For a long time I have been sizing vSphere clusters for customers and I am regularly asked how I work out the overcommitment ratio and calculate the suitable percentage of cluster resources to reserve for HA.

This calculator is focused on Virtual Server clusters, for VDI solutions I recommend the VDI Calculator by @andreleibovici.

This calculator allows this to be done quickly and easily while giving advice as to the availability level recommended for the cluster based on its size.

All you need to do is enter the follow details into the Yellow fields.

1. The Number of ESXi hosts

2. The total CPU sockets per host

3. Ghz per Core

4. Physical Cores per CPU Socket (Not Hyper-threads)

5. Total RAM per host

6. Total number of VMs

7. The Desired Availability Level (N+x)

Next enter the total number of vCPUs and vRAM assigned (or expected to be) assigned to VMs in the cluster.

The calculator will then output the following:

1. Total Cluster Resources

This is the total Physical Cores and physical RAM in the cluster.

2. The total Cluster Ghz

Self Explanatory

3. The percentage of Cluster Resources reserved for HA

This is calculated from the availability level specified.

Note: The recommended Availability level is calculator and displayed on the same line as the “Desired Availability Level”.

4. The total available Cluster Resources

This is the total cluster resources, minus the percentage of cluster resources reserved for HA. Note: This is not how vSphere HA calculates available cluster resources. This is a method I use which is conservative and ensures performance does not degrade in the event of the configured availability level.

Finally, it calculates

5. The Overcommitment Ratio for CPU and RAM

This is represented as a ratio, so a result of “1” means no overcommitment.

A result of “4” would be a 4:1 overcommitment or 400%.

The tool then shows a “Rule of Thumb” for overcommitment levels of the vSphere cluster.

Simply modify the number of hosts, Cores per Socket and RAM per host until you have the desired overcommitment levels, then you can Print the sizing chart for your design.

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