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vSphere HA admission control calculation for memory

By Duncan Epping, Principal Architect.

Someone asked me today how the calculation is done for memory when the "Percentage Based" admission control policy was selected. As hopefully all of you know by now, the percentage based admission control policy does not work with fixed slot sizes. (More details to be found in the Availability Guide and the Best Practices white paper.)The percentage based admission control policy is a per virtual machine mechanism, hence the reason I always prefer this admission control policy as it is most flexible.

So how is the memory part calculated? Memory is calculated by taking the total amount of resources in a cluster and from this the virtualization overhead like agents and the VMkernel is subtracted. That will give you the total amount of resources for workloads in the cluster. If you have set your percentage to for admission control to 25 then this is also subtracted from the total as available for virtual machines, as this is reserved for a possible fail-over. What is now left is what can be "claimed" by virtual machines.

With claimed I am referring to the fact that if virtual machines are powered-on the memory resources that are reserved (including the memory overhead) for that virtual machine is taken from the total amount of available resources. To give an example:

Cluster has 100GB of memory resources available. The percentage reserved for fail-over is 10. There is some additional minor memory overhead this is also subtracted. In total 89GB of memory is available for workloads. A virtual machine with 8GB of memory and a 2GB memory reservation is powered on. From the 89GB that 2GB memory reservation is subtracted and the memory overhead of 300MB is subtracted. Now there is 86.7GB for workloads.

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