Admission control is a policy used by vSphere HA to ensure failover capacity within a cluster.

Raising the number of potential host failures to tolerate will increase the availability restraints and capacity reserved. You can reserve a percentage of Persistent Memory for Host failover capacity. This is actual storage capacity that is blocked and must be considered for host power off.

Under Edit Cluster Settings you can select Admission Control to specify the number of failures the host will tolerate.

If you select CPU/Memory reservation defined by:

  • Cluster resource percentage, some amount of persistent memory capacity in the cluster is dedicated for failover purpose even if the virtual machines in the cluster are not using persistent memory currently. This percentage can either be specified through an override, or it is automatically calculated according to the host failures to tolerate setting. When PMem admission control is enabled, PMem capacity is reserved across the cluster even if there are VMs using PMem as disks.
  • Slot Policy (powered-on VMs), persistent memory admission control overrides the Slot Policy with the Cluster Resource Percentage policy, for persistent memory resources only. The percentage value is automatically calculated from the host failures cluster tolerates setting and cannot be overriden.
  • Dedicated failover hosts, the persistent memory of the dedicated failover hosts is dedicated for failover purpose and you cannot provision virtual machines with persistent memory on these hosts.
Note: After you select an admission control policy, you must also click the Reserve Persistent Memory failover capacity checkbox to enable PMem admission control.