Based on kernel version 4.8. Page generated on 2016-10-06 23:10 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 evicted 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 (mq) 29 --------------- 30 31 This policy is now an alias for smq (see below). 32 33 The following tunables are accepted, but have no effect: 34 35 'sequential_threshold <#nr_sequential_ios>' 36 'random_threshold <#nr_random_ios>' 37 'read_promote_adjustment <value>' 38 'write_promote_adjustment <value>' 39 'discard_promote_adjustment <value>' 40 41 Stochastic multiqueue (smq) 42 --------------------------- 43 44 This policy is the default. 45 46 The stochastic multi-queue (smq) policy addresses some of the problems 47 with the multiqueue (mq) policy. 48 49 The smq policy (vs mq) offers the promise of less memory utilization, 50 improved performance and increased adaptability in the face of changing 51 workloads. smq also does not have any cumbersome tuning knobs. 52 53 Users may switch from "mq" to "smq" simply by appropriately reloading a 54 DM table that is using the cache target. Doing so will cause all of the 55 mq policy's hints to be dropped. Also, performance of the cache may 56 degrade slightly until smq recalculates the origin device's hotspots 57 that should be cached. 58 59 Memory usage: 60 The mq policy used a lot of memory; 88 bytes per cache block on a 64 61 bit machine. 62 63 smq uses 28bit indexes to implement it's data structures rather than 64 pointers. It avoids storing an explicit hit count for each block. It 65 has a 'hotspot' queue, rather than a pre-cache, which uses a quarter of 66 the entries (each hotspot block covers a larger area than a single 67 cache block). 68 69 All this means smq uses ~25bytes per cache block. Still a lot of 70 memory, but a substantial improvement nontheless. 71 72 Level balancing: 73 mq placed entries in different levels of the multiqueue structures 74 based on their hit count (~ln(hit count)). This meant the bottom 75 levels generally had the most entries, and the top ones had very 76 few. Having unbalanced levels like this reduced the efficacy of the 77 multiqueue. 78 79 smq does not maintain a hit count, instead it swaps hit entries with 80 the least recently used entry from the level above. The overall 81 ordering being a side effect of this stochastic process. With this 82 scheme we can decide how many entries occupy each multiqueue level, 83 resulting in better promotion/demotion decisions. 84 85 Adaptability: 86 The mq policy maintained a hit count for each cache block. For a 87 different block to get promoted to the cache it's hit count has to 88 exceed the lowest currently in the cache. This meant it could take a 89 long time for the cache to adapt between varying IO patterns. 90 91 smq doesn't maintain hit counts, so a lot of this problem just goes 92 away. In addition it tracks performance of the hotspot queue, which 93 is used to decide which blocks to promote. If the hotspot queue is 94 performing badly then it starts moving entries more quickly between 95 levels. This lets it adapt to new IO patterns very quickly. 96 97 Performance: 98 Testing smq shows substantially better performance than mq. 99 100 cleaner 101 ------- 102 103 The cleaner writes back all dirty blocks in a cache to decommission it. 104 105 Examples 106 ======== 107 108 The syntax for a table is: 109 cache <metadata dev> <cache dev> <origin dev> <block size> 110 <#feature_args> [<feature arg>]* 111 <policy> <#policy_args> [<policy arg>]* 112 113 The syntax to send a message using the dmsetup command is: 114 dmsetup message <mapped device> 0 sequential_threshold 1024 115 dmsetup message <mapped device> 0 random_threshold 8 116 117 Using dmsetup: 118 dmsetup create blah --table "0 268435456 cache /dev/sdb /dev/sdc \ 119 /dev/sdd 512 0 mq 4 sequential_threshold 1024 random_threshold 8" 120 creates a 128GB large mapped device named 'blah' with the 121 sequential threshold set to 1024 and the random_threshold set to 8.