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If the network ping investigation returns that it cannot detect the status of the host, CloudStack HA then relies on the hypervisor specific investigation. For VMware, there is no such investigation as the hypervisor host handles its own HA. For XenServer and KVM, CloudStack HA deploys a monitoring script that writes the current timestamp on to a heartbeat file on shared storage.  If the timestamp cannot be written, the hypervisor host self-fences by rebooting itself.   For these two hypervisors, CloudStack HA sends a CheckOnHostCommand to a neighboring hypervisor host that shares the same storage. The neighbor then checks on the heartbeat file on shared storage and see if the heartbeat is no longer being written. If the heartbeat is still being written, the host reports that the host in question is still alive. If the heartbeat file’s timestamp is lagging behind, after an acceptable timeout value, the host reports that the host in question is down and HA is started on the VMs on that host.

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