Resource Allocation Best Practices

Specify resource allocation settings (shares, reservation, and limit) that are appropriate for your ESX/ESXi environment.

The following guidelines can help you achieve better performance for your virtual infrastructure.

If you expect frequent changes to the total available resources, use shares to allocate resources across virtual machines. If you use shares and then you upgrade the host, the number of shares does not change. For example, each virtual machine stays at the same priority even though each share represents a larger amount of memory or CPU.
Use reservations to specify the minimum acceptable amount of CPU or memory, not the amount that you want to have available. The host assigns additional resources as available based on the number of shares, estimated demand, and the limit for your virtual machine. The amount of resources specified by a reservation does not change when you modify the environment, such as by adding or removing virtual machines.
When specifying reservations for virtual machines, do not commit all resources. Plan to leave an appropriate portion unreserved, because when you move closer to reserving all system capacity, it becomes increasingly difficult to change reservations and the resource pool hierarchy.
For further details, see the vSphere documentation at www.vmware.com.