alistair23-linux/Documentation/admin-guide/device-mapper/snapshot.rst

==============================
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.


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

takes the same table arguments as the snapshot target except it only
works with persistent snapshots.  This target assumes the role of the
"snapshot-origin" target and must not be loaded if the "snapshot-origin"
is still present for <origin>.

Creates a merging snapshot that takes control of the changed chunks
stored in the <COW device> of an existing snapshot, through a handover
procedure, and merges these chunks back into the <origin>.  Once merging
has started (in the background) the <origin> may be opened and the merge
will continue while I/O is flowing to it.  Changes to the <origin> are
deferred until the merging snapshot's corresponding chunk(s) have been
merged.  Once merging has started the snapshot device, associated with
the "snapshot" target, will return -EIO when accessed.


How snapshot is used by LVM2
============================
When you create the first LVM2 snapshot of a volume, four dm devices are used:

1) a device containing the original mapping table of the source volume;
2) a device used as the <COW device>;
3) a "snapshot" device, combining #1 and #2, which is the visible snapshot
   volume;
4) the "original" volume (which uses the device number used by the original
   source volume), whose table is replaced by a "snapshot-origin" mapping
   from device #1.

A fixed naming scheme is used, so with the following commands::

  lvcreate -L 1G -n base volumeGroup
  lvcreate -L 100M --snapshot -n snap volumeGroup/base

we'll have this situation (with volumes in above order)::

  # dmsetup table|grep volumeGroup

  volumeGroup-base-real: 0 2097152 linear 8:19 384
  volumeGroup-snap-cow: 0 204800 linear 8:19 2097536
  volumeGroup-snap: 0 2097152 snapshot 254:11 254:12 P 16
  volumeGroup-base: 0 2097152 snapshot-origin 254:11

  # ls -lL /dev/mapper/volumeGroup-*
  brw-------  1 root root 254, 11 29 ago 18:15 /dev/mapper/volumeGroup-base-real
  brw-------  1 root root 254, 12 29 ago 18:15 /dev/mapper/volumeGroup-snap-cow
  brw-------  1 root root 254, 13 29 ago 18:15 /dev/mapper/volumeGroup-snap
  brw-------  1 root root 254, 10 29 ago 18:14 /dev/mapper/volumeGroup-base


How snapshot-merge is used by LVM2
==================================
A merging snapshot assumes the role of the "snapshot-origin" while
merging.  As such the "snapshot-origin" is replaced with
"snapshot-merge".  The "-real" device is not changed and the "-cow"
device is renamed to <origin name>-cow to aid LVM2's cleanup of the
merging snapshot after it completes.  The "snapshot" that hands over its
COW device to the "snapshot-merge" is deactivated (unless using lvchange
--refresh); but if it is left active it will simply return I/O errors.

A snapshot will merge into its origin with the following command::

  lvconvert --merge volumeGroup/snap

we'll now have this situation::

  # dmsetup table|grep volumeGroup

  volumeGroup-base-real: 0 2097152 linear 8:19 384
  volumeGroup-base-cow: 0 204800 linear 8:19 2097536
  volumeGroup-base: 0 2097152 snapshot-merge 254:11 254:12 P 16

  # ls -lL /dev/mapper/volumeGroup-*
  brw-------  1 root root 254, 11 29 ago 18:15 /dev/mapper/volumeGroup-base-real
  brw-------  1 root root 254, 12 29 ago 18:16 /dev/mapper/volumeGroup-base-cow
  brw-------  1 root root 254, 10 29 ago 18:16 /dev/mapper/volumeGroup-base


How to determine when a merging is complete
===========================================
The snapshot-merge and snapshot status lines end with:

  <sectors_allocated>/<total_sectors> <metadata_sectors>

Both <sectors_allocated> and <total_sectors> include both data and metadata.
During merging, the number of sectors allocated gets smaller and
smaller.  Merging has finished when the number of sectors holding data
is zero, in other words <sectors_allocated> == <metadata_sectors>.

Here is a practical example (using a hybrid of lvm and dmsetup commands)::

  # lvs
    LV      VG          Attr   LSize Origin  Snap%  Move Log Copy%  Convert
    base    volumeGroup owi-a- 4.00g
    snap    volumeGroup swi-a- 1.00g base  18.97

  # dmsetup status volumeGroup-snap
  0 8388608 snapshot 397896/2097152 1560
                                    ^^^^ metadata sectors

  # lvconvert --merge -b volumeGroup/snap
    Merging of volume snap started.

  # lvs volumeGroup/snap
    LV      VG          Attr   LSize Origin  Snap%  Move Log Copy%  Convert
    base    volumeGroup Owi-a- 4.00g          17.23

  # dmsetup status volumeGroup-base
  0 8388608 snapshot-merge 281688/2097152 1104

  # dmsetup status volumeGroup-base
  0 8388608 snapshot-merge 180480/2097152 712

  # dmsetup status volumeGroup-base
  0 8388608 snapshot-merge 16/2097152 16

Merging has finished.

::

  # lvs
    LV      VG          Attr   LSize Origin  Snap%  Move Log Copy%  Convert
    base    volumeGroup owi-a- 4.00g