Patent ID: 8433674

Claim:
A file sharing server, which comprises at least one computer and is connected to a plurality of storage devices, the server comprising: a storage management section; and a file migration control section, wherein the storage management section is configured to arrange the plurality of storage devices into a hierarchy of tiers, manage each of the hierarchized tiers by assigning thereto a corresponding priority in accordance with either characteristics or performance of the plurality of storage devices, create for each of the hierarchized tiers a first and second list of files that are migration candidates, each of the lists including a plurality of entries which respectively contain an ID and access count of a file, when a maximum number of the entries for each of the lists is predetermined; in response to an I/O request to a file in one of the hierarchized tiers from the computer, when the file is not in the lists for the tier, add an entry of the file to the first list for the tier, and then when the first list to which the entry added is overflowing, delete an entry of a file which is the least recently accessed from the overflowing first list; and when the file is in the first list for the tier, increment the access count of the file, and then when the incremented access count exceeds a predetermined threshold of the first list, move the entry of the accessed file to the second list, wherein the file migration control section is configured to migrate the files on the second list concurrently to a tier higher in the hierarchy at a determined timing; and wherein the storage management section is configured to delete the migrated files from the list.