Disclosed is a storage system having storage devices from which storage areas are specified. Virtualization apparatuses allocate the storage areas as virtual volumes and processes I/O requests with respect to the virtual volumes. A controller is operable to change the allocation of storage areas to the virtual volumes. The controller is configured to send a request to some of the virtualization apparatuses to temporarily suspend processing of their I/O. When a virtualization apparatus receives such a request, it completes its pending I/O and temporarily suspends subsequent I/O requests, and sends a completion report to the controller. The controller then changes the allocation of storage areas to the virtual volumes.