Observe the following best practices for the configuration and usage of admission control for vSphere HA.
The following recommendations are best practices for vSphere HA admission control.
Select the Percentage of Cluster Resources Reserved admission control policy. This policy offers the most flexibility in terms of host and virtual machine sizing. When configuring this policy, choose a percentage for CPU and memory that reflects the number of host failures you want to support. For example, if you want vSphere HA to set aside resources for two host failures and have ten hosts of equal capacity in the cluster, then specify 20% (2/10).
Ensure that you size all cluster hosts equally. For the Host Failures Cluster Tolerates policy, an unbalanced cluster results in excess capacity being reserved to handle failures because vSphere HA reserves capacity for the largest hosts. For the Percentage of Cluster Resources Policy, an unbalanced cluster requires that you specify larger percentages than would otherwise be necessary to reserve enough capacity for the anticipated number of host failures.
If you plan to use the Host Failures Cluster Tolerates policy, try to keep virtual machine sizing requirements similar across all configured virtual machines. This policy uses slot sizes to calculate the amount of capacity needed to reserve for each virtual machine. The slot size is based on the largest reserved memory and CPU needed for any virtual machine. When you mix virtual machines of different CPU and memory requirements, the slot size calculation defaults to the largest possible, which limits consolidation.
If you plan to use the Specify Failover Hosts policy, decide how many host failures to support and then specify this number of hosts as failover hosts. If the cluster is unbalanced, the designated failover hosts should be at least the same size as the non-failover hosts in your cluster. This ensures that there is adequate capacity in case of failure.