159 строки
6.6 KiB
Plaintext
159 строки
6.6 KiB
Plaintext
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 (mq)
|
|
---------------
|
|
|
|
This policy has been deprecated in favor of the smq policy (see below).
|
|
|
|
The multiqueue policy has three sets of 16 queues: one set for entries
|
|
waiting for the cache and another two for those in the cache (a set for
|
|
clean entries and a set for dirty entries).
|
|
|
|
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>'
|
|
'random_threshold <#nr_random_ios>'
|
|
'read_promote_adjustment <value>'
|
|
'write_promote_adjustment <value>'
|
|
'discard_promote_adjustment <value>'
|
|
|
|
The sequential threshold indicates the number of contiguous I/Os
|
|
required before a stream is treated as sequential. Once a stream is
|
|
considered sequential it will bypass the cache. 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 I/Os are probably better left on the origin device
|
|
since spindles tend to have good sequential I/O bandwidth. The
|
|
io_tracker counts contiguous I/Os to try to spot when the I/O is in one
|
|
of these sequential modes. But there are use-cases for wanting to
|
|
promote sequential blocks to the cache (e.g. fast application startup).
|
|
If sequential threshold is set to 0 the sequential I/O detection is
|
|
disabled and sequential I/O will no longer implicitly bypass the cache.
|
|
Setting the random threshold to 0 does _not_ disable the random I/O
|
|
stream detection.
|
|
|
|
Internally the mq policy determines a promotion threshold. If the hit
|
|
count of a block not in the cache goes above this threshold it gets
|
|
promoted to the cache. The read, write and discard promote adjustment
|
|
tunables allow you to tweak the promotion threshold by adding a small
|
|
value based on the io type. They default to 4, 8 and 1 respectively.
|
|
If you're trying to quickly warm a new cache device you may wish to
|
|
reduce these to encourage promotion. Remember to switch them back to
|
|
their defaults after the cache fills though.
|
|
|
|
Stochastic multiqueue (smq)
|
|
---------------------------
|
|
|
|
This policy is the default.
|
|
|
|
The stochastic multi-queue (smq) policy addresses some of the problems
|
|
with the multiqueue (mq) policy.
|
|
|
|
The smq policy (vs mq) offers the promise of less memory utilization,
|
|
improved performance and increased adaptability in the face of changing
|
|
workloads. SMQ also does not have any cumbersome tuning knobs.
|
|
|
|
Users may switch from "mq" to "smq" simply by appropriately reloading a
|
|
DM table that is using the cache target. Doing so will cause all of the
|
|
mq policy's hints to be dropped. Also, performance of the cache may
|
|
degrade slightly until smq recalculates the origin device's hotspots
|
|
that should be cached.
|
|
|
|
Memory usage:
|
|
The mq policy uses a lot of memory; 88 bytes per cache block on a 64
|
|
bit machine.
|
|
|
|
SMQ uses 28bit indexes to implement it's data structures rather than
|
|
pointers. It avoids storing an explicit hit count for each block. It
|
|
has a 'hotspot' queue rather than a pre cache which uses a quarter of
|
|
the entries (each hotspot block covers a larger area than a single
|
|
cache block).
|
|
|
|
All these mean smq uses ~25bytes per cache block. Still a lot of
|
|
memory, but a substantial improvement nontheless.
|
|
|
|
Level balancing:
|
|
MQ places entries in different levels of the multiqueue structures
|
|
based on their hit count (~ln(hit count)). This means the bottom
|
|
levels generally have the most entries, and the top ones have very
|
|
few. Having unbalanced levels like this reduces the efficacy of the
|
|
multiqueue.
|
|
|
|
SMQ does not maintain a hit count, instead it swaps hit entries with
|
|
the least recently used entry from the level above. The over all
|
|
ordering being a side effect of this stochastic process. With this
|
|
scheme we can decide how many entries occupy each multiqueue level,
|
|
resulting in better promotion/demotion decisions.
|
|
|
|
Adaptability:
|
|
The MQ policy maintains a hit count for each cache block. For a
|
|
different block to get promoted to the cache it's hit count has to
|
|
exceed the lowest currently in the cache. This means it can take a
|
|
long time for the cache to adapt between varying IO patterns.
|
|
Periodically degrading the hit counts could help with this, but I
|
|
haven't found a nice general solution.
|
|
|
|
SMQ doesn't maintain hit counts, so a lot of this problem just goes
|
|
away. In addition it tracks performance of the hotspot queue, which
|
|
is used to decide which blocks to promote. If the hotspot queue is
|
|
performing badly then it starts moving entries more quickly between
|
|
levels. This lets it adapt to new IO patterns very quickly.
|
|
|
|
Performance:
|
|
Testing SMQ shows substantially better performance than MQ.
|
|
|
|
cleaner
|
|
-------
|
|
|
|
The cleaner writes back all dirty blocks in a cache to decommission it.
|
|
|
|
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.
|