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