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4.7.14 LVM snapshotting
This option is effective only for Linux operating systems. This option is effective for both disk-level
and file-level backup of volumes managed by Linux Logical Volume Manager (LVM). Such volumes are
also called logical volumes.
This option defines how a snapshot of a logical volume is taken. Acronis Backup can do this on its
own or rely on Linux Logical Volume Manager (LVM). Use of a snapshot ensures a time-consistent
backup of a volume whose data may change during the backup process.
The preset is: Acronis Backup
Select one of the following:
Acronis Backup
Acronis Backup will take the snapshot on its own. With this setting, a backup normally proceeds
faster and unallocated space on the volume group is not required. Therefore, we recommend
changing the preset only if you are experiencing problems with backing up logical volumes.
Logical volume manager
Acronis Backup will use the snapshot taken by LVM. This snapshot is stored on unallocated space of
the volume group. If the unallocated space is missing, Acronis Backup will take the snapshot on its
own.
A detailed explanation of LVM snapshotting
Once a volume snapshot is taken, and the data starts changing, the old data has to be kept
somewhere until it is saved to the backup.
Acronis keeps the old data mostly in the RAM. (During a file-level backup, the software may
create a temporary file in /tmp if the old data size significantly grows.)
LVM needs a temporary logical volume (a snapshot logical volume) for keeping the old data (see
http://tldp.org/HOWTO/LVM-HOWTO/snapshots_backup.html). Writing this data onto the
volume causes a large number of disk I/O operations. This is why the backup is usually slower
when a snapshot is taken by LVM.
If you choose to use LVM, Acronis Backup creates a snapshot logical volume on its own. The software
proceeds as follows:
1. Checks the backed-up volume size (not data size, but volume size).
2. Calculates 10 percent of that size—say, 10 GB.
3. Checks if there is at least 10 GB of unallocated space on the corresponding volume group.
4. If there is, creates a 10-GB snapshot logical volume (using the lvcreate -s command) and
starts backing up. Otherwise, the software backs up using the Acronis snapshot.
5. Deletes the snapshot volume as soon as it is no longer needed.
Multiple logical volumes are backed up one by one. For each of them, the software creates a
separate snapshot volume of the corresponding size. Only one snapshot volume is stored at a time.
If the data changes extremely fast while the software is keeping the snapshot, the snapshot logical
volume will run out of space and the backup will fail. That is why you can change the 10 percent
default value to any value up to 100 percent (which guarantees success) in
/etc/Acronis/BackupAndRecovery.config.
