In recent years, dynamic storage tiering has been proposed as one of the storage virtualization technologies. In this storage system, a volume is divided into sub-volumes, which can be migrated among different performance storage media according to the frequency of I/O operations, etc. This technology is expected to reduce the total storage media cost. But there is no method to assure a certain service level for the storage system. We discuss five requirements: (1) performance management of arbitrary volume granularity, (2) system scalability, (3) adaptability to unknown environments, (4) general versatility, (5) reducing a total storage media cost. To satisfy these five requirements, we propose a virtual volume clustering method and implement it in a simulation environment. By analyzing the simulation results, we summarize the three considerations for further improvement: (1) wide variation in target response time, (2) maintenance of service level, (3) migration of a large capacity volume. Based on these three considerations, we improve our proposal method. We show that our improved method is effective by an evaluation in a simulation environment. © 2011 IEEE.