Vmware Slot Size Policy Tool
Vmware Slot Size Policy Guide

If I go to run time info of cluster in vmware I can it has used 32MHz 1, Virtual CPU and 40MB memory slot size. Then it says total slot is 691 and failover slot is 422. I tried both by using slot size of 32Mhz and 40MB memory and also using highest assigned cpu and memory in the powered on VMs but none of these calculation give the vlaue as in. When you select Slot Policy (Powered-on VMs) Admission Control will start resources calculations and VMs slots. If you have significant VMs or big Reservation Pools, the VM slot will also be large, and could not fit in the host failure capacity (if you have 1 or 2 host failover cluster). VMware streamlines the journey for organizations to become digital businesses that deliver better experiences to their customers and empower employees to do their best work. Our software spans App Modernization, Cloud, Networking & Security and Digital Workspace.

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

If no reservation is set anywhere HA will use the highest Memory Overhead in your cluster as the slot size for memory. This is where the amount of vCPUs come in to play, the more vCPUs you add to a virtual machine the higher will your memory overhead be. In vCenter Server 4.0, the slot size is shown in the vSphere Client on the Summary tab of the cluster. For more information, see the VMware Availability Guide. In vCenter Server 4.1 and 5.x, the slot size information is shown in the Summary tab of the cluster by clicking Advanced Runtime Info.

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.

Vmware Slot Size Policy Tool

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.

Vmware Slot Size Policy Guide

View the Advanced Runtime Info pane that appears in the vSphere HA section of the cluster's Monitor tab 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 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.