Method and apparatus for reducing the amount of data in a storage system

Groups of volumes having a certain amount of commonality of data are identified within a storage system. For each group, the process defines a base volume, such as the volume containing the most commonality with the other volumes. For each volume in the group, the process extracts differential data between the base volume and each other volume, saves the differential data in a pool volume, and updates a mapping table. Commonality between volumes in the group is managed in the base volume, the pool volume, and the mapping table. Then the original volumes except the base volume and the pool volume may be deleted, and the original volumes are replaced with virtual volumes accessible through the mapping table. As a result, the amount of data in a storage system can be reduced while maintaining data integrity and accessibility.