Patent ID: 8402238

Claim:
A storage apparatus that performs writing and reading of data to a plurality of physical drives classified into hierarchies, in accordance with data I/O requests sent from an external device, provides to the external device a storage area of any of the plurality of physical drives in a unit of a logical unit that is a logical storage area, configures the logical unit by assigning a physical page that is a unit of a storage area of any of the plurality of physical drives, to a logical page that is a unit of a storage area of the logical unit, performs a first migration of the logical page by reassigning a first physical page to the logical page, the first physical page being the physical page provided by any of the plurality of physical drives of a first hierarchy that is the hierarchy higher than a second hierarchy that is the hierarchy of any of the plurality of physical drives providing a second physical page that is the physical page currently assigned to the logical page, and making contents of the second physical page identical with contents of the first physical page, associates the second physical page with the first physical page and manages the two, performs a second migration of the logical page by reassigning the second physical page to the logical page, when the first physical page is associated to the second physical page, at a time the second migration is performed by reassigning the physical page provided by any of the plurality of physical drives of the second hierarchy, to the logical page to which the first physical page is assigned, a channel control unit that communicates with the external device; a drive control unit that writes data to any of the plurality of physical drives or reads data from any of the plurality of physical drives; a cache memory that stores therein data to be written to any of the plurality of physical drives and data read out from any of the plurality of physical drives; and a processor control unit that transfers data between at least any one of the channel control unit, the drive control unit and the cache memory.