Documentation/admin-guide/device-mapper/cache-policies.rst

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Documentation/admin-guide/device-mapper/cache-policies.rst
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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 evicted
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 (mq)
---------------

This policy is now an alias for smq (see below).

The following tunables are accepted, but have no effect::

	'sequential_threshold <#nr_sequential_ios>'
	'random_threshold <#nr_random_ios>'
	'read_promote_adjustment <value>'
	'write_promote_adjustment <value>'
	'discard_promote_adjustment <value>'

Stochastic multiqueue (smq)
---------------------------

This policy is the default.

The stochastic multi-queue (smq) policy addresses some of the problems
with the multiqueue (mq) policy.

The smq policy (vs mq) offers the promise of less memory utilization,
improved performance and increased adaptability in the face of changing
workloads.  smq also does not have any cumbersome tuning knobs.

Users may switch from "mq" to "smq" simply by appropriately reloading a
DM table that is using the cache target.  Doing so will cause all of the
mq policy's hints to be dropped.  Also, performance of the cache may
degrade slightly until smq recalculates the origin device's hotspots
that should be cached.

Memory usage
^^^^^^^^^^^^

The mq policy used a lot of memory; 88 bytes per cache block on a 64
bit machine.

smq uses 28bit indexes to implement its data structures rather than
pointers.  It avoids storing an explicit hit count for each block.  It
has a 'hotspot' queue, rather than a pre-cache, which uses a quarter of
the entries (each hotspot block covers a larger area than a single
cache block).

Annotation

Implementation Notes