VMware HA clusters enable a collection of ESXi hosts to work together so that, as a group, they provide higher levels of availability for virtual machines than each ESXi host could provide individually. When you plan the creation and usage of a new VMware HA cluster, the options you select affect the way that cluster responds to failures of hosts or virtual machines.
Before creating a VMware HA cluster, you should be aware of how VMware HA identifies host failures and isolation and responds to these situations. You also should know how admission control works so that you can choose the policy that best fits your failover needs. After a cluster has been established, you can customize its behavior with advanced attributes and optimize its performance by following recommended best practices.
When you create a VMware HA cluster, a single host is chosen as the master host to communicate with vCenter Server and to monitor the state of the other, slave, hosts and their virtual machines. Different types of host failures are possible and must be detected and appropriately dealt with. To do this, the master host must distinguish between a failed host and one that is in a network partition. Datastore heartbeating is used to do this.