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authorJoe Thornber <ejt@redhat.com>2013-03-01 22:45:51 +0000
committerAlasdair G Kergon <agk@redhat.com>2013-03-01 22:45:51 +0000
commitc6b4fcbad044e6fffcc75bba160e720eb8d67d17 (patch)
tree1fb20e6ca157ebfbf2c97ae022fc6ba3e0550dd6 /Documentation
parentdm persistent data: add bitset (diff)
downloadlinux-dev-c6b4fcbad044e6fffcc75bba160e720eb8d67d17.tar.xz
linux-dev-c6b4fcbad044e6fffcc75bba160e720eb8d67d17.zip
dm: add cache target
Add a target that allows a fast device such as an SSD to be used as a cache for a slower device such as a disk. A plug-in architecture was chosen so that the decisions about which data to migrate and when are delegated to interchangeable tunable policy modules. The first general purpose module we have developed, called "mq" (multiqueue), follows in the next patch. Other modules are under development. Signed-off-by: Joe Thornber <ejt@redhat.com> Signed-off-by: Heinz Mauelshagen <mauelshagen@redhat.com> Signed-off-by: Mike Snitzer <snitzer@redhat.com> Signed-off-by: Alasdair G Kergon <agk@redhat.com>
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+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+may be out of date or kept in sync with the copy on the cache device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'