Documentation/admin-guide/device-mapper/snapshot.rst

Source file repositories/reference/linux-study-clean/Documentation/admin-guide/device-mapper/snapshot.rst

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Documentation/admin-guide/device-mapper/snapshot.rst
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Device-mapper snapshot support
==============================

Device-mapper allows you, without massive data copying:

-  To create snapshots of any block device i.e. mountable, saved states of
   the block device which are also writable without interfering with the
   original content;
-  To create device "forks", i.e. multiple different versions of the
   same data stream.
-  To merge a snapshot of a block device back into the snapshot's origin
   device.

In the first two cases, dm copies only the chunks of data that get
changed and uses a separate copy-on-write (COW) block device for
storage.

For snapshot merge the contents of the COW storage are merged back into
the origin device.


There are three dm targets available:
snapshot, snapshot-origin, and snapshot-merge.

-  snapshot-origin <origin>

which will normally have one or more snapshots based on it.
Reads will be mapped directly to the backing device. For each write, the
original data will be saved in the <COW device> of each snapshot to keep
its visible content unchanged, at least until the <COW device> fills up.


-  snapshot <origin> <COW device> <persistent?> <chunksize>
   [<# feature args> [<arg>]*]

A snapshot of the <origin> block device is created. Changed chunks of
<chunksize> sectors will be stored on the <COW device>.  Writes will
only go to the <COW device>.  Reads will come from the <COW device> or
from <origin> for unchanged data.  <COW device> will often be
smaller than the origin and if it fills up the snapshot will become
useless and be disabled, returning errors.  So it is important to monitor
the amount of free space and expand the <COW device> before it fills up.

<persistent?> is P (Persistent) or N (Not persistent - will not survive
after reboot).  O (Overflow) can be added as a persistent store option
to allow userspace to advertise its support for seeing "Overflow" in the
snapshot status.  So supported store types are "P", "PO" and "N".

The difference between persistent and transient is with transient
snapshots less metadata must be saved on disk - they can be kept in
memory by the kernel.

When loading or unloading the snapshot target, the corresponding
snapshot-origin or snapshot-merge target must be suspended. A failure to
suspend the origin target could result in data corruption.

Optional features:

   discard_zeroes_cow - a discard issued to the snapshot device that
   maps to entire chunks to will zero the corresponding exception(s) in
   the snapshot's exception store.

   discard_passdown_origin - a discard to the snapshot device is passed
   down to the snapshot-origin's underlying device.  This doesn't cause
   copy-out to the snapshot exception store because the snapshot-origin
   target is bypassed.

   The discard_passdown_origin feature depends on the discard_zeroes_cow
   feature being enabled.

Annotation

Implementation Notes