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  may be out of date or kept in sync with the copy on the cache device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.  This metadata device may only
     be used by a single cache device.
  
  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 256KB - 1024KB.  The block size must be between 64
  (32KB) and 2097152 (1GB) and a multiple of 64 (32KB).
  
  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).
  
  Cache operating modes
  ---------------------
  
  The cache has three operating modes: writeback, writethrough and
  passthrough.
  
  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.
  
  If passthrough is selected, useful when the cache contents are not known
  to be coherent with the origin device, then all reads are served from
  the origin device (all reads miss the cache) and all writes are
  forwarded to the origin device; additionally, write hits cause cache
  block invalidates.  To enable passthrough mode the cache must be clean.
  Passthrough mode allows a cache device to be activated without having to
  worry about coherency.  Coherency that exists is maintained, although
  the cache will gradually cool as writes take place.  If the coherency of
  the cache can later be verified, or established through use of the
  "invalidate_cblocks" message, the cache device can be transitioned to
  writethrough or writeback mode while still warm.  Otherwise, the cache
  contents can be discarded prior to transitioning to the desired
  operating mode.
  
  A simple cleaner policy is provided, which will clean (write back) all
  dirty blocks in a cache.  Useful for decommissioning a cache or when
  shrinking a cache.  Shrinking the cache's fast device requires all cache
  blocks, in the area of the cache being removed, to be clean.  If the
  area being removed from the cache still contains dirty blocks the resize
  will fail.  Care must be taken to never reduce the volume used for the
  cache's fast device until the cache is clean.  This is of particular
  importance if writeback mode is used.  Writethrough and passthrough
  modes already maintain a clean cache.  Future support to partially clean
  the cache, above a specified threshold, will allow for keeping the cache
  warm and in writeback mode during resize.
  
  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 occurring 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 FLUSH or 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 volatile write
  cache.  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 or passthrough (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.
  
     passthrough	 : a degraded mode useful for various cache coherency
  		   situations (e.g., rolling back snapshots of
  		   underlying storage).	 Reads and writes always go to
  		   the origin.	If a write goes to a cached origin
  		   block, then the cache block is invalidated.
  		   To enable passthrough mode the cache must be clean.
  
  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
  ------
  
  <metadata block size> <#used metadata blocks>/<#total metadata blocks>
  <cache block size> <#used cache blocks>/<#total cache blocks>
  <#read hits> <#read misses> <#write hits> <#write misses>
  <#demotions> <#promotions> <#dirty> <#features> <features>*
  <#core args> <core args>* <policy name> <#policy args> <policy args>*
  
  metadata block size	 : Fixed block size for each metadata block in
  			     sectors
  #used metadata blocks	 : Number of metadata blocks used
  #total metadata blocks	 : Total number of metadata blocks
  cache block size	 : Configurable block size for the cache device
  			     in sectors
  #used cache blocks	 : Number of blocks resident in the cache
  #total cache blocks	 : Total number of cache 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
  #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 name		 : Name of the policy
  #policy args		 : Number of policy arguments to follow (must be even)
  policy args		 : Key/value pairs
  			     e.g. sequential_threshold
  
  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
  
  
  Invalidation is removing an entry from the cache without writing it
  back.  Cache blocks can be invalidated via the invalidate_cblocks
  message, which takes an arbitrary number of cblock ranges.  Each cblock
  range's end value is "one past the end", meaning 5-10 expresses a range
  of values from 5 to 9.  Each cblock must be expressed as a decimal
  value, in the future a variant message that takes cblock ranges
  expressed in hexidecimal may be needed to better support efficient
  invalidation of larger caches.  The cache must be in passthrough mode
  when invalidate_cblocks is used.
  
     invalidate_cblocks [<cblock>|<cblock begin>-<cblock end>]*
  
  E.g.
     dmsetup message my_cache 0 invalidate_cblocks 2345 3456-4567 5678-6789
  
  Examples
  ========
  
  The test suite can be found here:
  
  https://github.com/jthornber/device-mapper-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'