A storage system comprises a storage apparatus which includes a processor, storage disks, and a memory storing a page mapping table, a page mapping program, and a page-filename mapping program. A file system manages a file tree of files with filenames. The page mapping table specifies a relationship between data volumes in the storage apparatus and the storage disks and the file system, the data volumes each including pages, each page including segments, each segment including sectors. 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. 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.