When there is more than one host in a cluster, it's usually expected that these hosts are redundant. In other words, if one or more hosts fail, the remaining should still be able to accommodate all VMs.

For this reason, each time a new VM is powered on or more memory or CPU is added to a VM, the administrator needs to be sure that the required redundant capacity is still available.

VMware offers a feature called admission control, which helps to ensure that sufficient resources are available to provide the required reservations or redundancy.

Once enabled, admission control is automatic. It is available not only for hosts but also for resource pools and vSphere HA.

Admission control monitors the environment for certain actions and decides whether they should be allowed or not based on resources that will be available after the action. This is to make sure that once the action is allowed, there are still enough resources to provide the expected...