Using VMware HA with VMware DRS combines automatic failover with load balancing. This combination can result in faster rebalancing of virtual machines after VMware HA has moved virtual machines to different hosts.
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 might be comparatively lightly loaded. VMware HA uses the CPU and memory reservation to determine failover, but 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.
- DRS evacuates virtual machines from hosts and place the hosts in maintenance mode or standby mode regardless of the effect this might have on failover requirements.
- VMware DPM powers off hosts (place them in standby mode) even if doing so violates failover requirements.
The VMware HCI cluster includes VMware DRS and VMware HA features, so all the above remarks apply.
