VM’s host memory usage = VM’s guest memory size + VM’s overhead memory

Each VM running on an vSphere consumes some memory overhead additional to the current usage of its configured memory. This extra memory is needed by ESX for the internal datastructures like virtual machine frame buffer and mapping table for memory translation (mapping guest physical memory to the actual machine memory)

• Virtual machine frame buffer

A framebuffer is a video output device that drives a video display from a memory buffer containing a complete frame of data.

• Mapping table for memory translation  – Mapping guest physical memory to the actual machine memory)

The VMM is responsible for mapping guest physical memory to the actual machine memory, and it uses shadow page tables to accelerate the mappings. As depicted
by the red line in the diagram, the VMM uses TLB (translation lookaside buffer) hardware to map the virtual memory directly to the machine memory to avoid the two levels of translation on every access. When the guest OS changes the virtual memory to physical memory mapping, the VMM updates the shadow page tables to enable a direct lookup.

Static overhead

This is the minimum amount of memory needed to start/boot the VM. DRS and the VMkernel uses this metric for admission control and VMotion calculations. The destination host must be able to back the virtual machine reservation and the static overhead otherwise the VMotion will fail.

Dynamic overhead

When the VM is powered on, the virtual machine monitor (VMM) can request additional memory space. The VMM will request the space, but the VMkernel is not required to supply it. If the VMM does not obtain the extra memory space, the virtual machine will continue to function but this can lead to performance degradation. The VMkernel treats virtual machine overhead reservation the same as VM-level memory reservation and it will not reclaim this

Memory Overhead Table