US Patent 7506120 Data transfer virtualization system

When data in the first primary volume is migrated to the second primary volume, an access request from the host to the primary volume is transferred to the second primary volume. Further, the second storage system stores write data received from the host and data of the first primary volume received from the storage system, into the second primary volume. Out of data stored in the second primary volume, the second storage system sends data determined by management information received from the first storage system, to the third storage system.