HA Failover and Admission controls
vSphere HA might not be able to fail over virtual machines because of resource constraints. This can occur for several reasons
HA admission control is disabled and Distributed Power Management (DPM) is enabled. This can result in DPM consolidating virtual machines onto fewer hosts and placing the empty hosts in standby mode leaving insufficient powered-on capacity to perform a failover.
VM-Host affinity (required) rules might limit the hosts on which certain virtual machines can be placed. The VM-Host affinity rules that are required, cannot be violated. vSphere HA does not perform a failover if doing so would violate such a rule.
There might be sufficient aggregate resources but these can be fragmented across multiple hosts so that they can not be used by virtual machines for failover.
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.
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.
Only vSphere HA admission control can be disabled. If this admission control is disabled there is no guarantee on how many virtual machines can be restarted in event of failover.