177 lines
6.9 KiB
Plaintext
177 lines
6.9 KiB
Plaintext
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>
|
|
|
|
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.
|
|
|
|
|
|
* snapshot-merge <origin> <COW device> <persistent> <chunksize>
|
|
|
|
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
|