This white paper presents the results of a study centered on the characterization of I/O patterns of Microsoft Exchange Server when deployed on a storage subsystem based on EqualLogic SAN and in a dense server environment built on Dell PowerEdge blade servers. It will guide Exchange and SAN administrators to understand their messaging workload, and predict their SAN size requirements. The scope of this paper is a virtual infrastructure built on VMware vSphere connecting to the SAN directly from the virtual machines and storage sizing, leaving aside server sizing activity.