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