How does the primary host identify whether a secondary host is isolated or has failed in a vSphere HA cluster?

In a vSphere HA (High Availability) cluster, the primary host utilizes datastore heartbeating to determine whether a secondary host is isolated from the cluster or has experienced a failure. When a host is isolated or fails, it is critical for the primary host to identify the correct status of the secondary host to take appropriate actions, such as restarting virtual machines on other healthy hosts in the cluster.

Datastore heartbeating works by writing and reading heartbeats to and from the shared datastores that are accessible by the hosts in the cluster. If a primary host is unable to communicate with a secondary host over the network (indicating a potential network partition or isolation), it checks the datastore to see if the secondary host is still alive by looking for the presence of heartbeat files. If the secondary host's heartbeat is missing from the datastore, it indicates that the host is either isolated from the network or has failed entirely.

This method provides a more reliable way of monitoring host status compared to solely relying on network pings, as it accounts for scenarios where network communication may be disrupted but the host might still be operational at the datastore level. By using datastore heartbeats, vSphere HA can make informed decisions to maintain availability and recover workloads effectively within the cluster.