This white paper presents an overview of storage virtualisation at the controller level of a storage system. Virtualisation technology allows companies to respond quickly to ever-changing storage capacity requirements. Virtualisation has multiple definitions and many different implementations in a storage area network (SAN). This document describes how HP defines virtualisation technology and how it applies to the HP StorageWorks Enterprise Virtual Array (EVA) storage system. It explains tiered storage and the use of low-cost Fibre Attached Technology Adapted (FATA) drives and illustrates the cost benefit of FATA drives.