6.3 Using LVM-backed VBDs
A particularly appealing solution is to use LVM volumes as backing for
domain file-systems since this allows dynamic growing/shrinking of
volumes as well as snapshot and other features.
To initialize a partition to support LVM volumes:
# pvcreate /dev/sda10
Create a volume group named `vg' on the physical partition:
# vgcreate vg /dev/sda10
Create a logical volume of size 4GB named `myvmdisk1':
# lvcreate -L4096M -n myvmdisk1 vg
You should now see that you have a /dev/vg/myvmdisk1 Make a
filesystem, mount it and populate it, e.g.:
# mkfs -t ext3 /dev/vg/myvmdisk1
# mount /dev/vg/myvmdisk1 /mnt
# cp -ax / /mnt
# umount /mnt
Now configure your VM with the following disk configuration:
disk = [ 'phy:vg/myvmdisk1,sda1,w' ]
LVM enables you to grow the size of logical volumes, but you'll need
to resize the corresponding file system to make use of the new space.
Some file systems (e.g. ext3) now support online resize. See the LVM
manuals for more details.
You can also use LVM for creating copy-on-write (CoW) clones of LVM
volumes (known as writable persistent snapshots in LVM terminology).
This facility is new in Linux 2.6.8, so isn't as stable as one might
hope. In particular, using lots of CoW LVM disks consumes a lot of
dom0 memory, and error conditions such as running out of disk space
are not handled well. Hopefully this will improve in future.
To create two copy-on-write clones of the above file system you would
use the following commands:
# lvcreate -s -L1024M -n myclonedisk1 /dev/vg/myvmdisk1
# lvcreate -s -L1024M -n myclonedisk2 /dev/vg/myvmdisk1
Each of these can grow to have 1GB of differences from the master
volume. You can grow the amount of space for storing the differences
using the lvextend command, e.g.:
# lvextend +100M /dev/vg/myclonedisk1
Don't let the `differences volume' ever fill up otherwise LVM gets
rather confused. It may be possible to automate the growing process by
using dmsetup wait to spot the volume getting full and then
issue an lvextend.
In principle, it is possible to continue writing to the volume that
has been cloned (the changes will not be visible to the clones), but
we wouldn't recommend this: have the cloned volume as a `pristine'
file system install that isn't mounted directly by any of the virtual
machines.
