Linux: PingWales’ round-up of UKUUG Linux 2005 Day 3 includes this snippet:
As well as running (Virtual Machines), Xen allows them to be migrated on the fly. If a physical system is overloaded, or showing signs of failure, a virtual machine can be migrated to a spare node. This process takes time, but causes very little interruption to service. The machine state is first copied in its entirety, then the changes are copied repeatedly until there are a small enough number than the machine can be stopped, the remaining changes copied and the new version started. This usually provides a service interruption of under 100ms – a small enough jitter that people playing Quake 3 on a server in a virtual machine did not notice when it was moved to a different node.
Now that is cool.