Storage system comprising multiple storage apparatuses with both storage virtualization function and capacity virtualization function

A storage system comprising:

• a first storage apparatus, and
• a second storage apparatus, the first storage apparatus and second storage apparatus sharing an external volume, which is a logical volume provided by an external storage apparatus,
• wherein:
• the first storage apparatus comprises an external capacity pool, which is a storage area based on the external volume and is partitioned into multiple external pages that are sub-storage areas, and a first virtual volume, which is a virtual logical volume comprising multiple first virtual areas, and in a case of a write to a virtual area that has not been allocated a page, is configured to allocate an unallocated external page from the external capacity pool to the virtual area;
• the second storage apparatus comprises a second virtual volume, which is a virtual logical volume comprising multiple second virtual areas;
• and
• in a data migration from the first virtual volume to the second virtual volume, a data migration from a first virtual area to which an external page has been allocated to a second virtual area is completed in accordance with the following (a) and (b):
• (a) the first storage apparatus notifies the second storage apparatus of a migration source address, which is an address of the first virtual area, and an external address, which is an address of an external page allocated to the first virtual area;
• and
• (b) the second storage apparatus stores a corresponding relationship between the notified migration-source address and the notified external address;
• and
• wherein one of the first storage apparatus and the second storage apparatus;
• (c) receives a write request from a host;
• (d) in a case where a page has not been allocated to a write-destination virtual area, which is a virtual area that belongs to an address specified in the write request from the host, and an external page is to be allocated to the virtual area, determines whether or not the one of the first storage apparatus and the second storage apparatus has an authority for allocating an external page;
• (e) in a case where a result of the determination in (d) is affirmative, allocates an external page to the write-destination virtual area;
• and
• (f) in a case where a result of the determination in (d) is negative, sends an external page allocation request to the storage apparatus having the authority for allocating an external page.