vCenter Server uses admission control to ensure that sufficient resources are available in a cluster to provide failover protection and to ensure that virtual machine resource reservations are respected.
Three types of admission control are available.
Ensures that a host has sufficient resources to satisfy the reservations of all virtual machines running on it.
Ensures that a resource pool has sufficient resources to satisfy the reservations, shares, and limits of all virtual machines associated with it.
Ensures that sufficient resources in the cluster are reserved for virtual machine recovery in the event of host failure.
Admission control imposes constraints on resource usage and any action that would violate these constraints is not permitted. Examples of actions that could be disallowed include the following:
Powering on a virtual machine.
Migrating a virtual machine onto a host or into a cluster or resource pool.
Increasing the CPU or memory reservation of a virtual machine.
Of the three types of admission control, only vSphere HA admission control can be disabled. However, without it there is no assurance that the expected number of virtual machines can be restarted after a failure. Do not disable admission control, however you might need to do so temporarily, for the following reasons:
If you need to violate the failover constraints when there are not enough resources to support them--for example, if you are placing hosts in standby mode to test them for use with Distributed Power Management (DPM).
If an automated process needs to take actions that might temporarily violate the failover constraints (for example, as part of an upgrade directed by vSphere Update Manager).
If you need to perform testing or maintenance operations.
Admission control sets aside capacity, but when a failure occurs vSphere HA uses whatever capacity is available for virtual machine restarts. For example, vSphere HA places more virtual machines on a host than admission control would allow for user-initiated power ons.
When vSphere HA admission control is disabled, vSphere HA ensures that there are at least two powered-on hosts in the cluster even if DPM is enabled and can consolidate all virtual machines onto a single host. This is to ensure that failover is possible.