <#policy_args> [<policy arg>]*
+
+The syntax to send a message using the dmsetup command is:
+       dmsetup message <mapped device> 0 sequential_threshold 1024
+       dmsetup message <mapped device> 0 random_threshold 8
+
+Using dmsetup:
+       dmsetup create blah --table "0 268435456 cache /dev/sdb /dev/sdc \
+           /dev/sdd 512 0 mq 4 sequential_threshold 1024 random_threshold 8"
+       creates a 128GB large mapped device named 'blah' with the
+       sequential threshold set to 1024 and the random_threshold set to 8.