7 Creating An LVM-Based Guest

Debian Lenny KVM Host:

LVM-based guests have some advantages over image-based guests. They are not as heavy on hard disk IO, and they are easier to back up (using LVM snapshots).

To use LVM-based guests, you need a volume group that has some free space that is not allocated to any logical volume. In this example, I use the volume group /dev/vg0 with a size of approx. 465GB...

vgdisplay

server1:~# vgdisplay
  --- Volume group ---
  VG Name               vg0
  System ID
  Format                lvm2
  Metadata Areas        1
  Metadata Sequence No  3
  VG Access             read/write
  VG Status             resizable
  MAX LV                0
  Cur LV                2
  Open LV               2
  Max PV                0
  Cur PV                1
  Act PV                1
  VG Size               465.28 GB
  PE Size               4.00 MB
  Total PE              119112
  Alloc PE / Size       59842 / 233.76 GB
  Free  PE / Size       59270 / 231.52 GB
  VG UUID               gnUCYV-mYXj-qxpM-PEat-tdXS-wumf-6FK3rA

server1:~#

... that contains the logical volume /dev/vg0/root with a size of approx. 232GB and the logical volume /dev/vg0/swap_1 (about 1GB) - the rest is not allocated and can be used for KVM guests:

lvdisplay

server1:~# lvdisplay
  --- Logical volume ---
  LV Name                /dev/vg0/root
  VG Name                vg0
  LV UUID                kMYrHg-d0ox-yc6y-1eNR-lB2R-yMIn-WFgzSZ
  LV Write Access        read/write
  LV Status              available
  # open                 1
  LV Size                232.83 GB
  Current LE             59604
  Segments               1
  Allocation             inherit
  Read ahead sectors     auto
  - currently set to     256
  Block device           254:0

  --- Logical volume ---
  LV Name                /dev/vg0/swap_1
  VG Name                vg0
  LV UUID                SUI0uq-iTsy-7EnZ-INNz-gjvu-tqLD-rGSegE
  LV Write Access        read/write
  LV Status              available
  # open                 2
  LV Size                952.00 MB
  Current LE             238
  Segments               1
  Allocation             inherit
  Read ahead sectors     auto
  - currently set to     256
  Block device           254:1

server1:~#

I will now create the virtual machine vm11 as an LVM-based guest. I want vm11 to have 20GB of disk space, so I create the logical volume /dev/vg0/vm11 with a size of 20GB:

lvcreate -L20G -n vm11 vg0

Afterwards, we use the virt-install command again to create the guest:

virt-install --connect qemu:///system -n vm11 -r 512 --vcpus=2 --disk path=/dev/vg0/vm11 -c ~/debian-500-amd64-netinst.iso --vnc --noautoconsole --os-type linux --os-variant debianLenny --accelerate --network=bridge:br0 --hvm

Please note that instead of -f ~/vm11.qcow2 I use --disk path=/dev/vg0/vm11, and I don't need the -s switch to define the disk space anymore because the disk space is defined by the size of the logical volume vm11 (20GB).

Now follow chapter 5 to install that guest.

8 Converting Image-Based Guests To LVM-Based Guests

Debian Lenny KVM Host:

No let's assume we want to convert our image-based guest vm10 into an LVM-based guest. This is how we do it:

First make sure the guest is stopped:

virsh --connect qemu:///system

shutdown vm10

quit

Then create a logical volume (e.g. /dev/vg0/vm10) that has the same size as the image file - the image has 12GB, so the logical volume must have 12GB of size as well:

lvcreate -L12G -n vm10 vg0

Now you can convert the image:

qemu-img convert ~/vm10.qcow2 -O raw /dev/vg0/vm10

Afterwards you can delete the disk image:

rm -f ~/vm10.qcow2

Now we must open the guest's xml configuration file /etc/libvirt/qemu/vm10.xml...

vi /etc/libvirt/qemu/vm10.xml

... and change the following section...

[...]
    <disk type='file' device='disk'>
      <source file='/root/vm10.qcow2'/>
      <target dev='vda' bus='virtio'/>
    </disk>
[...]

... so that it looks as follows:

[...]
    <disk type='file' device='disk'>
      <source file='/dev/vg0/vm10'/>
      <target dev='vda' bus='virtio'/>
    </disk>
[...]

Afterwards we must redefine the guest:

virsh --connect qemu:///system

define /etc/libvirt/qemu/vm10.xml

Still on the virsh shell, we can start the guest...

start vm10

... and leave the virsh shell:

quit

9 Links

• KVM: http://kvm.qumranet.com/
• Debian: http://www.debian.org/
• Ubuntu: http://www.ubuntu.com/