dm: add clone target

Add the dm-clone target, which allows cloning of arbitrary block
devices.

dm-clone produces a one-to-one copy of an existing, read-only source
device into a writable destination device: It presents a virtual block
device which makes all data appear immediately, and redirects reads and
writes accordingly.

The main use case of dm-clone is to clone a potentially remote,
high-latency, read-only, archival-type block device into a writable,
fast, primary-type device for fast, low-latency I/O. The cloned device
is visible/mountable immediately and the copy of the source device to
the destination device happens in the background, in parallel with user
I/O.

When the cloning completes, the dm-clone table can be removed altogether
and be replaced, e.g., by a linear table, mapping directly to the
destination device.

For further information and examples of how to use dm-clone, please read
Documentation/admin-guide/device-mapper/dm-clone.rst

Suggested-by: Vangelis Koukis <vkoukis@arrikto.com>
Co-developed-by: Ilias Tsitsimpis <iliastsi@arrikto.com>
Signed-off-by: Ilias Tsitsimpis <iliastsi@arrikto.com>
Signed-off-by: Nikos Tsironis <ntsironis@arrikto.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

1 files changed, 333 insertions, 0 deletions

diff --git a/Documentation/admin-guide/device-mapper/dm-clone.rst b/Documentation/admin-guide/device-mapper/dm-clone.rst
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+.. SPDX-License-Identifier: GPL-2.0-only
+
+========
+dm-clone
+========
+
+Introduction
+============
+
+dm-clone is a device mapper target which produces a one-to-one copy of an
+existing, read-only source device into a writable destination device: It
+presents a virtual block device which makes all data appear immediately, and
+redirects reads and writes accordingly.
+
+The main use case of dm-clone is to clone a potentially remote, high-latency,
+read-only, archival-type block device into a writable, fast, primary-type device
+for fast, low-latency I/O. The cloned device is visible/mountable immediately
+and the copy of the source device to the destination device happens in the
+background, in parallel with user I/O.
+
+For example, one could restore an application backup from a read-only copy,
+accessible through a network storage protocol (NBD, Fibre Channel, iSCSI, AoE,
+etc.), into a local SSD or NVMe device, and start using the device immediately,
+without waiting for the restore to complete.
+
+When the cloning completes, the dm-clone table can be removed altogether and be
+replaced, e.g., by a linear table, mapping directly to the destination device.
+
+The dm-clone target reuses the metadata library used by the thin-provisioning
+target.
+
+Glossary
+========
+
+   Hydration
+     The process of filling a region of the destination device with data from
+     the same region of the source device, i.e., copying the region from the
+     source to the destination device.
+
+Once a region gets hydrated we redirect all I/O regarding it to the destination
+device.
+
+Design
+======
+
+Sub-devices
+-----------
+
+The target is constructed by passing three devices to it (along with other
+parameters detailed later):
+
+1. A source device - the read-only device that gets cloned and source of the
+   hydration.
+
+2. A destination device - the destination of the hydration, which will become a
+   clone of the source device.
+
+3. A small metadata device - it records which regions are already valid in the
+   destination device, i.e., which regions have already been hydrated, or have
+   been written to directly, via user I/O.
+
+The size of the destination device must be at least equal to the size of the
+source device.
+
+Regions
+-------
+
+dm-clone divides the source and destination devices in fixed sized regions.
+Regions are the unit of hydration, i.e., the minimum amount of data copied from
+the source to the destination device.
+
+The region size is configurable when you first create the dm-clone device. The
+recommended region size is the same as the file system block size, which usually
+is 4KB. The region size must be between 8 sectors (4KB) and 2097152 sectors
+(1GB) and a power of two.
+
+Reads and writes from/to hydrated regions are serviced from the destination
+device.
+
+A read to a not yet hydrated region is serviced directly from the source device.
+
+A write to a not yet hydrated region will be delayed until the corresponding
+region has been hydrated and the hydration of the region starts immediately.
+
+Note that a write request with size equal to region size will skip copying of
+the corresponding region from the source device and overwrite the region of the
+destination device directly.
+
+Discards
+--------
+
+dm-clone interprets a discard request to a range that hasn't been hydrated yet
+as a hint to skip hydration of the regions covered by the request, i.e., it
+skips copying the region's data from the source to the destination device, and
+only updates its metadata.
+
+If the destination device supports discards, then by default dm-clone will pass
+down discard requests to it.
+
+Background Hydration
+--------------------
+
+dm-clone copies continuously from the source to the destination device, until
+all of the device has been copied.
+
+Copying data from the source to the destination device uses bandwidth. The user
+can set a throttle to prevent more than a certain amount of copying occurring at
+any one time. Moreover, dm-clone takes into account user I/O traffic going to
+the devices and pauses the background hydration when there is I/O in-flight.
+
+A message `hydration_threshold <#regions>` can be used to set the maximum number
+of regions being copied, the default being 1 region.
+
+dm-clone employs dm-kcopyd for copying portions of the source device to the
+destination device. By default, we issue copy requests of size equal to the
+region size. A message `hydration_batch_size <#regions>` can be used to tune the
+size of these copy requests. Increasing the hydration batch size results in
+dm-clone trying to batch together contiguous regions, so we copy the data in
+batches of this many regions.
+
+When the hydration of the destination device finishes, a dm event will be sent
+to user space.
+
+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
+dm-clone device 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.
+
+Target Interface
+================
+
+Constructor
+-----------
+
+  ::
+
+   clone <metadata dev> <destination dev> <source dev> <region size>
+         [<#feature args> [<feature arg>]* [<#core args> [<core arg>]*]]
+
+ ================ ==============================================================
+ metadata dev     Fast device holding the persistent metadata
+ destination dev  The destination device, where the source will be cloned
+ source dev       Read only device containing the data that gets cloned
+ region size      The size of a region in sectors
+
+ #feature args    Number of feature arguments passed
+ feature args     no_hydration or no_discard_passdown
+
+ #core args       An even number of arguments corresponding to key/value pairs
+                  passed to dm-clone
+ core args        Key/value pairs passed to dm-clone, e.g. `hydration_threshold
+                  256`
+ ================ ==============================================================
+
+Optional feature arguments are:
+
+ ==================== =========================================================
+ no_hydration         Create a dm-clone instance with background hydration
+                      disabled
+ no_discard_passdown  Disable passing down discards to the destination device
+ ==================== =========================================================
+
+Optional core arguments are:
+
+ ================================ ==============================================
+ hydration_threshold <#regions>   Maximum number of regions being copied from
+                                  the source to the destination device at any
+                                  one time, during background hydration.
+ hydration_batch_size <#regions>  During background hydration, try to batch
+                                  together contiguous regions, so we copy data
+                                  from the source to the destination device in
+                                  batches of this many regions.
+ ================================ ==============================================
+
+Status
+------
+
+  ::
+
+   <metadata block size> <#used metadata blocks>/<#total metadata blocks>
+   <region size> <#hydrated regions>/<#total regions> <#hydrating regions>
+   <#feature args> <feature args>* <#core args> <core args>*
+   <clone metadata mode>
+
+ ======================= =======================================================
+ 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
+ region size             Configurable region size for the device in sectors
+ #hydrated regions       Number of regions that have finished hydrating
+ #total regions          Total number of regions to hydrate
+ #hydrating regions      Number of regions currently hydrating
+ #feature args           Number of feature arguments to follow
+ feature args            Feature arguments, e.g. `no_hydration`
+ #core args              Even number of core arguments to follow
+ core args               Key/value pairs for tuning the core, e.g.
+                         `hydration_threshold 256`
+ clone metadata mode     ro if read-only, rw if read-write
+
+                         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'. If the metadata
+                         mode changes, a dm event will be sent to user space.
+ ======================= =======================================================
+
+Messages
+--------
+
+  `disable_hydration`
+      Disable the background hydration of the destination device.
+
+  `enable_hydration`
+      Enable the background hydration of the destination device.
+
+  `hydration_threshold <#regions>`
+      Set background hydration threshold.
+
+  `hydration_batch_size <#regions>`
+      Set background hydration batch size.
+
+Examples
+========
+
+Clone a device containing a file system
+---------------------------------------
+
+1. Create the dm-clone device.
+
+   ::
+
+    dmsetup create clone --table "0 1048576000 clone $metadata_dev $dest_dev \
+      $source_dev 8 1 no_hydration"
+
+2. Mount the device and trim the file system. dm-clone interprets the discards
+   sent by the file system and it will not hydrate the unused space.
+
+   ::
+
+    mount /dev/mapper/clone /mnt/cloned-fs
+    fstrim /mnt/cloned-fs
+
+3. Enable background hydration of the destination device.
+
+   ::
+
+    dmsetup message clone 0 enable_hydration
+
+4. When the hydration finishes, we can replace the dm-clone table with a linear
+   table.
+
+   ::
+
+    dmsetup suspend clone
+    dmsetup load clone --table "0 1048576000 linear $dest_dev 0"
+    dmsetup resume clone
+
+   The metadata device is no longer needed and can be safely discarded or reused
+   for other purposes.
+
+Known issues
+============
+
+1. We redirect reads, to not-yet-hydrated regions, to the source device. If
+   reading the source device has high latency and the user repeatedly reads from
+   the same regions, this behaviour could degrade performance. We should use
+   these reads as hints to hydrate the relevant regions sooner. Currently, we
+   rely on the page cache to cache these regions, so we hopefully don't end up
+   reading them multiple times from the source device.
+
+2. Release in-core resources, i.e., the bitmaps tracking which regions are
+   hydrated, after the hydration has finished.
+
+3. During background hydration, if we fail to read the source or write to the
+   destination device, we print an error message, but the hydration process
+   continues indefinitely, until it succeeds. We should stop the background
+   hydration after a number of failures and emit a dm event for user space to
+   notice.
+
+Why not...?
+===========
+
+We explored the following alternatives before implementing dm-clone:
+
+1. Use dm-cache with cache size equal to the source device and implement a new
+   cloning policy:
+
+   * The resulting cache device is not a one-to-one mirror of the source device
+     and thus we cannot remove the cache device once cloning completes.
+
+   * dm-cache writes to the source device, which violates our requirement that
+     the source device must be treated as read-only.
+
+   * Caching is semantically different from cloning.
+
+2. Use dm-snapshot with a COW device equal to the source device:
+
+   * dm-snapshot stores its metadata in the COW device, so the resulting device
+     is not a one-to-one mirror of the source device.
+
+   * No background copying mechanism.
+
+   * dm-snapshot needs to commit its metadata whenever a pending exception
+     completes, to ensure snapshot consistency. In the case of cloning, we don't
+     need to be so strict and can rely on committing metadata every time a FLUSH
+     or FUA bio is written, or periodically, like dm-thin and dm-cache do. This
+     improves the performance significantly.
+
+3. Use dm-mirror: The mirror target has a background copying/mirroring
+   mechanism, but it writes to all mirrors, thus violating our requirement that
+   the source device must be treated as read-only.
+
+4. Use dm-thin's external snapshot functionality. This approach is the most
+   promising among all alternatives, as the thinly-provisioned volume is a
+   one-to-one mirror of the source device and handles reads and writes to
+   un-provisioned/not-yet-cloned areas the same way as dm-clone does.
+
+   Still:
+
+   * There is no background copying mechanism, though one could be implemented.
+
+   * Most importantly, we want to support arbitrary block devices as the
+     destination of the cloning process and not restrict ourselves to
+     thinly-provisioned volumes. Thin-provisioning has an inherent metadata
+     overhead, for maintaining the thin volume mappings, which significantly
+     degrades performance.
+
+   Moreover, cloning a device shouldn't force the use of thin-provisioning. On
+   the other hand, if we wish to use thin provisioning, we can just use a thin
+   LV as dm-clone's destination device.