vCenter Admission Control is a measure that is used to monitor and maintain the configured failover capacity in an HA cluster. It does so by restricting the number of operations that, if allowed, will reduce the failover capacity. Any operation that violates the resource constraints that are imposed by the admission control policy will not be permitted. Some of these operations include a virtual machine power-on operation, a vMotion operation, and a change in the CPU/memory reservation of the VM.
There are two aspects to configuring the failover capacity on a cluster:
- Host Failures Cluster Tolerates
- Failover Capacity Calculation Method
Host Failures Cluster Tolerates defaults to one, and it defines the number of hosts that can fail while still allowing the cluster to restart VMs and provide resources for the workloads.
Failover capacity is calculated using any of the following methods and the number of host failures to tolerate:
- Cluster resource percentage
- Slot policy (powered-on VMs)
- Dedicated failover hosts
Let's delve a bit deeper into how each of the preceding options affects the failover capacity.