Based on kernel version 3.15.4. Page generated on 2014-07-07 09:00 EST.
1 Guidance for writing policies 2 ============================= 3 4 Try to keep transactionality out of it. The core is careful to 5 avoid asking about anything that is migrating. This is a pain, but 6 makes it easier to write the policies. 7 8 Mappings are loaded into the policy at construction time. 9 10 Every bio that is mapped by the target is referred to the policy. 11 The policy can return a simple HIT or MISS or issue a migration. 12 13 Currently there's no way for the policy to issue background work, 14 e.g. to start writing back dirty blocks that are going to be evicte 15 soon. 16 17 Because we map bios, rather than requests it's easy for the policy 18 to get fooled by many small bios. For this reason the core target 19 issues periodic ticks to the policy. It's suggested that the policy 20 doesn't update states (eg, hit counts) for a block more than once 21 for each tick. The core ticks by watching bios complete, and so 22 trying to see when the io scheduler has let the ios run. 23 24 25 Overview of supplied cache replacement policies 26 =============================================== 27 28 multiqueue 29 ---------- 30 31 This policy is the default. 32 33 The multiqueue policy has three sets of 16 queues: one set for entries 34 waiting for the cache and another two for those in the cache (a set for 35 clean entries and a set for dirty entries). 36 37 Cache entries in the queues are aged based on logical time. Entry into 38 the cache is based on variable thresholds and queue selection is based 39 on hit count on entry. The policy aims to take different cache miss 40 costs into account and to adjust to varying load patterns automatically. 41 42 Message and constructor argument pairs are: 43 'sequential_threshold <#nr_sequential_ios>' 44 'random_threshold <#nr_random_ios>' 45 'read_promote_adjustment <value>' 46 'write_promote_adjustment <value>' 47 'discard_promote_adjustment <value>' 48 49 The sequential threshold indicates the number of contiguous I/Os 50 required before a stream is treated as sequential. The random threshold 51 is the number of intervening non-contiguous I/Os that must be seen 52 before the stream is treated as random again. 53 54 The sequential and random thresholds default to 512 and 4 respectively. 55 56 Large, sequential ios are probably better left on the origin device 57 since spindles tend to have good bandwidth. The io_tracker counts 58 contiguous I/Os to try to spot when the io is in one of these sequential 59 modes. 60 61 Internally the mq policy maintains a promotion threshold variable. If 62 the hit count of a block not in the cache goes above this threshold it 63 gets promoted to the cache. The read, write and discard promote adjustment 64 tunables allow you to tweak the promotion threshold by adding a small 65 value based on the io type. They default to 4, 8 and 1 respectively. 66 If you're trying to quickly warm a new cache device you may wish to 67 reduce these to encourage promotion. Remember to switch them back to 68 their defaults after the cache fills though. 69 70 cleaner 71 ------- 72 73 The cleaner writes back all dirty blocks in a cache to decommission it. 74 75 Examples 76 ======== 77 78 The syntax for a table is: 79 cache <metadata dev> <cache dev> <origin dev> <block size> 80 <#feature_args> [<feature arg>]* 81 <policy> <#policy_args> [<policy arg>]* 82 83 The syntax to send a message using the dmsetup command is: 84 dmsetup message <mapped device> 0 sequential_threshold 1024 85 dmsetup message <mapped device> 0 random_threshold 8 86 87 Using dmsetup: 88 dmsetup create blah --table "0 268435456 cache /dev/sdb /dev/sdc \ 89 /dev/sdd 512 0 mq 4 sequential_threshold 1024 random_threshold 8" 90 creates a 128GB large mapped device named 'blah' with the 91 sequential threshold set to 1024 and the random_threshold set to 8.