Admission Control explained

The Admission Control feature in vSphere HA ensures that sufficent resources are available in a HA cluster to provide failover protection. You can use Admission Control to determine whether a user will be allowed to power on more VMs than the HA cluster has the capacity to support.

Admission Control ensures that resources will always be available on the remaining hosts in the HA cluster to power on the virtual machines that were running on a failed host. If you enable this feature, the VM power-on operations that violate availability constraints will be disallowed.

To better understand the Admission Control concept, consider the following example. Let’s say that we have a cluster of four identical ESXi hosts running identically configured virtual machines. A cluster acts as a single pool of resources and the VMs consume a total of 75% its resources. The cluster is configured for a single ESXi host failure.

Let’s say that we want to power on one more VM. This means that the resource consumption will increase above 75%. If Admission Control is enabled, we will not be able to power on the new VM. Why? Well, each host of our four hosts in the cluster is equal to 25% of the cluster capacity. Because the cluster is at the limit of the capacity it can support if one host fails, Admission Control will prevent us from starting more VMs than it has resources to protect.

If Admission Control was disabled, we would be able to power on VMs until all of the cluster resources are allocated. But if an ESXi host fails, it’s possible that some of the VMs would not be able to be restarted because there wouldn’t be sufficient resources to power on all the VMs.