Migrate Xen based VM (domU) to KVM based VM

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Xen VMs do not have bootable disks. I.E. there is typically no grub config to worry about as a Xen kernel is used to boot DomU's. KVM however "typically" allows guest VMs to run unmodified kernels and as a result typically have disks and partitions including a /boot and grub mbr.

Method 1

KVM however can boot a Xen (disk based or LVM) VM using a kernel and initrd config.

OrigXen: lvcreate -L1G -s -n vmdisksnap /dev/vg0/vm01-disk              //Create a LVM Snapshot
NewKVM : lvcreate -n deb05-disk --size 3g vg0                           //Create a new LV with same size
NewKVM : lvcreate -n deb05-swap --size 512M vg0                         //Create swap LV as was done on the Xen host originally
NewKVM : nc -l -p 7000 | dd of=/dev/vg0/deb05-disk bs=2M                   //use net cat and dd with bs=2M to speed things up.
OrigXen: dd if=/dev/vg0/vmdisksnap bs=2M | nc x.x.x.x 7000 -q 10        //using port 7000 for nc
scp initrd.img-2.6.26-2-686-bigmem to new host                          //scp across original kernel and initrd image.
scp vmlinuz-2.6.26-2-686-bigmem to new host
cp /etc/libvirt/qemu/deb04.xml /etc/libvirt/qemu/deb05.xml              //copy an existing libvirt xml domain. We are going to edit it by hand instead of going virt-install
vi /etc/libvirt/qemu/deb05.xml
//delete the uuid line.
//change the name. Delete the mac address line.
//add the jernel and initrd options.
//add a cmdline root=/dev/vda and an optional console=ttyS0 allowing you to go virsh console deb05

<domain type='kvm'>
 <name>deb05</name>
 <memory>196608</memory>
 <currentMemory>196608</currentMemory>
 <vcpu>1</vcpu>
 <os>
   <type arch='i686' machine='pc'>hvm</type>
   <kernel>/root/vmlinuz-2.6.26-2-686-bigmem</kernel>
   <initrd>/root/initrd.img-2.6.26-2-686-bigmem</initrd>
   <cmdline>root=/dev/vda console=ttyS0</cmdline>
   <boot dev='hd'/>
 </os>
 <features>
   <acpi/>
   <apic/>
   <pae/>
 </features>
 <clock offset='utc'/>
 <on_poweroff>destroy</on_poweroff>
 <on_reboot>restart</on_reboot>
 <on_crash>restart</on_crash>
 <devices>
   <emulator>/usr/bin/kvm</emulator>
   <disk type='block' device='disk'>
     <source dev='/dev/vg0/deb05-disk'/>
     <target dev='vda' bus='virtio'/>
   </disk>
   <disk type='block' device='disk'>
     <source dev='/dev/vg0/deb05-swap'/>
     <target dev='vdb' bus='virtio'/>
   </disk>
   <interface type='bridge'>
     <source bridge='br0'/>
     <model type='virtio'/>
   </interface>
   <serial type='pty'>
     <source path='/dev/pts/8'/>
     <target port='0'/>
   </serial>
   <console type='pty' tty='/dev/pts/8'>
     <source path='/dev/pts/8'/>
     <target port='0'/>
   </console>
   <input type='mouse' bus='ps2'/>
   <graphics type='vnc' port='-1' autoport='yes' keymap='en-us'/>
 </devices>
</domain>

Notes

Make sure that the kernel specified is contained on the VM disk under /lib/modules/2.6.... otherwise iptables etc. won't work.

The above config uses a separate network br0 bridge. See: KVM_Setup_on_Debian_Lenny

For faster dd's see: http://serverfault.com/questions/147935/how-to-determine-the-best-byte-size-for-the-dd-command

For LVM snapshots, see: http://tldp.org/HOWTO/LVM-HOWTO/snapshots_backup.html http://arstechnica.com/open-source/news/2004/10/linux-20041013.ars http://sysadminsjourney.com/content/2008/11/29/backup-lamp-stacks-lvm-snapshots

On the original Xen host, you can go:

lvremove /dev/vg0/vmdisksnap
lvscan  //shows all LVs

Other tidying up to be done is to: vi /etc/fstab and replace in vda and vdb for swap. Update the IP and gateway in /etc/network/interface

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