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.
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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
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