====== Migrate a VM to a specific hypervisor ======

If the VM was created using 'boot from volume', or if there is a shared file system among compute nodes, you may use live migration where you can specify the target compute nodes.

Otherwise you can refer to the following instructions (taken from https://raymii.org/s/articles/Openstack_-_(Manually)_migrating_(KVM)_Nova_Compute_Virtual_Machines.html#Manual_migration_to_a_specific_compute_node). 


This procedure was tested on a Mitaka based Cloud.
 
**Use at your own risk !!
**


This example manually migrates a VM from compute-30.cloud.pd.infn.it to compute-34.cloud.pd.infn.it 

Shut down the VM first:

<code bash>
nova stop $VM_UUID
</code>

Also detach any volumes:

<code bash>
nova volume-detach $VM_UUID $VOLUME_UUID
</code>

Use the nova show command to see the specific hypervisor the VM is running on:

<code bash>
nova show UUID | grep hypervisor
</code>

Example output:

<code bash>
| OS-EXT-SRV-ATTR:hypervisor_hostname  | compute-30.cloud.pd.infn.it    |
</code>


Check also the instance name:

<code bash>
nova show UUID | grep instance
</code>

Example output:

<code bash>
| OS-EXT-SRV-ATTR:instance_name        | instance-00000b08                                                                |
</code>

Login to that hypervisor via SSH. Navigate to the folder where this instance is located, in our case, /var/lib/nova/instances/$UUID.

The instance is booted from an image based root disk, named disk. qemu in our case diffs the root disk from the image the VM was created from. Therefore the new hypervisor also needs that backing image. Find out which file is the backing image:

<code bash>
cd /var/lib/nova/instances/UUID/
qemu-img info disk # disk is the filename of the instance root disk

</code>

Example output:

<code>
  image: disk
  file format: qcow2
  virtual size: 32G (34359738368 bytes)
  disk size: 1.3G
  cluster_size: 65536
  backing file: /var/lib/nova/instances/_base/d004f7f8d3f79a053fad5f9e54a4aed9e2864561
  Format specific information:
      compat: 1.1
      lazy refcounts: false 
</code>      
      
      
The file /var/lib/nova/instances/_base/d004f7f8d3f79a053fad5f9e54a4aed9e2864561 is the backing disk. 


Check the target hypervisor for the existence of that image. If it is not there, copy that file to the target hypervisor first:

<code bash>
rsync -r --progress /var/lib/nova/instances/_base/d004f7f8d3f79a053fad5f9e54a4aed9e2864561 -e ssh compute-34:/var/lib/nova/instances/_base/d004f7f8d3f79a053fad5f9e54a4aed9e2864561
</code>

On the target hypervisor, set the correct permissions:

<code bash>
chown nova:nova /var/lib/nova/instances/_base/d004f7f8d3f79a053fad5f9e54a4aed9e2864561
</code>

Copy the instance folder to the new hypervisor:

<code bash>
cd /var/lib/nova/instances/
rsync -r --progress $VM_UUID -e ssh compute-34:/var/lib/nova/instances/
</code>

Set the correct permissions on the folder on the target hypervisor:

<code bash>


chown nova:nova /var/lib/nova-compute/instances/$VM_UUID
chown nova:nova /var/lib/nova-compute/instances/$VM_UUID/*
</code>


Log in to your database server. Start up a MySQL command prompt in the nova database:

<code bash>
mysql nova
</code>

Execute the following command to update the nova database with the new hypervisor for this VM:

<code bash>
update instances set node='compute-34.cloud.pd.infn.it', host=node where uuid='$VM_UUID';
</code>

Use the nova show command to see if the new hypervisor is set. If so, start the VM:

<code bash>
nova start $VM_UUID
</code>

Attach any volumes that were detached earlier:

<code bash>
nova volume-attach $VM_UUID $VOLUME_UUID
</code>

On the source hypervisor (compute-30), delete the instance from libvirt:

<code bash>
virsh undefine instance-00000b08
</code>