When VMware HA performs failover and restarts virtual machines on different hosts, its first priority is the immediate availability of all virtual machines. After the virtual machines have been restarted, those hosts on which they were powered on might be heavily loaded, while other hosts are comparatively lightly loaded. VMware HA uses the CPU and memory reservation to determine failover, while the actual usage might be higher.

In a cluster using DRS and VMware HA with admission control turned on, virtual machines might not be evacuated from hosts entering maintenance mode because of resources reserved to maintain the failover level. You must manually migrate the virtual machines off of the hosts using VMotion.

When VMware HA admission control is disabled, failover resource constraints are not passed on to DRS and VMware Distributed Power Management (DPM). The constraints are not enforced.