You can configure vSphere HA to tolerate a specified number of host failures. With the Host Failures Cluster Tolerates admission control policy, vSphere HA ensures that a specified number of hosts can fail and sufficient resources remain in the cluster to fail over all the virtual machines from those hosts.
The Advanced Runtime Info box might display a smaller number of available slots in the cluster than you expect. When you select the Host Failures Cluster Tolerates admission control policy, the Advanced Runtime Info link appears in the vSphere HA section of the cluster's Summary tab in the vSphere Client or as a box in the cluster's Monitor tab of the vSphere Web Client.
This box displays information about the cluster, including the number of slots available to power on additional virtual machines in the cluster. This number might be smaller than expected under certain conditions. Cause Slot size is calculated using the largest reservations plus the memory overhead of any powered on virtual machines in the cluster.
However, vSphere HA admission control considers only the resources on a host that are available for virtual machines. This amount is less than the total amount of physical resources on the host, because there is some overhead. Solution Reduce the virtual machine reservations if possible, use vSphere HA advanced options to reduce the slot size, or use a different admission control policy.
Download link to the PPT: Mythbusters_Dutch_VMUG_2012.pptx