Monday, July 26, 2010

(LVM) snapshot and how do I use it?

What is a Logical Volume Manager (LVM) snapshot and how do I use it?
Logical Volume Manager (LVM) provides the ability to take a snapshot of any logical volume for the purpose of obtaining a backup of a partition in a consistent state. As applications may access files or databases on a partition during a backup some files may be backed up in one state, while later files are backed up after an update has been made, leading to an inconsistent backup.
Traditionally the solution has been to mount the partition read-only, apply table-level write locks to databases or shut down the database engine etc.; all measures which adversely impact availability (but not as much as data loss without a backup will). With LVM snapshots it is possible to obtain a consistent backup without compromising availability.
Please note that this information is only valid for partitions that have been created using LVM. LVM snapshots cannot be used with non-LVM filesystems.
The LVM snapshot works by logging the changes to the filesystem to the snapshot partition, rather than mirroring the partition. Thus when you create a snapshot partition you do not need to use space equal to the size of the partition that you are taking a snapshot of, but rather the amount of changes that it will undergo during the lifetime of the snapshot. This is a function of both how much data is being written to the partition and also how long you intend keeping the LVM snapshot. The longer you leave it, the more changes there are likely to be on the file system and the more the snapshot partition will fill up with change information. The higher the rate of change on the partition the shorter the lifespan of the snapshot. If the amount of changes on the LVM partition exceed the size of the snapshot then the snapshot is released.
Now we will show an example of how to make an LVM snapshot. Here we create a logical volume of 500MB to use to take a snapshot. This will allow 500MB of changes on the volume we are taking a snapshot of during the lifetime of the snapshot.
The following command will create /dev/ops/dbbackup as a snapshot of /dev/ops/databases.
# lvcreate -L500M -s -n dbbackup /dev/ops/databases
lvcreate -- WARNING: the snapshot must be disabled if it gets full
lvcreate -- INFO: using default snapshot chunk size of 64 KB for "/dev/ops/dbbackup"
lvcreate -- doing automatic backup of "ops"
lvcreate -- logical volume "/dev/ops/dbbackup" successfully created
Now we create the mount point and mount the snapshot.
# mkdir /mnt/ops/dbbackup
# mount /dev/ops/dbbackup /mnt/ops/dbbackup
mount: block device /dev/ops/dbbackup is write-protected, mounting read-only
After performing the backup of the snapshot partition we release the snapshot. The snapshot will be automatically released when it fills up, but maintaining incurs a system overhead in the meantime.
# umount /mnt/ops/dbbackup
# lvremove /dev/ops/dbbackup
lvremove -- do you really want to remove "/dev/ops/dbbackup"? [y/n]: y
lvremove -- doing automatic backup of volume group "ops"
lvremove -- logical volume "/dev/ops/dbbackup" successfully removed
For more information about using LVM you can consult the Linux Documentation Project's HOW-TO on the subject at