Although vSphere High Availability can also be used to to protect against VM- and application-level failures, it is primarily used to protect agains ESXi host failures. If an ESXi host crashes or doesn’t see network traffic coming from other hosts in the cluster, the VMs that were running on the affected host will be restarted on other hosts in the cluster.
To implement vSphere HA, the following requirements must be met:
- all ESXi hosts in a vSphere HA cluster must have access to the same shared storage locations used by all VMs on the cluster. This includes all Fibre Channel, FCoE, iSCSI, and NFS datastores.
- all hosts in a vSphere HA cluster must have an identical virtual networking configuration.
- If a new switch is added to one host, the same new switch must be added to all hosts.
- all hosts must be licensed for vSphere HA.
vSphere HA is configured on a cluster. When vSphere HA is enabled, a service called Fault Domain Manager is started on each host in the cluster. This service starts agents on each ESXi host that participate in an election to pick a vSphere HA master. The master will manage the HA domain and monitors slave hosts. By default, the host that can access the greatest number of datastores will become the master.