Understanding Total Slots, Used Slots & Available slots in VMware HA Slots

Posted by Mohammed Raffic on September 1, 2012 in HA Slots, High Availability | 347 Views | 3 Responses

This post is the follow up post for my previous post on VMware HA Slot Calculation. In that post, i have explained the step by step procedure for how to calculate the HA slot information. This post clarifies more on the Total Slots, Used Slots & Available slots in VMware HA Slot calculation. I strongly recommend to read my previous post on HA slot calculation to fully understand the concepts of this post.

From the above snapshot,

Total Slots in Cluster = 234

Used Slots  = 6

Available slots  = 150 

What is that? How does it calculate and our mind will think of the below calculation

Available slots = Total slot – Used slots i,e  Available slots = 234 – 6 = 228. It should come as 228 as available slots but why Available slots is 150 in the above snapshot. Is that wrong or VMware did something wrong in HA slot calculation? Absolutely NOT. Below is the method of calculation for Total Slots, Used Slots & Available slots.

Available Slots  = (Total Slots -Used Slots) – Slots reserved for failover capacity 

How to Calculate Slots reserved for failover ?

It is basically coming from you Admission Control Policy of HA cluster. I have enabled admission control and configured Admission control Policy as ” Host Failures Cluster Tolerates” is equal to 1 . So It will make sure the resources always be available in the cluster as a reserved capacity to tolerate the 1 ESX host failure in my 3 node HA cluster.So, It has to reserve some capacity for fail over purposes.

Lets Come back to the Calculation

Total Available Slots in the Cluster = 234

No of Hots in HA cluster = 3

Total Available slots per ESX host = 234 /3 = 78 Slots Per Host

“Host Failures Cluster Tolerates” Admission control Policy = 1 host Failure. So, 1 host failure should be tolerated in the cluster by reserving 78 Slots for fail over purposes.

Available Slots  = (Total Slots -Used Slots) – Slots reserved for fail over by admission control policy

Available Slots  = (234 – 6)  - 78 

Available Slots  = 150 Slots

I hope this post will clear doubts regarding Total Slots, Used Slots & Available slots in VMWare HA slot calculation. If you are looking for more information on Admission Control policy, Please visit my blog post on HA interview questions.

Thanks For Reading!!!!

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