Rolling pool upgrade allows the host software to be upgraded without incurring downtime. An upgrade is a sequence of operations:
The following diagram shows an upgrade of a 4 host pool:
The xapi master host must be upgraded first. A new xapi version can always talk to an old xapi, but an old xapi may not be able to talk to a a new one. Once the master is upgraded the rest of the hosts may be upgraded in any order. When the upgrade is complete all VMs should still be running (provided there was enough spare capacity during the procedure) but may now be running on different hosts.