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