admin-guide/device-mapper/dm-clone.rst

1 .. SPDX-License-Identifier: GPL-2.0-only
4 dm-clone
10 dm-clone is a device mapper target which produces a one-to-one copy of an
11 existing, read-only source device into a writable destination device: It
15 The main use case of dm-clone is to clone a potentially remote, high-latency,
16 read-only, archival-type block device into a writable, fast, primary-type device
17 for fast, low-latency I/O. The cloned device is visible/mountable immediately
18 and the copy of the source device to the destination device happens in the
21 For example, one could restore an application backup from a read-only copy,
26 When the cloning completes, the dm-clone table can be removed altogether and be
27 replaced, e.g., by a linear table, mapping directly to the destination device.
29 The dm-clone target reuses the metadata library used by the thin-provisioning
36      The process of filling a region of the destination device with data from
38      source to the destination device.
40 Once a region gets hydrated we redirect all I/O regarding it to the destination
46 Sub-devices
47 -----------
52 1. A source device - the read-only device that gets cloned and source of the
55 2. A destination device - the destination of the hydration, which will become a
58 3. A small metadata device - it records which regions are already valid in the
59    destination device, i.e., which regions have already been hydrated, or have
62 The size of the destination device must be at least equal to the size of the
66 -------
68 dm-clone divides the source and destination devices in fixed sized regions.
70 the source to the destination device.
72 The region size is configurable when you first create the dm-clone device. The
77 Reads and writes from/to hydrated regions are serviced from the destination
87 destination device directly.
90 --------
92 dm-clone interprets a discard request to a range that hasn't been hydrated yet
94 skips copying the region's data from the source to the destination device, and
97 If the destination device supports discards, then by default dm-clone will pass
101 --------------------
103 dm-clone copies continuously from the source to the destination device, until
106 Copying data from the source to the destination device uses bandwidth. The user
108 any one time. Moreover, dm-clone takes into account user I/O traffic going to
109 the devices and pauses the background hydration when there is I/O in-flight.
114 dm-clone employs dm-kcopyd for copying portions of the source device to the
115 destination device. By default, we issue copy requests of size equal to the
118 dm-clone trying to batch together contiguous regions, so we copy the data in
121 When the hydration of the destination device finishes, a dm event will be sent
124 Updating on-disk metadata
125 -------------------------
127 On-disk metadata is committed every time a FLUSH or FUA bio is written. If no
129 dm-clone device behaves like a physical disk that has a volatile write cache. If
137 -----------
141    clone <metadata dev> <destination dev> <source dev> <region size>
146  destination dev  The destination device, where the source will be cloned
154                   passed to dm-clone
155  core args        Key/value pairs passed to dm-clone, e.g. `hydration_threshold
162  no_hydration         Create a dm-clone instance with background hydration
164  no_discard_passdown  Disable passing down discards to the destination device
171                                   the source to the destination device at any
175                                   from the source to the destination device in
180 ------
202  clone metadata mode     ro if read-only, rw if read-write
204                          In serious cases where even a read-only mode is deemed
211 --------
214       Disable the background hydration of the destination device.
217       Enable the background hydration of the destination device.
229 ---------------------------------------
231 1. Create the dm-clone device.
235     dmsetup create clone --table "0 1048576000 clone $metadata_dev $dest_dev \
238 2. Mount the device and trim the file system. dm-clone interprets the discards
243     mount /dev/mapper/clone /mnt/cloned-fs
244     fstrim /mnt/cloned-fs
246 3. Enable background hydration of the destination device.
252 4. When the hydration finishes, we can replace the dm-clone table with a linear
258     dmsetup load clone --table "0 1048576000 linear $dest_dev 0"
267 1. We redirect reads, to not-yet-hydrated regions, to the source device. If
274 2. Release in-core resources, i.e., the bitmaps tracking which regions are
278    destination device, we print an error message, but the hydration process
286 We explored the following alternatives before implementing dm-clone:
288 1. Use dm-cache with cache size equal to the source device and implement a new
291    * The resulting cache device is not a one-to-one mirror of the source device
294    * dm-cache writes to the source device, which violates our requirement that
295      the source device must be treated as read-only.
299 2. Use dm-snapshot with a COW device equal to the source device:
301    * dm-snapshot stores its metadata in the COW device, so the resulting device
302      is not a one-to-one mirror of the source device.
306    * dm-snapshot needs to commit its metadata whenever a pending exception
309      or FUA bio is written, or periodically, like dm-thin and dm-cache do. This
312 3. Use dm-mirror: The mirror target has a background copying/mirroring
314    the source device must be treated as read-only.
316 4. Use dm-thin's external snapshot functionality. This approach is the most
317    promising among all alternatives, as the thinly-provisioned volume is a
318    one-to-one mirror of the source device and handles reads and writes to
319    un-provisioned/not-yet-cloned areas the same way as dm-clone does.
326      destination of the cloning process and not restrict ourselves to
327      thinly-provisioned volumes. Thin-provisioning has an inherent metadata
331    Moreover, cloning a device shouldn't force the use of thin-provisioning. On
333    LV as dm-clone's destination device.