US Patent 8250327 Storage apparatus and its control method

In a storage apparatus and its control method including multiple first virtual volumes to be provided to a host system and multiple pools each having a memory capacity, and equipped with a function of dynamically allocating a storage area to the first virtual volumes from the pools associated with the first virtual volumes in accordance with the usage status of the first virtual volumes, the unused capacity in each of the pools is managed, and, when the unused capacity of one of the pools falls below a predetermined threshold value, a part of the unused capacity of the other pools is allocated to the one pool. It is thereby possible to realize a highly reliable storage apparatus and its control method.