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

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.

This problem can have several causes.

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).

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.

Click on the Advanced Runtime Info link that appears in the vSphere HA section of the cluster's Summary tab in the vSphere Client. This information box 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 Allocation tab 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 of these options, however, increase the risk of resource fragmentation.