Patent ID: 8180989

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 plurality of real storage areas 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, when the host system makes a request to write data in a logical area, to which a real storage area is not allocated before the request to write data, in the first volume, the control circuit allocates a real storage area in the second volume to the logical area in the first volume and registers the relationship between the logical area in the first volume and the real storage area in the second volume in the control table, 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 it to the host system, wherein a first real storage area storing a first data and included in the second volume is related to a first logical area in the first volume, wherein the control circuit detects a certain logical area in the first volume to which a certain real storage area storing data that is the same as the first data is allocated, and releases the certain real storage area from allocation to the certain logical area after which the released certain real storage area can be used for allocating to another logical area in the first volume or in another first volume as a destination of another write access request.