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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 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
modes.

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.