Virtualization With KVM On Ubuntu 11.04 - Page 3

Want to support HowtoForge? Become a subscriber!
Submitted by falko (Contact Author) (Forums) on Thu, 2011-05-05 16:18. ::

6 Creating An LVM-Based VM

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

To use LVM-based VMs, 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...


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               xGcqq6-kH4t-C3et-n31G-xb2S-1XJp-H07Apl


... that contains the logical volumes /dev/vg0/root with a size of approx. 100GB and /dev/vg0/swap_1 with a size of 1GB - the rest is not allocated and can be used for VMs:


root@server1:~# lvdisplay
  --- Logical volume ---
  LV Name                /dev/vg0/root
  VG Name                vg0
  LV UUID                LZrfMp-DbL5-rZEl-4m1l-tyxL-53bm-gCeiTg
  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           251:0

  --- Logical volume ---
  LV Name                /dev/vg0/swap_1
  VG Name                vg0
  LV UUID                Vlwyhq-cfeh-RthB-sM6A-Tt07-Ar5G-eFGjrV
  LV Write Access        read/write
  LV Status              available
  # open                 2
  LV Size                952.00 MiB
  Current LE             238
  Segments               1
  Allocation             inherit
  Read ahead sectors     auto
  - currently set to     256
  Block device           251:1


I will now create the virtual machine vm3 as an LVM-based VM. We can use the vmbuilder command again. vmbuilder knows the --raw option which allows to write the VM to a block device (e.g. /dev/vg0/vm3).

mkdir -p /var/lib/libvirt/images/vm3/mytemplates/libvirt
cp /etc/vmbuilder/libvirt/* /var/lib/libvirt/images/vm3/mytemplates/libvirt/

Make sure that you create all partitions in just one image file, so don't use --- in the vmbuilder.partition file:

vi /var/lib/libvirt/images/vm3/vmbuilder.partition

root 8000
swap 2000
/var 10000

vi /var/lib/libvirt/images/vm3/

# This script will run the first time the virtual machine boots
# It is ran as root.

# Expire the user account
passwd -e administrator

# Install openssh-server
apt-get update
apt-get install -qqy --force-yes openssh-server

As you see from the vmbuilder.partition file, the VM will use a max. of 20GB, so we create a logical volume called /dev/vg0/vm3 with a size of 20GB now:

lvcreate -L20G -n vm3 vg0

We can now create the new VM as follows:

cd /var/lib/libvirt/images/vm3/
vmbuilder kvm ubuntu --suite=natty --flavour=virtual --arch=amd64 --mirror= -o --libvirt=qemu:///system --ip= --gw= --part=vmbuilder.partition --raw=/dev/mapper/vg0-vm3 --templates=mytemplates --user=administrator --name=Administrator --pass=howtoforge --addpkg=vim-nox --addpkg=unattended-upgrades --addpkg=acpid --firstboot=/var/lib/libvirt/images/vm3/ --mem=256 --hostname=vm3 --bridge=br0

Please note that I use --raw=/dev/mapper/vg0-vm3 instead of --raw=/dev/vg0/vm3 switch - both /dev/vg0/vm3 and /dev/mapper/vg0-vm3 are symlinks that point to the same logical volume - /dev/dm-2 in my case - but when I used --raw=/dev/vg0/vm3 the vmbuilder process failed with the following error message...

2011-05-05 15:53:24,056 INFO    : Partition at beginning of disk - reserving first cylinder
2011-05-05 15:53:24,764 INFO    : device-mapper: deps ioctl failed: No such device or address
2011-05-05 15:53:24,765 INFO    : Cleaning up
2011-05-05 15:53:24,765 ERROR   : Process (['parted', '--script', '--', '/dev/vg0/vm3', 'mkpart', 'primary', 'ext2', '63s', '7999']) returned -11. stdout: , stderr: device-mapper: deps ioctl failed: No such device or address

Traceback (most recent call last):
  File "/usr/bin/vmbuilder", line 24, in <module>
  File "/usr/lib/python2.7/dist-packages/VMBuilder/contrib/", line 223, in main
  File "/usr/lib/python2.7/dist-packages/VMBuilder/", line 65, in install_os
    self.call_hooks('mount_partitions', self.chroot_dir)
  File "/usr/lib/python2.7/dist-packages/VMBuilder/", line 67, in call_hooks
    call_hooks(self, *args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/VMBuilder/", line 165, in call_hooks
    getattr(context, func, log_no_such_method)(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/VMBuilder/", line 88, in mount_partitions
  File "/usr/lib/python2.7/dist-packages/VMBuilder/", line 117, in partition
  File "/usr/lib/python2.7/dist-packages/VMBuilder/", line 295, in create
    run_cmd('parted', '--script', '--', disk.filename, 'mkpart', 'primary', self.parted_fstype(), partition_start, self.end)
  File "/usr/lib/python2.7/dist-packages/VMBuilder/", line 120, in run_cmd
    raise VMBuilderException, "Process (%s) returned %d. stdout: %s, stderr: %s" % (args.__repr__(), status, mystdout.buf, mystderr.buf)
VMBuilder.exception.VMBuilderException: Process (['parted', '--script', '--', '/dev/vg0/vm3', 'mkpart', 'primary', 'ext2', '63s', '7999']) returned -11. stdout: , stderr: device-mapper: deps ioctl failed: No such device or address


... while it succeeded with --raw=/dev/mapper/vg0-vm3 (I tested this multiple times). This looks like a bug in vmbuilder or parted to me.

You can now use virsh to manage the VM:

virsh --connect qemu:///system

Run the define command first...

define /etc/libvirt/qemu/vm3.xml

... before you start the VM:

start vm3


7 Links

Please do not use the comment function to ask for help! If you need help, please use our forum.
Comments will be published after administrator approval.
Submitted by Anonymous (not registered) on Wed, 2011-11-02 13:42.
Great tutorial; just one question. I don't get any errors when running the vmbuilder command, but when attempting to "define" a vm the error says there is no file or directory. It's all there in the /var/... directories, but the .xml profile is not being created in the /etc/.. directory. (I've searched the entire file system). Do you have any suggestions on where else to troubleshoot?  I'm using Ubuntu 11.04 on an 64-bit AMD dual core with KVM. The same hardware supported VMs fine under Ubuntu 9.04.