Sysadmin/Migrate virtual machine

How to migrate a file-based guest to LVM (online method)

• First, create the logical volume and attach it to the running VM:

treehouse# scp lightwave:/boot/{vmlinuz,initrd}-2.6.31-22-server /srv/vm/kernel/ubuntu/
treehouse# virsh vol-create-as treehouse lightwave 10G
treehouse# virsh attach-disk lightwave /dev/treehouse/lightwave-root vdc
treehouse# virsh console lightwave

• From within the VM, format the new volume and move things over

lightwave# mkfs.ext4 -L lightwave-root -O flex_bg,extent,uninit_bg,sparse_super /dev/vdc
lightwave# tune2fs -c -1 -i 0 /dev/vdc
lightwave# mount /dev/vdc /mnt
lightwave# rsync -PHAXphax --numeric-ids --delete / /mnt/
lightwave# vim /mnt/etc/fstab
lightwafe# umount /mnt
lightwave# ^D

• Then edit the VM configuration to make it boot without GRUB, using externally provided kernel and initrd:

treehouse# virsh edit lightwave

 <os>
   <type arch='x86_64' machine='pc-0.11'>hvm</type>
   <kernel>/srv/vm/kernel/ubuntu/vmlinuz-2.6.31-22-server</kernel>
   <initrd>/srv/vm/kernel/ubuntu/initrd.img-2.6.31-22-server</initrd>
   <cmdline>console=tty0 console=ttyS0,115200n8 vga=normal root=LABEL=lightwave-root ro</cmdline>
   <boot dev='hd'/>
 </os>
 ...
 <devices>
   ...
   <disk type='block' device='disk'>
     <source dev='/dev/treehouse/lightwave-root'/>
     <target dev='vda' bus='virtio'/>
   </disk>
   ...
 </devices>

• Reboot

#treehouse virsh destroy lightwave
#treehouse virsh start lightwave --console

How to migrate a file-based guest to LVM (offline method)

• Determine the offset of the root partition within the raw disk image:

fdisk -l -u vm.img

• Convert the start offset to bytes:

mount -o loop,offset=$((63 * 512)) vm.img /mnt/

• Then create an LV and copy over everything as in the online case

How to migrate an LVM-based guest to another host

(untested procedure)

First, shutdown the VM, then do:

mount /dev/treehouse/beamrider-root /mnt
ssh housetree mount /dev/housetree/beamrider-root /mnt
rsync -PHAXphax --delete --numeric-ids /mnt/ housetree:/mnt/
rsync -a /etc/libvirt/qemu/beamrider.xml housetree:/etc/libvirt/qemu/
umount /mnt
ssh housetree umount /mnt

Then log to $OTHERHOST and create the domain in virsh.

Downtime can be reduced to just a few minutes by doing a first rsync iteration from within the VM while it's still running.

Make sure you pass all the fancy switches to rsync or you'll loose information on the copy.

Alternative to convert file based VM to block based LVM (Assumes server is treehouse)

Stop the VM

sudo virsh stop name_of_vm

Convert the qcow2 file to a raw file

sudo qemu-img convert "name_of_vm".qcow2 -O raw "name_of_vm".raw

Create the LVM - Make sure the "size_of_vm" is equal or greater than the size of your raw image

sudo virsh vol-create-as treehouse "name_of_vm" "size_of_vm"

Copy the raw image to the LVM

sudo dd if="name_of_vm".raw of=/dev/treehouse/"name_of_vm" bs=1M

Edit the VM configuration

sudo virsh edit "name_of_vm"

   <devices>
     ...
     <disk type='block' device='disk'>      <----- change disk type to block
       <source dev='/dev/treehouse/"name_of_vm"'/>      <------ note that the source is now dev= and not file=
       <target dev='vda' bus='virtio'/>
     </disk>
     ...
   </devices>

Now start your VM and your all set