In a cluster-structured disk subsystem, when creating a volume for an online backup separately from a volume for a normal I/O, it is desirable to be able to achieve such a creation for any volume under subsystem. Further, with an increase in the capacity of the subsystem, it becomes more difficult for a user to determine where to place a volume to which data is to be copied. Thus, a cluster-structured storage system is provided in which it is possible to reference/renew snapshot control information in shared memory of other clusters and achieve a snapshot between clusters via an inter-cluster connecting mechanism. In this system, control is performed inside/outside the cluster, and a control is performed inside/outside the cluster, and a volume to which data is to be copied is suggested to the user.