Virtualization With KVM On An OpenSUSE 11.4 Server - Page 5

8 Creating An LVM-Based Guest

OpenSUSE 11.4 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

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               465.61 GiB
  PE Size               4.00 MiB
  Total PE              119195
  Alloc PE / Size       27136 / 106.00 GiB
  Free  PE / Size       92059 / 359.61 GiB
  VG UUID               jv7lY3-pHqK-4kCc-C9oJ-mczc-PL7R-bf9QJL

server1:~ #

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

lvdisplay

server1:~ # lvdisplay
  --- Logical volume ---
  LV Name                /dev/vg_server1/lv_root
  VG Name                vg_server1
  LV UUID                60W4MZ-VMn0-s0Tr-0B2r-1xc1-Pjmx-tRvaRd
  LV Write Access        read/write
  LV Status              available
  # open                 1
  LV Size                100.00 GiB
  Current LE             25600
  Segments               1
  Allocation             inherit
  Read ahead sectors     auto
  - currently set to     256
  Block device           253:0

  --- Logical volume ---
  LV Name                /dev/vg_server1/lv_swap
  VG Name                vg_server1
  LV UUID                1vByAo-pwVs-GmMJ-hxHi-jZqj-CWQj-2euYdU
  LV Write Access        read/write
  LV Status              available
  # open                 1
  LV Size                6.00 GiB
  Current LE             1536
  Segments               1
  Allocation             inherit
  Read ahead sectors     auto
  - currently set to     256
  Block device           253:1

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-6.0.0-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/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

OpenSUSE 11.4 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 - the image has 12GB, so the logical volume must have 12GB of size as well:

lvcreate -L12G -n vm10 vg_server1

Now convert the disk 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

 

Falko Timme

About Falko Timme

Falko Timme is an experienced Linux administrator and founder of Timme Hosting, a leading nginx business hosting company in Germany. He is one of the most active authors on HowtoForge since 2005 and one of the core developers of ISPConfig since 2000. He has also contributed to the O'Reilly book "Linux System Administration".

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