In many consolidated workloads, it is rare for all virtual machines to be actively using all of their memory simultaneously. Typically, some virtual machines are lightly loaded, while others are more heavily loaded, and relative activity levels generally vary over time. In such cases, it may be reasonable to overcommit memory to reduce hardware memory requirements.

ESX Server automatically transfers memory from idle virtual machines to virtual machines that actively need more memory in order to improve memory utilization.

You may also specify configuration parameters to preferentially devote space to important virtual machines.

The minimum size for a virtual machine defines a guaranteed lower bound on the amount of memory that it is allocated, even when memory is overcommitted. You can also use memory shares to specify the relative importance of different virtual machines. In any case, you should configure an appropriate minimum size for each virtual machine to ensure that each virtual machine can function effectively (without excessive paging), even when all virtual machines are active concurrently.

When memory is scarce, ESX Server dynamically reclaims space from some virtual machines based on importance and current working sets. For optimal performance, the server attempts to reclaim memory from a virtual machine via a VMware-supplied vmmemctl module running in the guest. This allows the guest operating system to invoke its own native memory management policies, causing it to swap to its own virtual disk only when necessary.

ESX Server also has its own swap file and may also swap memory from a virtual machine to the ESX Server swap file directly, without any involvement by the guest operating system.