In recent years, it has not been unusual for enterprise IT systems to have utilization rates of about 10％ or less for their disk units. So we can expect that storage virtualization will be effective for cost reduction as well as server virtualization. The term “storage virtualization” has recently been used in contexts such as the one mentioned above. But it originally meant a disconnection of storage’s physical topology from a logical view, and mapping the physical resources for a logical view of applications. Storage systems carry out such disconnection and mapping in an invisible manner. This paper explains virtualization in a block access layer and file aggregation layer as an example of how virtualization is adopted in an operational system.