1. In an information system that includes a host computer which issues an I/O request to a storage system, the storage system comprising: a storage apparatus which includes a processor, a plurality of storage disks, and a memory storing a page mapping table, a page mapping program, and a page-filename mapping program; and a file system which manages a file tree of files with filenames; wherein the page mapping table specifies a relationship between data volumes in the storage apparatus that are accessible from the host computer and the storage disks and the file system, the data volumes each including a plurality of pages, each page including a plurality of segments, each segment including a plurality of sectors; wherein the file tree has for each storage apparatus a hierarchy of directories and files based on relationships among the data volumes, the pages, and the segments; wherein the page mapping program and the page-filename mapping program are executable by the processor to specify, by page, a location of data contained in the I/O request by referring to the page mapping table and the file tree.