Patent ID: 8489844

Claim:
A storage system comprising: a plurality of storage devices including a plurality of first storage devices and a plurality of second storage devices, wherein a performance of said plurality of first storage devices is better than a performance of said plurality of second storage devices; and a controller being configured to manage a storage pool including a first tier storage area provided by said plurality of first storage devices and a second tier storage area provided by said plurality of second storage devices, and provide a virtual volume including a plurality of virtual storage areas including at least a first virtual storage area to which a first storage area in said first tier storage area is allocated and a second virtual storage area to which a second storage area in said second tier storage area is allocated; wherein: (A) if an I/O load on said first virtual storage area falls below a first threshold, said controller copies data in said first storage area, which is allocated to said first virtual storage area, to a third storage area in said second tier storage area, and allocates said third storage area instead of said first storage area to said first virtual storage area, (B) if an I/O load on said second virtual storage area exceeds a second threshold, which is set to be greater than said first threshold to reduce a number of data copy processes between a storage area in said first tier storage area and a storage area in said second tier storage area, said controller copies data in said second storage area, which is allocated to said second virtual storage area, to a fourth storage area in said first tier storage area, and allocates said fourth storage area instead of said second storage area to said second virtual storage area, and (C) if rate of allocated storage areas in first tier storage area to said virtual volume exceeds a third threshold, said controller copies data in said second storage area, which is allocated to said second virtual storage area, to the forth storage area in said first tier storage area, and allocates said forth storage area instead of said second storage area to said second virtual storage area.