A virtualization system, including: at least one first port coupled to at least one host system; at least one second port coupled to a plurality of storage systems; wherein the virtualization system is capable to control to perform processes of splitting a relationship between the first virtual volume and the second virtual volume; storing first differential information identifying data of a first write request; write data of first write request to a storage area of the disk drives related to the first logical volume; storing second differential information identifying data of a second write request, the data of the second request being written after the splitting step, receiving a differential copying request; to copy differential data from the first virtual volume to the second virtual volume, or to copy differential data from the second virtual volume to the first virtual volume.