Storage system and storage control device

A virtualization system, comprising:

• at least one first port coupled to at least one host system;
• at least one second port coupled to a plurality of storage systems, each of the storage systems comprising at least one logical volume related to at least a portion of a plurality of disk drives;
• and
• at least one controller forming a first virtual volume and a second virtual volume;
• 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;
• receiving a first write request, the first write request being sent from the host system for writing data to the first virtual volume;
• storing first differential information identifying data of the first write request;
• transferring the data of the first write request to a first logical volume of a first storage system of the storage systems, the first logical volume being related to the first virtual volume, so that the first storage system can write the data of the 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, the second differential information related to the second virtual volume, the second virtual volume being related to a second logical volume of a second storage system of the storage systems;
• receiving a differential copying request;
• if the differential copying request indicates to copy differential data from one of the first and second virtual volume to the other of the first and second virtual volume, (1) controlling to copy the data to the one of the first and second virtual volume based on the differential information, and (2) transferring the data to the other of the first and second logical volume of the storage systems, so that the storage system can write the data of the write request to a storage area of the disk drives.