Introduction
  ============
  
  This document describes a collection of device-mapper targets that
  between them implement thin-provisioning and snapshots.
  
  The main highlight of this implementation, compared to the previous
  implementation of snapshots, is that it allows many virtual devices to
  be stored on the same data volume.  This simplifies administration and
  allows the sharing of data between volumes, thus reducing disk usage.
  
  Another significant feature is support for an arbitrary depth of
  recursive snapshots (snapshots of snapshots of snapshots ...).  The
  previous implementation of snapshots did this by chaining together
  lookup tables, and so performance was O(depth).  This new
  implementation uses a single data structure to avoid this degradation
  with depth.  Fragmentation may still be an issue, however, in some
  scenarios.
  
  Metadata is stored on a separate device from data, giving the
  administrator some freedom, for example to:
  
  - Improve metadata resilience by storing metadata on a mirrored volume
    but data on a non-mirrored one.
  
  - Improve performance by storing the metadata on SSD.
  
  Status
  ======
  
  These targets are very much still in the EXPERIMENTAL state.  Please
  do not yet rely on them in production.  But do experiment and offer us
  feedback.  Different use cases will have different performance
  characteristics, for example due to fragmentation of the data volume.
  
  If you find this software is not performing as expected please mail
  dm-devel@redhat.com with details and we'll try our best to improve
  things for you.
  
  Userspace tools for checking and repairing the metadata are under
  development.
  
  Cookbook
  ========
  
  This section describes some quick recipes for using thin provisioning.
  They use the dmsetup program to control the device-mapper driver
  directly.  End users will be advised to use a higher-level volume
  manager such as LVM2 once support has been added.
  
  Pool device
  -----------
  
  The pool device ties together the metadata volume and the data volume.
  It maps I/O linearly to the data volume and updates the metadata via
  two mechanisms:
  
  - Function calls from the thin targets
  
  - Device-mapper 'messages' from userspace which control the creation of new
    virtual devices amongst other things.
  
  Setting up a fresh pool device
  ------------------------------
  
  Setting up a pool device requires a valid metadata device, and a
  data device.  If you do not have an existing metadata device you can
  make one by zeroing the first 4k to indicate empty metadata.
  
      dd if=/dev/zero of=$metadata_dev bs=4096 count=1
  
  The amount of metadata you need will vary according to how many blocks
  are shared between thin devices (i.e. through snapshots).  If you have
  less sharing than average you'll need a larger-than-average metadata device.
  
  As a guide, we suggest you calculate the number of bytes to use in the
  metadata device as 48 * $data_dev_size / $data_block_size but round it up
  to 2MB if the answer is smaller.  If you're creating large numbers of
  snapshots which are recording large amounts of change, you may find you
  need to increase this.
  
  The largest size supported is 16GB: If the device is larger,
  a warning will be issued and the excess space will not be used.
  
  Reloading a pool table
  ----------------------
  
  You may reload a pool's table, indeed this is how the pool is resized
  if it runs out of space.  (N.B. While specifying a different metadata
  device when reloading is not forbidden at the moment, things will go
  wrong if it does not route I/O to exactly the same on-disk location as
  previously.)
  
  Using an existing pool device
  -----------------------------
  
      dmsetup create pool \
  	--table "0 20971520 thin-pool $metadata_dev $data_dev \
  		 $data_block_size $low_water_mark"
  
  $data_block_size gives the smallest unit of disk space that can be
  allocated at a time expressed in units of 512-byte sectors.
  $data_block_size must be between 128 (64KB) and 2097152 (1GB) and a
  multiple of 128 (64KB).  $data_block_size cannot be changed after the
  thin-pool is created.  People primarily interested in thin provisioning
  may want to use a value such as 1024 (512KB).  People doing lots of
  snapshotting may want a smaller value such as 128 (64KB).  If you are
  not zeroing newly-allocated data, a larger $data_block_size in the
  region of 256000 (128MB) is suggested.
  
  $low_water_mark is expressed in blocks of size $data_block_size.  If
  free space on the data device drops below this level then a dm event
  will be triggered which a userspace daemon should catch allowing it to
  extend the pool device.  Only one such event will be sent.
  Resuming a device with a new table itself triggers an event so the
  userspace daemon can use this to detect a situation where a new table
  already exceeds the threshold.
  
  A low water mark for the metadata device is maintained in the kernel and
  will trigger a dm event if free space on the metadata device drops below
  it.
  
  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 thin-provisioning target 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.
  
  If data space is exhausted the pool will either error or queue IO
  according to the configuration (see: error_if_no_space).  If metadata
  space is exhausted or a metadata operation fails: the pool will error IO
  until the pool is taken offline and repair is performed to 1) fix any
  potential inconsistencies and 2) clear the flag that imposes repair.
  Once the pool's metadata device is repaired it may be resized, which
  will allow the pool to return to normal operation.  Note that if a pool
  is flagged as needing repair, the pool's data and metadata devices
  cannot be resized until repair is performed.  It should also be noted
  that when the pool's metadata space is exhausted the current metadata
  transaction is aborted.  Given that the pool will cache IO whose
  completion may have already been acknowledged to upper IO layers
  (e.g. filesystem) it is strongly suggested that consistency checks
  (e.g. fsck) be performed on those layers when repair of the pool is
  required.
  
  Thin provisioning
  -----------------
  
  i) Creating a new thinly-provisioned volume.
  
    To create a new thinly- provisioned volume you must send a message to an
    active pool device, /dev/mapper/pool in this example.
  
      dmsetup message /dev/mapper/pool 0 "create_thin 0"
  
    Here '0' is an identifier for the volume, a 24-bit number.  It's up
    to the caller to allocate and manage these identifiers.  If the
    identifier is already in use, the message will fail with -EEXIST.
  
  ii) Using a thinly-provisioned volume.
  
    Thinly-provisioned volumes are activated using the 'thin' target:
  
      dmsetup create thin --table "0 2097152 thin /dev/mapper/pool 0"
  
    The last parameter is the identifier for the thinp device.
  
  Internal snapshots
  ------------------
  
  i) Creating an internal snapshot.
  
    Snapshots are created with another message to the pool.
  
    N.B.  If the origin device that you wish to snapshot is active, you
    must suspend it before creating the snapshot to avoid corruption.
    This is NOT enforced at the moment, so please be careful!
  
      dmsetup suspend /dev/mapper/thin
      dmsetup message /dev/mapper/pool 0 "create_snap 1 0"
      dmsetup resume /dev/mapper/thin
  
    Here '1' is the identifier for the volume, a 24-bit number.  '0' is the
    identifier for the origin device.
  
  ii) Using an internal snapshot.
  
    Once created, the user doesn't have to worry about any connection
    between the origin and the snapshot.  Indeed the snapshot is no
    different from any other thinly-provisioned device and can be
    snapshotted itself via the same method.  It's perfectly legal to
    have only one of them active, and there's no ordering requirement on
    activating or removing them both.  (This differs from conventional
    device-mapper snapshots.)
  
    Activate it exactly the same way as any other thinly-provisioned volume:
  
      dmsetup create snap --table "0 2097152 thin /dev/mapper/pool 1"
  
  External snapshots
  ------------------
  
  You can use an external _read only_ device as an origin for a
  thinly-provisioned volume.  Any read to an unprovisioned area of the
  thin device will be passed through to the origin.  Writes trigger
  the allocation of new blocks as usual.
  
  One use case for this is VM hosts that want to run guests on
  thinly-provisioned volumes but have the base image on another device
  (possibly shared between many VMs).
  
  You must not write to the origin device if you use this technique!
  Of course, you may write to the thin device and take internal snapshots
  of the thin volume.
  
  i) Creating a snapshot of an external device
  
    This is the same as creating a thin device.
    You don't mention the origin at this stage.
  
      dmsetup message /dev/mapper/pool 0 "create_thin 0"
  
  ii) Using a snapshot of an external device.
  
    Append an extra parameter to the thin target specifying the origin:
  
      dmsetup create snap --table "0 2097152 thin /dev/mapper/pool 0 /dev/image"
  
    N.B. All descendants (internal snapshots) of this snapshot require the
    same extra origin parameter.
  
  Deactivation
  ------------
  
  All devices using a pool must be deactivated before the pool itself
  can be.
  
      dmsetup remove thin
      dmsetup remove snap
      dmsetup remove pool
  
  Reference
  =========
  
  'thin-pool' target
  ------------------
  
  i) Constructor
  
      thin-pool <metadata dev> <data dev> <data block size (sectors)> \
  	      <low water mark (blocks)> [<number of feature args> [<arg>]*]
  
      Optional feature arguments:
  
        skip_block_zeroing: Skip the zeroing of newly-provisioned blocks.
  
        ignore_discard: Disable discard support.
  
        no_discard_passdown: Don't pass discards down to the underlying
  			   data device, but just remove the mapping.
  
        read_only: Don't allow any changes to be made to the pool
  		 metadata.
  
        error_if_no_space: Error IOs, instead of queueing, if no space.
  
      Data block size must be between 64KB (128 sectors) and 1GB
      (2097152 sectors) inclusive.
  
  
  ii) Status
  
      <transaction id> <used metadata blocks>/<total metadata blocks>
      <used data blocks>/<total data blocks> <held metadata root>
      [no_]discard_passdown ro|rw
  
      transaction id:
  	A 64-bit number used by userspace to help synchronise with metadata
  	from volume managers.
  
      used data blocks / total data blocks
  	If the number of free blocks drops below the pool's low water mark a
  	dm event will be sent to userspace.  This event is edge-triggered and
  	it will occur only once after each resume so volume manager writers
  	should register for the event and then check the target's status.
  
      held metadata root:
  	The location, in blocks, of the metadata root that has been
  	'held' for userspace read access.  '-' indicates there is no
  	held root.
  
      discard_passdown|no_discard_passdown
  	Whether or not discards are actually being passed down to the
  	underlying device.  When this is enabled when loading the table,
  	it can get disabled if the underlying device doesn't support it.
  
      ro|rw|out_of_data_space
  	If the pool encounters certain types of device failures it will
  	drop into a read-only metadata mode in which no changes to
  	the pool metadata (like allocating new blocks) are permitted.
  
  	In serious cases where even a read-only mode is deemed unsafe
  	no further I/O will be permitted and the status will just
  	contain the string 'Fail'.  The userspace recovery tools
  	should then be used.
  
      error_if_no_space|queue_if_no_space
  	If the pool runs out of data or metadata space, the pool will
  	either queue or error the IO destined to the data device.  The
  	default is to queue the IO until more space is added or the
  	'no_space_timeout' expires.  The 'no_space_timeout' dm-thin-pool
  	module parameter can be used to change this timeout -- it
  	defaults to 60 seconds but may be disabled using a value of 0.
  
      needs_check
  	A metadata operation has failed, resulting in the needs_check
  	flag being set in the metadata's superblock.  The metadata
  	device must be deactivated and checked/repaired before the
  	thin-pool can be made fully operational again.  '-' indicates
  	needs_check is not set.
  
  iii) Messages
  
      create_thin <dev id>
  
  	Create a new thinly-provisioned device.
  	<dev id> is an arbitrary unique 24-bit identifier chosen by
  	the caller.
  
      create_snap <dev id> <origin id>
  
  	Create a new snapshot of another thinly-provisioned device.
  	<dev id> is an arbitrary unique 24-bit identifier chosen by
  	the caller.
  	<origin id> is the identifier of the thinly-provisioned device
  	of which the new device will be a snapshot.
  
      delete <dev id>
  
  	Deletes a thin device.  Irreversible.
  
      set_transaction_id <current id> <new id>
  
  	Userland volume managers, such as LVM, need a way to
  	synchronise their external metadata with the internal metadata of the
  	pool target.  The thin-pool target offers to store an
  	arbitrary 64-bit transaction id and return it on the target's
  	status line.  To avoid races you must provide what you think
  	the current transaction id is when you change it with this
  	compare-and-swap message.
  
      reserve_metadata_snap
  
          Reserve a copy of the data mapping btree for use by userland.
          This allows userland to inspect the mappings as they were when
          this message was executed.  Use the pool's status command to
          get the root block associated with the metadata snapshot.
  
      release_metadata_snap
  
          Release a previously reserved copy of the data mapping btree.
  
  'thin' target
  -------------
  
  i) Constructor
  
      thin <pool dev> <dev id> [<external origin dev>]
  
      pool dev:
  	the thin-pool device, e.g. /dev/mapper/my_pool or 253:0
  
      dev id:
  	the internal device identifier of the device to be
  	activated.
  
      external origin dev:
  	an optional block device outside the pool to be treated as a
  	read-only snapshot origin: reads to unprovisioned areas of the
  	thin target will be mapped to this device.
  
  The pool doesn't store any size against the thin devices.  If you
  load a thin target that is smaller than you've been using previously,
  then you'll have no access to blocks mapped beyond the end.  If you
  load a target that is bigger than before, then extra blocks will be
  provisioned as and when needed.
  
  ii) Status
  
       <nr mapped sectors> <highest mapped sector>
  
  	If the pool has encountered device errors and failed, the status
  	will just contain the string 'Fail'.  The userspace recovery
  	tools should then be used.