Storage system including multiple storage apparatuses and pool virtualization method

A storage system, comprising:

• a plurality of storage apparatuses including a first storage apparatus and a second storage apparatus,
• wherein said first storage apparatus is configured to have a first virtual volume composed of a plurality of virtual segments,
• at least said second storage apparatus is configured to have a pool composed of a plurality of real pages,
• said plurality of storage apparatuses are each configured to manage one or more pools including at least said pool in said second storage apparatus as one virtual pool,
• said virtual pool is composed of a plurality of virtual pages, each virtual page corresponding to any of the real pages, and
• said first storage apparatus is configured to receive a write command that specifies an address belonging to an unallocated virtual segment to which no virtual page is allocated, allocate a free virtual page from said virtual pool to said unallocated virtual segment, and write data accompanying said write command to the real page corresponding to the allocated virtual page.