dm cache: add mq policy

A cache policy that uses a multiqueue ordered by recent hit
count to select which blocks should be promoted and demoted.
This is meant to be a general purpose policy.  It prioritises
reads over writes.

Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

1 files changed, 72 insertions, 0 deletions

diff --git a/Documentation/device-mapper/cache-policies.txt b/Documentation/device-mapper/cache-policies.txt
new file mode 100644
index 000000000000..731879f97b80
--- /dev/null
+++ b/Documentation/device-mapper/cache-policies.txt
@@ -0,0 +1,72 @@
+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.