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We have implemented a 2-node HA Xen cluster, which consists of two physical machines (hosts,) and runs several virtual servers (guests) each, for our company's internal services (mail, web applications, development, etc.) When one host gets down unexpectedly, the other host physically kills it (STONITH - power down or reset) and then takes over all the guests the failed host was running. When we want to shutdown a host machine for maintenance (to replace a fan, add disk or memory, etc.), we just type the usual shutdown command, and the guests are automatically live-migrated to the other host. Since the guest servers keep running throughout the migration process, except for the less than a second pause, users would never even notice the event.