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[dm-devel] [RFC PATCH 0/1] dm: add clone target

This patch adds the dm-clone target, which allows cloning of arbitrary
block devices.

dm-clone produces a one-to-one copy of an existing, read-only device
(origin) into a writable device (clone): 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 origin device to
the clone 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 clone

dm-clone is optimized for small, random writes, with size equal to
dm-clone's block/region size, e.g., 4K.

For more information regarding dm-clone's operation, please read the
attached documentation.

A preliminary test suite for dm-clone can be found at

Nikos Tsironis (1):
  dm: add clone target

 Documentation/device-mapper/dm-clone.rst |  334 +++++
 drivers/md/Kconfig                       |   13 +
 drivers/md/Makefile                      |    2 +
 drivers/md/dm-clone-metadata.c           |  991 +++++++++++++
 drivers/md/dm-clone-metadata.h           |  158 +++
 drivers/md/dm-clone-target.c             | 2244 ++++++++++++++++++++++++++++++
 6 files changed, 3742 insertions(+)
 create mode 100644 Documentation/device-mapper/dm-clone.rst
 create mode 100644 drivers/md/dm-clone-metadata.c
 create mode 100644 drivers/md/dm-clone-metadata.h
 create mode 100644 drivers/md/dm-clone-target.c


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