Troubleshooting vSphere HA Admission Control

vCenter Server uses admission control to ensure that sufficient resources in a vSphere HA cluster are

Reserved for virtual machine recovery in the event of host failure. If vSphere HA admission control does not function properly, there is no assurance that all virtual machines in the cluster can be restarted after a host failure.

Red Cluster Due to Insufficient Fail over Resources

When you use the Host Failures Cluster Tolerates admission control policy, vSphere HA clusters might

Become invalid (red) due to insufficient fail over resources.

Problem

If you select the Host Failures Cluster Tolerates admission control policy and certain problems arise, the

cluster turns red.

 

Cause

This problem can arise when hosts in the cluster are disconnected, in maintenance mode, not responding, or

Have a vSphere HA error. Disconnected and maintenance mode hosts are typically caused by user action.

Unresponsive or error-possessing hosts usually result from a more serious problem, for example, hosts or

Agents have failed or a networking problem exists.

Another possible cause of this problem is if your cluster contains any virtual machines that have much

Larger memory or CPU reservations than the others. The Host Failures Cluster Tolerates admission control

Policy is based on the calculation on a slot size consisting of two components, the CPU and memory

Reservations of a virtual machine. If the calculation of this slot size is skewed by outlier virtual machines, the

Admission control policy can become too restrictive and result in a red cluster.

VMware, Inc.  37

Solution

Check that all hosts in the cluster are healthy, that is, connected, not in maintenance mode and free of

vSphere HA errors. vSphere HA admission control only considers resources from healthy hosts.

Unable to Power On Virtual Machine Due to Insufficient Failover Resources

You might get a not enough failover resources fault when trying to power on a virtual machine in a

vSphere HA cluster.

 

Problem

If you select the Host Failures Cluster Tolerates admission control policy and certain problems arise, you

Might be prevented from powering on a virtual machine due to insufficient resources.

Cause

This problem can have several causes.

n Hosts in the cluster are disconnected, in maintenance mode, not responding, or have a vSphere HA

Error.

Disconnected and maintenance mode hosts are typically caused by user action. Unresponsive or error possessing hosts usually result from a more serious problem, for example, hosts or agents have failed or

a networking problem exists).

n Cluster contains virtual machines that have much larger memory or CPU reservations than the others.

The Host Failures Cluster Tolerates admission control policy is based on the calculation on a slot size

Comprised of two components, the CPU and memory reservations of a virtual machine. If the

Calculation of this slot size is skewed by outlier virtual machines, the admission control policy can

Become too restrictive and result in the inability to power on virtual machines.

No free slots in the cluster.

Problems occur if there are no free slots in the cluster or if powering on a virtual machine causes the slot

size to increase because it has a larger reservation than existing virtual machines. In either case, you

Should use the vSphere HA advanced options to reduce the slot size, use a different admission control

Policy, or modify the policy to tolerate fewer host failures.

Solution

View the Advanced Runtime Infopane that appears in the vSphere HA section of the cluster’s Monitortab

in the vSphere Web Client. This information pane shows the slot size and how many available slots there are

in the cluster. If the slot size appears too high, click on the Resource Allocationtab of the cluster and sort

the virtual machines by reservation to determine which have the largest CPU and memory reservations. If

there are outlier virtual machines with much higher reservations than the others, consider using a different

vSphere HA admission control policy (such as the Percentage of Cluster Resources Reserved admission

control policy) or use the vSphere HA advanced options to place an absolute cap on the slot size. Both