From f283635281132af7bc7b90af3c105b8c0f73b9c7 Mon Sep 17 00:00:00 2001 From: Joe Thornber Date: Fri, 1 Mar 2013 22:45:51 +0000 Subject: 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 Signed-off-by: Alasdair G Kergon --- Documentation/device-mapper/cache-policies.txt | 72 ++++++++++++++++++++++++++ 1 file changed, 72 insertions(+) create mode 100644 Documentation/device-mapper/cache-policies.txt (limited to 'Documentation/device-mapper') 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 + <#feature_args> []* + <#policy_args> []* + +The syntax to send a message using the dmsetup command is: + dmsetup message 0 sequential_threshold 1024 + dmsetup message 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. -- cgit v1.2.3-59-g8ed1b