To calculate the amount of memory required to support the current workloads, the following formula is used:
The memory overhead of a virtual machine must also be accounted for when calculating memory requirements. The amount of memory required for an overhead depends on the configuration of the virtual machine.
The number of vCPUs allocated to the virtual machine, the amount of memory allocated to the virtual machine, and the virtual hardware configured for the virtual machine, will all have an impact on the amount of memory required for overhead:
Typically, the memory overhead required for a virtual machine is between 20 MB and 150 MB. Memory overhead estimations based on the amount of RAM and the number of vCPUs can be found in the vSphere documentation, as follows:
- vSphere 6.0: https://pubs.vmware.com/vsphere-60/topic/com.vmware.vsphere.resmgmt.doc/GUID-B42C72C1-F8D5-40DC-93D1-FB31849B1114.html
- vSphere 6.5/6.7: https://docs.vmware.com/en/VMware-vSphere/6.7/com.vmware.vsphere.resmgmt.doc/GUID-4954A03F-E1F4-46C7-A3E7-947D30269E34.html?hWord=N4IghgNiBcILYFM4HsBOBPABMgbg1AFgmACYgC+QA
This may seem like a small amount of memory, but over dozens (or even hundreds) of virtual machines, it can have a significant impact on the amount of total memory required:
To calculate the total memory required, future growth must be considered. When memory is calculated for growth, the memory overhead that's required to support the additional virtual machines must also be considered.
The maximum utilization threshold must also be determined for memory resources. This threshold defines the maximum percentage of the total memory resources that will be consumed. If the maximum utilization threshold is 75%, an additional 25% of memory resources will need to be added to calculate the total memory resources required:
875 GB of memory is required to support the current workloads, the future growth, and a maximum utilization threshold of 75%.