Each virtual machine consumes memory based on its configured size, plus additional overhead memory for virtualization.

The configured size is a construct maintained by the virtualization layer for the virtual machine. It is the amount of memory that is presented to the guest operating system, but it is independent of the amount of physical RAM that is allocated to the virtual machine, which depends on the resource settings (shares, reservation, limit) explained below.

For example, consider a virtual machine with a configured size of 1GB. When the guest operating system boots, it detects that it is running on a dedicated machine with 1GB of physical memory. The actual amount of physical host memory allocated to the virtual machine depends on its memory resource settings and memory contention on the ESX/ESXi host. In some cases, the virtual machine might be allocated the full 1GB. In other cases, it might receive a smaller allocation. Regardless of the actual allocation, the guest operating system continues to behave as though it is running on a dedicated machine with 1GB of physical memory.


Specify the relative priority for a virtual machine if more than the reservation is available.


Is a guaranteed lower bound on the amount of physical memory that the host reserves for the virtual machine, even when memory is overcommitted. Set the reservation to a level that ensures the virtual machine has sufficient memory to run efficiently, without excessive paging.

After a virtual machine has accessed its full reservation, it is allowed to retain that amount of memory and this memory is not reclaimed, even if the virtual machine becomes idle. For example, some guest operating systems (for example, Linux) might not access all of the configured memory immediately after booting. Until the virtual machines accesses its full reservation, VMkernel can allocate any unused portion of its reservation to other virtual machines. However, after the guest’s workload increases and it consumes its full reservation, it is allowed to keep this memory.


Is an upper bound on the amount of physical memory that the host can allocate to the virtual machine. The virtual machine’s memory allocation is also implicitly limited by its configured size.

Overhead memory includes space reserved for the virtual machine frame buffer and various virtualization data structures.