338 lines
13 KiB
ReStructuredText
338 lines
13 KiB
ReStructuredText
=====
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Cache
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=====
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Introduction
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============
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dm-cache is a device mapper target written by Joe Thornber, Heinz
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Mauelshagen, and Mike Snitzer.
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It aims to improve performance of a block device (eg, a spindle) by
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dynamically migrating some of its data to a faster, smaller device
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(eg, an SSD).
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This device-mapper solution allows us to insert this caching at
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different levels of the dm stack, for instance above the data device for
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a thin-provisioning pool. Caching solutions that are integrated more
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closely with the virtual memory system should give better performance.
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The target reuses the metadata library used in the thin-provisioning
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library.
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The decision as to what data to migrate and when is left to a plug-in
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policy module. Several of these have been written as we experiment,
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and we hope other people will contribute others for specific io
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scenarios (eg. a vm image server).
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Glossary
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========
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Migration
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Movement of the primary copy of a logical block from one
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device to the other.
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Promotion
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Migration from slow device to fast device.
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Demotion
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Migration from fast device to slow device.
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The origin device always contains a copy of the logical block, which
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may be out of date or kept in sync with the copy on the cache device
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(depending on policy).
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Design
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======
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Sub-devices
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-----------
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The target is constructed by passing three devices to it (along with
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other parameters detailed later):
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1. An origin device - the big, slow one.
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2. A cache device - the small, fast one.
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3. A small metadata device - records which blocks are in the cache,
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which are dirty, and extra hints for use by the policy object.
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This information could be put on the cache device, but having it
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separate allows the volume manager to configure it differently,
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e.g. as a mirror for extra robustness. This metadata device may only
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be used by a single cache device.
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Fixed block size
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----------------
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The origin is divided up into blocks of a fixed size. This block size
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is configurable when you first create the cache. Typically we've been
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using block sizes of 256KB - 1024KB. The block size must be between 64
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sectors (32KB) and 2097152 sectors (1GB) and a multiple of 64 sectors (32KB).
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Having a fixed block size simplifies the target a lot. But it is
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something of a compromise. For instance, a small part of a block may be
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getting hit a lot, yet the whole block will be promoted to the cache.
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So large block sizes are bad because they waste cache space. And small
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block sizes are bad because they increase the amount of metadata (both
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in core and on disk).
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Cache operating modes
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---------------------
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The cache has three operating modes: writeback, writethrough and
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passthrough.
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If writeback, the default, is selected then a write to a block that is
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cached will go only to the cache and the block will be marked dirty in
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the metadata.
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If writethrough is selected then a write to a cached block will not
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complete until it has hit both the origin and cache devices. Clean
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blocks should remain clean.
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If passthrough is selected, useful when the cache contents are not known
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to be coherent with the origin device, then all reads are served from
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the origin device (all reads miss the cache) and all writes are
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forwarded to the origin device; additionally, write hits cause cache
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block invalidates. To enable passthrough mode the cache must be clean.
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Passthrough mode allows a cache device to be activated without having to
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worry about coherency. Coherency that exists is maintained, although
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the cache will gradually cool as writes take place. If the coherency of
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the cache can later be verified, or established through use of the
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"invalidate_cblocks" message, the cache device can be transitioned to
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writethrough or writeback mode while still warm. Otherwise, the cache
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contents can be discarded prior to transitioning to the desired
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operating mode.
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A simple cleaner policy is provided, which will clean (write back) all
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dirty blocks in a cache. Useful for decommissioning a cache or when
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shrinking a cache. Shrinking the cache's fast device requires all cache
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blocks, in the area of the cache being removed, to be clean. If the
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area being removed from the cache still contains dirty blocks the resize
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will fail. Care must be taken to never reduce the volume used for the
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cache's fast device until the cache is clean. This is of particular
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importance if writeback mode is used. Writethrough and passthrough
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modes already maintain a clean cache. Future support to partially clean
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the cache, above a specified threshold, will allow for keeping the cache
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warm and in writeback mode during resize.
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Migration throttling
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--------------------
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Migrating data between the origin and cache device uses bandwidth.
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The user can set a throttle to prevent more than a certain amount of
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migration occurring at any one time. Currently we're not taking any
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account of normal io traffic going to the devices. More work needs
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doing here to avoid migrating during those peak io moments.
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For the time being, a message "migration_threshold <#sectors>"
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can be used to set the maximum number of sectors being migrated,
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the default being 2048 sectors (1MB).
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Updating on-disk metadata
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-------------------------
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On-disk metadata is committed every time a FLUSH or FUA bio is written.
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If no such requests are made then commits will occur every second. This
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means the cache behaves like a physical disk that has a volatile write
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cache. If power is lost you may lose some recent writes. The metadata
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should always be consistent in spite of any crash.
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The 'dirty' state for a cache block changes far too frequently for us
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to keep updating it on the fly. So we treat it as a hint. In normal
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operation it will be written when the dm device is suspended. If the
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system crashes all cache blocks will be assumed dirty when restarted.
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Per-block policy hints
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----------------------
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Policy plug-ins can store a chunk of data per cache block. It's up to
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the policy how big this chunk is, but it should be kept small. Like the
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dirty flags this data is lost if there's a crash so a safe fallback
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value should always be possible.
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Policy hints affect performance, not correctness.
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Policy messaging
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----------------
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Policies will have different tunables, specific to each one, so we
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need a generic way of getting and setting these. Device-mapper
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messages are used. Refer to cache-policies.txt.
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Discard bitset resolution
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-------------------------
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We can avoid copying data during migration if we know the block has
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been discarded. A prime example of this is when mkfs discards the
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whole block device. We store a bitset tracking the discard state of
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blocks. However, we allow this bitset to have a different block size
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from the cache blocks. This is because we need to track the discard
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state for all of the origin device (compare with the dirty bitset
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which is just for the smaller cache device).
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Target interface
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================
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Constructor
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-----------
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::
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cache <metadata dev> <cache dev> <origin dev> <block size>
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<#feature args> [<feature arg>]*
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<policy> <#policy args> [policy args]*
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================ =======================================================
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metadata dev fast device holding the persistent metadata
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cache dev fast device holding cached data blocks
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origin dev slow device holding original data blocks
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block size cache unit size in sectors
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#feature args number of feature arguments passed
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feature args writethrough or passthrough (The default is writeback.)
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policy the replacement policy to use
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#policy args an even number of arguments corresponding to
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key/value pairs passed to the policy
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policy args key/value pairs passed to the policy
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E.g. 'sequential_threshold 1024'
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See cache-policies.txt for details.
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================ =======================================================
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Optional feature arguments are:
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==================== ========================================================
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writethrough write through caching that prohibits cache block
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content from being different from origin block content.
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Without this argument, the default behaviour is to write
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back cache block contents later for performance reasons,
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so they may differ from the corresponding origin blocks.
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passthrough a degraded mode useful for various cache coherency
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situations (e.g., rolling back snapshots of
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underlying storage). Reads and writes always go to
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the origin. If a write goes to a cached origin
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block, then the cache block is invalidated.
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To enable passthrough mode the cache must be clean.
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metadata2 use version 2 of the metadata. This stores the dirty
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bits in a separate btree, which improves speed of
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shutting down the cache.
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no_discard_passdown disable passing down discards from the cache
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to the origin's data device.
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==================== ========================================================
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A policy called 'default' is always registered. This is an alias for
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the policy we currently think is giving best all round performance.
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As the default policy could vary between kernels, if you are relying on
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the characteristics of a specific policy, always request it by name.
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Status
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------
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::
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<metadata block size> <#used metadata blocks>/<#total metadata blocks>
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<cache block size> <#used cache blocks>/<#total cache blocks>
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<#read hits> <#read misses> <#write hits> <#write misses>
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<#demotions> <#promotions> <#dirty> <#features> <features>*
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<#core args> <core args>* <policy name> <#policy args> <policy args>*
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<cache metadata mode>
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========================= =====================================================
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metadata block size Fixed block size for each metadata block in
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sectors
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#used metadata blocks Number of metadata blocks used
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#total metadata blocks Total number of metadata blocks
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cache block size Configurable block size for the cache device
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in sectors
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#used cache blocks Number of blocks resident in the cache
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#total cache blocks Total number of cache blocks
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#read hits Number of times a READ bio has been mapped
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to the cache
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#read misses Number of times a READ bio has been mapped
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to the origin
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#write hits Number of times a WRITE bio has been mapped
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to the cache
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#write misses Number of times a WRITE bio has been
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mapped to the origin
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#demotions Number of times a block has been removed
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from the cache
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#promotions Number of times a block has been moved to
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the cache
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#dirty Number of blocks in the cache that differ
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from the origin
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#feature args Number of feature args to follow
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feature args 'writethrough' (optional)
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#core args Number of core arguments (must be even)
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core args Key/value pairs for tuning the core
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e.g. migration_threshold
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policy name Name of the policy
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#policy args Number of policy arguments to follow (must be even)
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policy args Key/value pairs e.g. sequential_threshold
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cache metadata mode ro if read-only, rw if read-write
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In serious cases where even a read-only mode is
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deemed unsafe no further I/O will be permitted and
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the status will just contain the string 'Fail'.
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The userspace recovery tools should then be used.
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needs_check 'needs_check' if set, '-' if not set
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A metadata operation has failed, resulting in the
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needs_check flag being set in the metadata's
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superblock. The metadata device must be
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deactivated and checked/repaired before the
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cache can be made fully operational again.
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'-' indicates needs_check is not set.
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========================= =====================================================
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Messages
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--------
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Policies will have different tunables, specific to each one, so we
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need a generic way of getting and setting these. Device-mapper
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messages are used. (A sysfs interface would also be possible.)
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The message format is::
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<key> <value>
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E.g.::
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dmsetup message my_cache 0 sequential_threshold 1024
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Invalidation is removing an entry from the cache without writing it
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back. Cache blocks can be invalidated via the invalidate_cblocks
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message, which takes an arbitrary number of cblock ranges. Each cblock
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range's end value is "one past the end", meaning 5-10 expresses a range
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of values from 5 to 9. Each cblock must be expressed as a decimal
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value, in the future a variant message that takes cblock ranges
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expressed in hexadecimal may be needed to better support efficient
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invalidation of larger caches. The cache must be in passthrough mode
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when invalidate_cblocks is used::
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invalidate_cblocks [<cblock>|<cblock begin>-<cblock end>]*
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E.g.::
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dmsetup message my_cache 0 invalidate_cblocks 2345 3456-4567 5678-6789
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Examples
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========
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The test suite can be found here:
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https://github.com/jthornber/device-mapper-test-suite
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::
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dmsetup create my_cache --table '0 41943040 cache /dev/mapper/metadata \
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/dev/mapper/ssd /dev/mapper/origin 512 1 writeback default 0'
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dmsetup create my_cache --table '0 41943040 cache /dev/mapper/metadata \
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/dev/mapper/ssd /dev/mapper/origin 1024 1 writeback \
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mq 4 sequential_threshold 1024 random_threshold 8'
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