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

vSphere HA Design Basics

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

The master ESXi host detects different types of failure:

  • ESXi host failure, for example an unexpected power failure
  • ESXi host network isolation or connectivity failure
  • Loss of storage connectivity
  • Problems with virtual machine OS availability
Table 1. Design Decisions on vSphere HA

Decision ID

Design Decision

Design Justification

Design Implication


Use vSphere HA to protect all virtual machines against failures.

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

You must provide sufficient resources on the remaining hosts 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 ESXi hosts.

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

vSphere HA Admission Control Policy Configuration

The vSphere HA Admission Control Policy allows an administrator to configure how the cluster determines available resources. In a smaller vSphere HA cluster, a larger proportion of the cluster resources are reserved to accommodate ESXi 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 ESXi hosts can fail and sufficient resources remain in the cluster to fail over all the virtual machines from those ESXi hosts.
Percentage of cluster resources reserved
vSphere HA reserves a specified percentage of aggregate CPU and memory resources for failover.
Specify Failover Hosts
When an ESXi host fails, vSphere HA attempts to restart its virtual machines on any of the specified failover ESXi hosts. If restart is not possible, for example, the failover ESXi hosts have insufficient resources or have failed as well, then vSphere HA attempts to restart the virtual machines on other ESXi hosts in the cluster.