VMware HA Slot is the default  admission control option prior to vSphere 6.5. Slot Size is defined as the memory and CPU resources that satisfy the reservation requirements for any powered-on virtual machines in the HA cluster.This article is just to cover how the HA slots are calculated step by step not more than that. If you want to read the detailed information about HA, Please refer HA Deep Dive by Duncan Epping.

As per VMWare’s Definition,

“VMware HA slots is a logical representation of the memory and CPU resources that satisfy the requirements for any powered-on virtual machine in the cluster.”

If you have configured reservations at VM level, It influence the HA slot calculation. Highest memory reservation and highest CPU reservation of the VM in your cluster determines the slot size for the cluster.

Here is the Example,

If you have the VM configured with the highest memory reservation of 8192 MB (8 GB) and  highest CPU reservation of 4096 MHZ. among the other VM’s  in the cluster, then the slot size for memory is 8192 MB and slot size for CPU is 4096 MHZ. in the cluster.

If no VM level reservation is configured , Minimum CPU size of 256 MHZ and memory size of 0 MB +  VM memory overhead will be considered as CPU and Memory slot size.

Calculation for Number of HA Slots in cluster :-

Once we got the Slot size for memory and CPU by the above method , Use the below calculation

Num of CPU Slots  = Total available CPU resource of ESX or cluster   /  CPU Slot Size

Num of memory slots = Total available memory resource of ESX or cluster minus memory used for service console & ESX system /  Memory Slot size

Let’s take a Example,  

I have 3 host on the cluster and 6 Virtual machine is running on the cluster and Each host capacity as follows

RAM = 50 GB per Host

CPU = 8 X 2.666 GHZ  per host

Cluster RAM Resources = 50 X 3 = 150 GB – Memory for service console and system = 143 GB

Cluster CPU resources = 8 X 2.6 X 3 =  63 GHZ (63000 MHZ) of total CPU capacity in the cluster – CPU Capacity used by the ESX System = 60384 MHZ

I don’t have any memory  or CPU reservation in my cluster, So,  the default CPU slot size 256 MHZ and one of my Virtual machine is assigned with 8 vcpu and its memory overhead is  344.98 MB (which is the highest overhead among my 6 virtual machines in the cluster)

Let’s calculate the num of  CPU  & Memory slots

Num of CPU Slots  = Total available CPU resource of cluster /  CPUSlot size in MHZ

No of CPU Slots = 60384 MHZ / 256 MHZ = 235.875 Approx

Num of Memory Slots =  Total available Memory resource of cluster  /  memory Slot Size  in MB

Num of Memory Slots =  146432 / 345 =  424 Approx

The most restrictive number among CPU and Memory slots determines the amount of slots for this cluster. We have 235 slots available for  CPU and 424 Slots available for Memory. So the most restrictive number is 235.

So, Total number of slots for my cluster is 235 Approx. Please find the  below snapshot

Please refer my blog post on Understanding Total Slots, Used Slots & Available slots in VMware HA Slot to understand Total Slots, Used Slots & Available slots. I hope this article is helpful to you to understand HA slots calculation. Thanks for Reading!!. Be social and share it in social media, if you feel worth sharing it.