mirror of
https://github.com/torvalds/linux.git
synced 2024-11-14 16:12:02 +00:00
8735a81347
A simple cache policy that writes back all data to the origin. This is used to decommission a dm cache by emptying it. Signed-off-by: Heinz Mauelshagen <mauelshagen@redhat.com> Signed-off-by: Joe Thornber <ejt@redhat.com> Signed-off-by: Alasdair G Kergon <agk@redhat.com>
78 lines
2.9 KiB
Plaintext
78 lines
2.9 KiB
Plaintext
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.
|
|
|
|
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.
|