HA allows VMs to automatically restart on another host in the resource pool in the event of a host failure. This functionality ensures that the amount of downtime for a VM is limited to just the amount of time it takes to restart the VM on another host. The recommended minimum amount of servers in a HA-enabled deployment is three, this allows us to maximize the availability of our deployment.
Heartbeats are used by the resource pool to determine whether a host is alive or reachable. Citrix XenServer uses two mechanisms for heartbeats, both are covered as follows: