Lines Matching +full:data +full:- +full:mirror
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
12 presents a virtual block device which makes all data appear immediately, and
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
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
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
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
66 -------
68 dm-clone divides the source and destination devices in fixed sized regions.
69 Regions are the unit of hydration, i.e., the minimum amount of data copied from
72 The region size is configurable when you first create the dm-clone device. The
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
118 dm-clone trying to batch together contiguous regions, so we copy the data in
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 -----------
147 source dev Read only device containing the data that gets 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
174 together contiguous regions, so we copy data
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 --------
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
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
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.
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.