Method of data migration

A method, apparatus and computer program for controlling data migration in an information processing system which includes a central processing unit (CPU), a new storage system connected to the CPU and an old storage system connected to the new storage system. In the information processing system data migration is conducted to transfer data from the old storage system to the new storage system. The invention operates by permitting access by the CPU to the storage systems during data migration. When an access by the CPU is generated the invention determines whether the access is to a region where data migration has been completed or to a region where data migration has not been completed. If the access is to a region where data migration has been completed, then processing of the access is handled by the new storage system. If the access is to a region where data migration has not been completed, then processing of the access is handled by the old storage system causing data related to the access to be transferred from the old storage system to the new storage system. The speed of data migration can be adjusted based upon the utilization of the resources of the information processing system and information of the priority of access to the new storage system by the CPU.