In one line - if we enable admission control in HA, it won’t allow us to put host in maintenance mode or standby, if its has VM in it, reason refer below.
If you want to put the host in maintenance mode for testing purpose or upgrade VMware recommends move the VMS manually to other host or disable available constraints for time being so that VMs will be automatically moved out to other VMs.
It’s recommended to use enable available constraints by VMware.
Implications of enabling VMware HA strict admission control
When VMware HA strict admission control is enabled, DRS and VMware DPM protect the availability of failover capacity at all times, and only take actions or make recommendations that are consistent with ensuring the availability of that capacity.
If we enable admission control below are advantage and disadvantage
Advantage:
Protect the availability of failover capacity at all times, and only take actions or make recommendations that are consistent with ensuring the availability of that capacity.
Disadvantage:
DRS does not evacuate virtual machines from a host for the purpose of placing it in maintenance mode or standby mode if placing the host in this state would violate failover requirements. You can still manually evacuate virtual machines in order to place hosts in maintenance mode or standby mode. If you violate failover requirements by doing this, however, the cluster turns red.
If you want to put the host in maintenance mode for testing purpose or upgrade VMware recommends move the VMS manually to other host or disable available constraints for time being so that VMs will be automatically moved out to other VMs.
It’s recommended to use enable available constraints by VMware.
Implications of enabling VMware HA strict admission control
When VMware HA strict admission control is enabled, DRS and VMware DPM protect the availability of failover capacity at all times, and only take actions or make recommendations that are consistent with ensuring the availability of that capacity.
If we enable admission control below are advantage and disadvantage
Advantage:
Protect the availability of failover capacity at all times, and only take actions or make recommendations that are consistent with ensuring the availability of that capacity.
Disadvantage:
DRS does not evacuate virtual machines from a host for the purpose of placing it in maintenance mode or standby mode if placing the host in this state would violate failover requirements. You can still manually evacuate virtual machines in order to place hosts in maintenance mode or standby mode. If you violate failover requirements by doing this, however, the cluster turns red.
