8 Creating An LVM-Based Guest From The Command Line

Debian Squeeze 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

root@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 GiB
  PE Size               4.00 MiB
  Total PE              119112
  Alloc PE / Size       24079 / 94.06 GiB
  Free  PE / Size       95033 / 371.22 GiB
  VG UUID               UfMJrb-eu18-NlYw-PGx1-wM5f-4d7S-x10hpw

root@server1:~#

... that contains the logical volume /dev/vg0/root with a size of approx. 93GB and the logical volume /dev/vg0/swap_1 (about 1GB) - the rest is not allocated and can be used for KVM guests:

lvdisplay

root@server1:~# lvdisplay
  --- Logical volume ---
  LV Name                /dev/vg0/root
  VG Name                vg0
  LV UUID                ydV63n-e57f-T3vu-xcBD-OZ7u-gXPf-QoGxVk
  LV Write Access        read/write
  LV Status              available
  # open                 1
  LV Size                93.13 GiB
  Current LE             23841
  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                0H0BAx-VeBU-pH4T-CpnD-7oFI-6ulP-krl66i
  LV Write Access        read/write
  LV Status              available
  # open                 1
  LV Size                952.00 MiB
  Current LE             238
  Segments               1
  Allocation             inherit
  Read ahead sectors     auto
  - currently set to     256
  Block device           254:1

root@server1:~#

I will now create the virtual machine vm12 as an LVM-based guest. I want vm12 to have 20GB of disk space, so I create the logical volume /dev/vg0/vm12 with a size of 20GB:

lvcreate -L20G -n vm12 vg0

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

virt-install --connect qemu:///system -n vm12 -r 512 --vcpus=2 --disk path=/dev/vg0/vm12 -c /var/lib/libvirt/images/debian-6.0.0-amd64-netinst.iso --vnc --noautoconsole --os-type linux --os-variant debiansqueeze --accelerate --network=bridge:br0 --hvm

Please note that instead of --disk path=/var/lib/libvirt/images/vm12.img,size=20 I use --disk path=/dev/vg0/vm12, and I don't need to define the disk space anymore because the disk space is defined by the size of the logical volume vm12 (20GB).

Now follow chapter 5 to install that guest.

9 Converting Image-Based Guests To LVM-Based Guests

Debian Squeeze 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 there are two ways of converting the image:

1. The first one is as follows:

   qemu-img convert /var/lib/libvirt/images/vm10.img -O host_device /dev/vg0/vm10

2. OR you do it like this:

   qemu-img convert /var/lib/libvirt/images/vm10.img -O raw /var/lib/libvirt/images/vm10.raw
   dd if=/var/lib/libvirt/images/vm10.raw of=/dev/vg0/vm10
   rm -f /var/lib/libvirt/images/vm10.raw

(The command

qemu-img convert /var/lib/libvirt/images/vm10.img -O raw /dev/vg0/vm10

does not work; you will get the following error message:

qemu-img: Error while formatting '/dev/vg0/vm10'

)

Afterwards you can delete the disk image:

rm -f /var/lib/libvirt/images/vm10.img

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'>
      <driver name='qemu' type='raw'/>
      <source file='/var/lib/libvirt/images/vm10.img'/>
      <target dev='vda' bus='virtio'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x04' function='0x0'/>
    </disk>
[...]

... so that it looks as follows:

[...]
    <disk type='block' device='disk'>
      <driver name='qemu' type='raw'/>
      <source dev='/dev/vg0/vm10'/>
      <target dev='vda' bus='virtio'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x04' function='0x0'/>
    </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

10 Links

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