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 primary ESXi host. The primary ESXi host communicates with the vCenter Server system and monitors the virtual machines and secondary ESXi hosts in the cluster.
The primary 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
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.
- 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.