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diff --git a/Documentation/device-mapper/cache.txt b/Documentation/device-mapper/cache.txt new file mode 100644 index 0000000..f50470a --- /dev/null +++ b/Documentation/device-mapper/cache.txt @@ -0,0 +1,243 @@ +Introduction +============ + +dm-cache is a device mapper target written by Joe Thornber, Heinz +Mauelshagen, and Mike Snitzer. + +It aims to improve performance of a block device (eg, a spindle) by +dynamically migrating some of its data to a faster, smaller device +(eg, an SSD). + +This device-mapper solution allows us to insert this caching at +different levels of the dm stack, for instance above the data device for +a thin-provisioning pool. Caching solutions that are integrated more +closely with the virtual memory system should give better performance. + +The target reuses the metadata library used in the thin-provisioning +library. + +The decision as to what data to migrate and when is left to a plug-in +policy module. Several of these have been written as we experiment, +and we hope other people will contribute others for specific io +scenarios (eg. a vm image server). + +Glossary +======== + + Migration - Movement of the primary copy of a logical block from one + device to the other. + Promotion - Migration from slow device to fast device. + Demotion - Migration from fast device to slow device. + +The origin device always contains a copy of the logical block, which +may be out of date or kept in sync with the copy on the cache device +(depending on policy). + +Design +====== + +Sub-devices +----------- + +The target is constructed by passing three devices to it (along with +other parameters detailed later): + +1. An origin device - the big, slow one. + +2. A cache device - the small, fast one. + +3. A small metadata device - records which blocks are in the cache, + which are dirty, and extra hints for use by the policy object. + This information could be put on the cache device, but having it + separate allows the volume manager to configure it differently, + e.g. as a mirror for extra robustness. + +Fixed block size +---------------- + +The origin is divided up into blocks of a fixed size. This block size +is configurable when you first create the cache. Typically we've been +using block sizes of 256k - 1024k. + +Having a fixed block size simplifies the target a lot. But it is +something of a compromise. For instance, a small part of a block may be +getting hit a lot, yet the whole block will be promoted to the cache. +So large block sizes are bad because they waste cache space. And small +block sizes are bad because they increase the amount of metadata (both +in core and on disk). + +Writeback/writethrough +---------------------- + +The cache has two modes, writeback and writethrough. + +If writeback, the default, is selected then a write to a block that is +cached will go only to the cache and the block will be marked dirty in +the metadata. + +If writethrough is selected then a write to a cached block will not +complete until it has hit both the origin and cache devices. Clean +blocks should remain clean. + +A simple cleaner policy is provided, which will clean (write back) all +dirty blocks in a cache. Useful for decommissioning a cache. + +Migration throttling +-------------------- + +Migrating data between the origin and cache device uses bandwidth. +The user can set a throttle to prevent more than a certain amount of +migration occuring at any one time. Currently we're not taking any +account of normal io traffic going to the devices. More work needs +doing here to avoid migrating during those peak io moments. + +For the time being, a message "migration_threshold <#sectors>" +can be used to set the maximum number of sectors being migrated, +the default being 204800 sectors (or 100MB). + +Updating on-disk metadata +------------------------- + +On-disk metadata is committed every time a REQ_SYNC or REQ_FUA bio is +written. If no such requests are made then commits will occur every +second. This means the cache behaves like a physical disk that has a +write cache (the same is true of the thin-provisioning target). If +power is lost you may lose some recent writes. The metadata should +always be consistent in spite of any crash. + +The 'dirty' state for a cache block changes far too frequently for us +to keep updating it on the fly. So we treat it as a hint. In normal +operation it will be written when the dm device is suspended. If the +system crashes all cache blocks will be assumed dirty when restarted. + +Per-block policy hints +---------------------- + +Policy plug-ins can store a chunk of data per cache block. It's up to +the policy how big this chunk is, but it should be kept small. Like the +dirty flags this data is lost if there's a crash so a safe fallback +value should always be possible. + +For instance, the 'mq' policy, which is currently the default policy, +uses this facility to store the hit count of the cache blocks. If +there's a crash this information will be lost, which means the cache +may be less efficient until those hit counts are regenerated. + +Policy hints affect performance, not correctness. + +Policy messaging +---------------- + +Policies will have different tunables, specific to each one, so we +need a generic way of getting and setting these. Device-mapper +messages are used. Refer to cache-policies.txt. + +Discard bitset resolution +------------------------- + +We can avoid copying data during migration if we know the block has +been discarded. A prime example of this is when mkfs discards the +whole block device. We store a bitset tracking the discard state of +blocks. However, we allow this bitset to have a different block size +from the cache blocks. This is because we need to track the discard +state for all of the origin device (compare with the dirty bitset +which is just for the smaller cache device). + +Target interface +================ + +Constructor +----------- + + cache <metadata dev> <cache dev> <origin dev> <block size> + <#feature args> [<feature arg>]* + <policy> <#policy args> [policy args]* + + metadata dev : fast device holding the persistent metadata + cache dev : fast device holding cached data blocks + origin dev : slow device holding original data blocks + block size : cache unit size in sectors + + #feature args : number of feature arguments passed + feature args : writethrough. (The default is writeback.) + + policy : the replacement policy to use + #policy args : an even number of arguments corresponding to + key/value pairs passed to the policy + policy args : key/value pairs passed to the policy + E.g. 'sequential_threshold 1024' + See cache-policies.txt for details. + +Optional feature arguments are: + writethrough : write through caching that prohibits cache block + content from being different from origin block content. + Without this argument, the default behaviour is to write + back cache block contents later for performance reasons, + so they may differ from the corresponding origin blocks. + +A policy called 'default' is always registered. This is an alias for +the policy we currently think is giving best all round performance. + +As the default policy could vary between kernels, if you are relying on +the characteristics of a specific policy, always request it by name. + +Status +------ + +<#used metadata blocks>/<#total metadata blocks> <#read hits> <#read misses> +<#write hits> <#write misses> <#demotions> <#promotions> <#blocks in cache> +<#dirty> <#features> <features>* <#core args> <core args>* <#policy args> +<policy args>* + +#used metadata blocks : Number of metadata blocks used +#total metadata blocks : Total number of metadata blocks +#read hits : Number of times a READ bio has been mapped + to the cache +#read misses : Number of times a READ bio has been mapped + to the origin +#write hits : Number of times a WRITE bio has been mapped + to the cache +#write misses : Number of times a WRITE bio has been + mapped to the origin +#demotions : Number of times a block has been removed + from the cache +#promotions : Number of times a block has been moved to + the cache +#blocks in cache : Number of blocks resident in the cache +#dirty : Number of blocks in the cache that differ + from the origin +#feature args : Number of feature args to follow +feature args : 'writethrough' (optional) +#core args : Number of core arguments (must be even) +core args : Key/value pairs for tuning the core + e.g. migration_threshold +#policy args : Number of policy arguments to follow (must be even) +policy args : Key/value pairs + e.g. 'sequential_threshold 1024 + +Messages +-------- + +Policies will have different tunables, specific to each one, so we +need a generic way of getting and setting these. Device-mapper +messages are used. (A sysfs interface would also be possible.) + +The message format is: + + <key> <value> + +E.g. + dmsetup message my_cache 0 sequential_threshold 1024 + +Examples +======== + +The test suite can be found here: + +https://github.com/jthornber/thinp-test-suite + +dmsetup create my_cache --table '0 41943040 cache /dev/mapper/metadata \ + /dev/mapper/ssd /dev/mapper/origin 512 1 writeback default 0' +dmsetup create my_cache --table '0 41943040 cache /dev/mapper/metadata \ + /dev/mapper/ssd /dev/mapper/origin 1024 1 writeback \ + mq 4 sequential_threshold 1024 random_threshold 8' |
