Recently my team and I ran into incident with and error while patching esxi servers using VMware Update Manager(VUM).  When attempting o remediate the following error message was shown:

“cannot remediate host because it is part of HA Admission Control enabled Cluster”

Cause:

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.

Admission control imposes constraints on resource usage and any action that would violate these constraints is not permitted. If an automated process needs to take actions, it might temporarily violate the failover constraints.

 

Solution:

Before patching of the ESXi Servers that are part of the HA Cluster, make sure you have disabled “Admission Control”. Once server has been patched you can re-enable Admission Control on the cluster.

 

Steps to disable Admission Control

  • Right-click the cluster and click Edit Settings.
  • Under Cluster Features, click VMware HA.
  • Under Admission Control, select Disable: Power on VMs that violate availability constraints.
  • Click OK

This can also be disabled in the VMware Update Manager remediation wizard. When you remediate check the option “Disable High Availability admission control if it is enabled for any of the selected clusters.