[peterwwillis]

If they have one spare xen host they can live migrate all guests from one host to the spare, patch the original host, reboot, then live migrate the spare's guests back to the original, and repeat. Patching them all and rebooting them all at once might be quicker though.

[mikeash]

Do you need to migrate them back? Would be faster to just use the newly patched original host as the new spare, and repeat like that.

[jroll]

This becomes a physics problem; it's a race between how quickly VMs can be migrated and when the embargo is lifted.

[avsm]

The XenServer toolstack supports exactly this mode of operation in a pool of physical hosts via a 'host evacuate' operation that live relocates VMs away and brings them back once the host upgrade is complete.

http://docs.vmd.citrix.com/XenServer/5.0.0/1.0/en_gb/install...