VMware vSphere High Availability (vSphere HA) protects your virtual machines in case of host failure by restarting virtual machines on other hosts in the cluster when a host fails.

During configuration of the cluster, the hosts elect a master host. The master host communicates with the vCenter Server system and monitors the virtual machines and secondary hosts in the cluster.

The master hosts detects different types of failure:

Host failure, for example an unexpected power failure

Host network isolation or connectivity failure

Loss of storage connectivity

Problems with virtual machine OS availability

vSphere HA Design Decisions

Decision ID

Design Decision

Design Justification

Design Implication


Use vSphere HA to protect all clusters against failures.

vSphere HA supports a robust level of protection for both host and virtual machine availability.

Sufficient resources on the remaining host are required to so that virtual machines can be migrated to those hosts in the event of a host outage.


Set vSphere HA Host Isolation Response to Power Off.

vSAN requires that the HA Isolation Response be set to Power Off and to restart VMs on available hosts.

VMs are powered off in case of a false positive and a host is declared isolated incorrectly.

The vSphere HA Admission Control Policy allows an administrator to configure how the cluster judges available resources. In a smaller vSphere HA cluster, a larger proportion of the cluster resources are reserved to accommodate host failures, based on the selected policy.

The following policies are available:

Host failures the cluster tolerates.

vSphere HA ensures that a specified number of hosts can fail and sufficient resources remain in the cluster to fail over all the virtual machines from those hosts.

Percentage of cluster resources reserved.

Percentage of cluster resources reserved. vSphere HA ensures that a specified percentage of aggregate CPU and memory resources are reserved for failover.

Specify Failover Hosts.

When a host fails, vSphere HA attempts to restart its virtual machines on any of the specified failover hosts. If restart is not possible, for example the failover hosts have insufficient resources or have failed as well, then vSphere HA attempts to restart the virtual machines on other hosts in the cluster.