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11.4. Taking a Backup Using Snapshots

Following on from the previous example we now want to use the extra space in the "ops" volume group to make a database backup every evening. To ensure that the data that goes onto the tape is consistent we use an LVM snapshot logical volume.

This type of volume is a read-only copy of another volume that contains all the data that was in the volume at the time the snapshot was created. This means we can back up that volume without having to worry about data being changed while the backup is going on, and we don't have to take the database volume offline while the backup is taking place.

11.4.1. Create the snapshot volume

There is a little over 500 Megabytes of free space in the "ops" volume group, so we will use all of it to allocate space for the snapshot logical volume. A snapshot volume can be as large or a small as you like but it must be large enough to hold all the changes that are likely to happen to the original volume during the lifetime of the snapshot. So here, allowing 500 megabytes of changes to the database volume which should be plenty.

# lvcreate -L592M -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



If the snapshot is of an XFS filesystem, the xfs_freeze command should be used to quiesce the filesystem before creating the snapshot. (if the filesystem is mounted)

# xfs_freeze -f /mnt/point; lvcreate -L592M -s -n dbbackup /dev/ops/databases; xfs_freeze -u /mnt/point 

WarningFull snapshot are automatically disabled

If the snapshot logical volume becomes full it will become unusable so it is vitally important to allocate enough space.

11.4.2. Mount the snapshot volume

We can now create a mount-point and mount the volume

# mkdir /mnt/ops/dbbackup
# mount /dev/ops/dbbackup /mnt/ops/dbbackup
mount: block device /dev/ops/dbbackup is write-protected, mounting read-only

If you are using XFS as the filesystem you will need to add the nouuid option to the mount command:

# mount /dev/ops/dbbackup /mnt/ops/dbbackup -onouuid,ro


Previously, the norecovery option was suggested to allow the mounting of XFS snapshots. It has been recommended not to use this option, but to instead use xfs_freeze to quiesce the filesystem before creating the snapshot.

11.4.3. Do the backup

I assume you will have a more sophisticated backup strategy than this!

# tar -cf /dev/rmt0 /mnt/ops/dbbackup
tar: Removing leading `/' from member names

11.4.4. Remove the snapshot

When the backup has finished you can now unmount the volume and remove it from the system. You should remove snapshot volume when you have finished with them because they take a copy of all data written to the original volume and this can hurt performance.

# 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

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