The VM Monitoring service, which is serviced by VMware Tools, evaluates whether each VM in the cluster is running or not. Regular heartbeats and I/O activity from the VMware Tools process will be checked by the VM Monitoring service to determine the running guest.
Sometimes, VM heartbeats or I/O activity is not received by the VM Monitoring service because the guest operating system fails or VMware Tools is not being allocated time to complete its tasks. If the VM Monitoring service does not hear those heartbeats, then it declares that the VM has failed and the VM is rebooted to restore service.
The VM Monitoring service also monitors a VM's I/O activity just to avoid unnecessary resets. If there are no heartbeats received within the failure interval, the I/O stats interval (a cluster-level attribute) is checked. The I/O stats interval (by default, 120 seconds) determines whether any disk or network activity has occurred for the VM during the previous 2 minutes. If not...