Thumb rules for memory allocation across virtual machines

Here are some thumb rules (best practices) for capacity planning of memory (RAM) allocation within your virtual infrastructure. These rules typically work 90 times of 100, however there would be exceptions:

  1. If you have over committed your available physical memory on the hypervisor, then the maximum memory configured on any single virtual machine should ideally be less than 40% of the total physical memory. The total memory configured on a single virtual machine in exceptional cases may exceed up to 60%.
  2. Over commitment should typically be not more 30% of the total physical memory on aggregate virtual machine basis. Remember only powered on virtual machines are considered for over-commitment.
  3. On a per virtual machine basis never over commit more than the available physical memory on the hypervisor.
  4. Its a good idea to reserve 30% of virtual machine memory for server virtualization projects, or increase it to accommodate active memory.
  5. Ensure memory assigned is vNUMA balanced.

On physical machines its a good practice to buy all the physical RAM from one single vendor with identical specifications. Physical RAM should be purchased in configuration to populate all the DIMM slots across all the physical sockets.