The present invention aims at preventing the access performance of a distributed memory system by accessing via cross-over ownership a track mapping information formed as a hierarchical memory. In the process of assigning a real data storage area to a virtual volume, at first, a page from a pool is assigned, and thereafter, a track is assigned from said page. The page is composed of multiple tracks into which track data assigned at host write operation timings are stored sequentially from the top. A mapping information of the virtual volume and the page is stored in a control information page that differs from the track data, and the mapping information is stored in the control information page which could only be accessed by a microprocessor having the ownership of the virtual volume.