Admission Control and HA initiated restarts

Today I attended the vSphere 5.0: Overview training. This is the course where you can learn all the basic stuff that is necessary to operate vSphere Cluster. One of the discussed things was the Admission Control and its influence on powering up machines in our virtual infrastructure.

Now I can see how much misunderstanding arose around this mechanism and its connection with High Availability and possible hosts failures. Responsibility for Admission Control lies in the vCenter server and it is role is to not power up new machines when the configured constraints are violated. In case of the host failure the restarts are initiated by the host, not the vCenter server so all restarts, even if resources are low, will occur.

Let’s make a short quote from one of the best books about VMware HA written by Frank Denneman and Duncan Epping where they describe this mechanism in details:

“Admission Control will not disallow HA initiated restarts. HA initiated restarts are done on a host level and not through vCenter.”