[dm-devel] [RFC PATCH v2 0/1] dm: add clone target

[Nikos Tsironis]

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 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.

dm-clone is optimized for small, random writes, with size equal to
dm-clone's 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
https://github.com/arrikto/device-mapper-test-suite/tree/feature-dm-clone

Changes in v2:
  - Remove needless empty newlines.
  - Use the term "region" consistently and never call it a block.
  - Rename "origin" device to "source" device and "clone" device to
    "destination" device.
  - Express "hydration_threshold" in multiples of a region.
  - Rename "hydration_block_size" to "hydration_batch_size" and express
    it in multiples of a region.
  - Add missing "static" keyword to __load_bitset_in_core() and
    __metadata_commit().
  - clone_message(): Don't print misleading error message about
    "unsupported message" in case of correct messages with wrong number
    of arguments.
  - Rename module parameter "clone_copy_throttle" to
    "clone_hydration_throttle" to be consistent with its description.

I also updated the test suite to reflect these changes.

v1: https://www.redhat.com/archives/dm-devel/2019-July/msg00088.html

Nikos Tsironis (1):
  dm: add clone target

 Documentation/device-mapper/dm-clone.rst |  333 +++++
 drivers/md/Kconfig                       |   14 +
 drivers/md/Makefile                      |    2 +
 drivers/md/dm-clone-metadata.c           |  963 +++++++++++++
 drivers/md/dm-clone-metadata.h           |  158 +++
 drivers/md/dm-clone-target.c             | 2190 ++++++++++++++++++++++++++++++
 6 files changed, 3660 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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2.11.0