8 Creating An LVM-Based Guest From The Command Line

Fedora 17 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/vg_server1 with a size of approx. 465GB...

vgdisplay

[root@server1 ~]# vgdisplay
  --- Volume group ---
  VG Name               vg_server1
  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       26500 / 103.52 GiB
  Free  PE / Size       92612 / 361.77 GiB
  VG UUID               ZXWn5k-oVkA-ibuC-ip8x-edLx-3DMw-UrYMXg

[root@server1 ~]#

... that contains the logical volumes /dev/vg_server1/LogVol00 with a size of approx. 100GB and /dev/vg_server1/LogVol01 (about 6GB) - the rest is not allocated and can be used for KVM guests:

lvdisplay

[root@server1 ~]# lvdisplay
  --- Logical volume ---
  LV Path                /dev/vg_server1/LogVol01
  LV Name                LogVol01
  VG Name                vg_server1
  LV UUID                uUpXY3-yGfZ-X6bc-3D1u-gB4E-CfKE-vDcNfw
  LV Write Access        read/write
  LV Creation host, time server1.example.com, 2012-08-21 13:45:32 +0200
  LV Status              available
  # open                 1
  LV Size                5.86 GiB
  Current LE             1500
  Segments               1
  Allocation             inherit
  Read ahead sectors     auto
  - currently set to     256
  Block device           253:0

  --- Logical volume ---
  LV Path                /dev/vg_server1/LogVol00
  LV Name                LogVol00
  VG Name                vg_server1
  LV UUID                FN1404-Aczo-9dfA-CnNI-IKn0-L2hW-Aix0rV
  LV Write Access        read/write
  LV Creation host, time server1.example.com, 2012-08-21 13:45:33 +0200
  LV Status              available
  # open                 1
  LV Size                97.66 GiB
  Current LE             25000
  Segments               1
  Allocation             inherit
  Read ahead sectors     auto
  - currently set to     256
  Block device           253:1

[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/vg_server1/vm12 with a size of 20GB:

lvcreate -L20G -n vm12 vg_server1

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/vg_server1/vm12 -c /var/lib/libvirt/images/debian-6.0.5-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/vg_server1/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

Fedora 17 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/vg_server1/vm10) that has the same size as the image file. To find out the size of the image, type in ...

ls -l /var/lib/libvirt/images/

[root@server1 ~]# ls -l /var/lib/libvirt/images/
total 13819392
-rw-r--r-- 1 qemu qemu   177209344 May 12 22:41 debian-6.0.5-amd64-netinst.iso
-rw------- 1 root root 12884901888 Aug 21 15:37 vm10.img
-rw------- 1 qemu qemu 12884901888 Aug 21 15:51 vm11.img
[root@server1 ~]#

As you see, vm10.img has a size of exactly 12884901888 bytes. To create a logical volume of exactly the same size, we must specify -L 12884901888b (please don't forget the b at the end which tells lvcreate to use bytes - otherwise it would assume megabytes):

lvcreate -L 12884901888b -n vm10 vg_server1

Now we convert the image:

qemu-img convert /var/lib/libvirt/images/vm10.img -O raw /dev/vg_server1/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/vg_server1/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/
• Fedora: http://fedoraproject.org/
• Debian: http://www.debian.org/