Configuring vSphere HA is a part of the cluster configuration in vSphere Web Client; click on the EDIT... button in the vSphere Availability area, as shown in the following screenshot:

Configuration in the basic state is straightforward; click on the Turn ON vSphere HA checkbox and then OK:

Under the Recent Tasks area, you can see Configuring vSphere HA on each host, so wait for success on all hosts of the cluster:

When you click on Turn ON on every ESXi host in the cluster, a special agent called Fault Domain Manager (FDM) is installed. A log specific to the FDM agent is available on each host in the /var/log/fdm.log file.

Now we have to enable HA and protect VMs. When a host fails, vSphere HA restarts the VMs on other hosts in the cluster. In each cluster, there is a master host that is responsible for managing all other slave hosts, and the master node reports the current state of the cluster to the vCenter server. The vCenter server is used to configure the vSphere HA, but the master node is responsible for HA events, so even if your vCenter server is not available, the vSphere HA will work as expected.

To configure vSphere HA, we have the following requirements:

• Right license: Minimal Essential Plus
• Two hosts minimal: 64 hosts, which is the maximum per cluster in vSphere 6.0 and 6.5
• At least two share datastores: These are required for datastore heartbeats, but you can also have a single shared datastore (in this case, you need an advanced option to remove the warning on the datastore numbers)
• vCenter Server: Needed to configure vSphere HA, but is not involved in actual HA failovers