Datastore heartbeating is enabled by default and chooses two heartbeat datastores per host. It does so by making sure that it selects the datastores that are not backed by the same NFS server or storage, and are mapped to at least two ESXi hosts. Datastore heartbeating works by using VMFS's filesystem locking mechanism. It creates files—what VMware calls heartbeat files, which are kept open in order to keep a filesystem lock active on that file. Every host in the HA cluster gets a corresponding heartbeat file. In the event of network isolation, the master checks the liveliness of the isolated host by checking whether there is an active lock on its heartbeat file. The heartbeat files are stored in a directory called .vSphere-HA, which can be located on the root of the heartbeat datastore, as you can see in the following screenshot:

There are three different heartbeat datastore selection policies:

• Automatically select datastores that are accessible from the hosts: This selects two heartbeat datastores from all of the datastores that can be seen by the ESXi host in the cluster.
• Use datastores only from the specified list: This selects only the datastores that you specify as the heartbeat datastores. In other words, you can manually assign two desired heartbeat datastores from the entire list. It is important to note that, with this setting, you also set just one datastore as the heartbeat datastore. If you select more than one datastore, then HA will automatically select any two from the specified datastores.
• Use datastores from the specified list and complement automatically if needed: This selects two heartbeat datastores from the specified list, but it can also automatically select datastores if you specify less than two or more than two datastores. For instance, if you were to select just one datastore from the list, then HA will use the specified datastore and automatically select the second datastore. If you were to select more than two datastores, then HA will automatically select two of the three specified datastores as heartbeat datastores.