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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
+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
+This policy is the default.
+The multiqueue policy has two sets of 16 queues: one set for entries
+waiting for the cache and another one for those in the cache.
+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>' and
+ 'random_threshold <#nr_random_ios>'.
+The sequential threshold indicates the number of contiguous I/Os
+required before a stream is treated as sequential. 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 ios are probably better left on the origin device
+since spindles tend to have good bandwidth. The io_tracker counts
+contiguous I/Os to try to spot when the io is in one of these sequential
+The cleaner writes back all dirty blocks in a cache to decommission it.
+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
+ 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.