Patent ID: 7681002

Claim:
A storage controller having a control circuit for controlling the migration of data between a host system and a storage apparatus, the migration being performed in response to a data write request or data read request sent from the host system to the storage apparatus, said control circuit comprising: a first volume accessible by the host system, having no real storage area in the storage apparatus allocated thereto; a second volume having a real storage area allocated thereto; and a control table defining the correspondence relationships between logical areas in the first volume the host system accesses and the real storage areas in the second volume, wherein when the host system makes a request to write data in a logical area in the first volume, the control circuit establishes correspondences between the logical area in the first volume and the real storage area in the second volume in the control table, the real storage area in the second volume being mapped onto the logical areas of the first volume, the real storage area in the second volume and the logical areas of the first volume being mapped in units of pages, each page being defined as a collection of slots, when the host system makes a request to read data from the logical area in the first volume, the control circuit reads, by referring to the control table, data from the real storage area in the second volume corresponding to the read-requested logical area and provides the requested data to the host system, and in response to a host system's request to delete the data, including when a file system in the host system deletes the data by deleting the management information for that data, the control circuit releases the real storage area in the second volume from its correspondence relationship with the logical area that is the data storage destination by storing specified data in the real storage area that is to be released from the correspondences, wherein, in response to a command from a specific program in the host system, the control circuit writes ‘0’ as the specified data in all areas of the real storage area that is to be released from the correspondences, wherein the control circuit keeps the real storage area where the specified data has been written in the second volume as a reserved storage area, when the control circuit judges that no real storage area in the second volume is registered in the control table as corresponding to the host-accessed logical area in the first volume, the control circuit provides the reserved storage area to the host system, wherein, when the control circuit searches the second volume for a page having ‘0’ set for all bits, the control circuit deletes the page having ‘0’ set for all bits from the mapping table when one of ( 1 ) the creation of a remote copy pair, ( 2 ) a command sent from an operator, or ( 3 ) a predetermined timing, occurs.