Storage system providing virtual volumes

A storage system comprising:

• a plurality of pools, each of which includes a plurality of tiers corresponding to a plurality of RAID groups, wherein one of the tiers has higher performance than another of the tiers;
• a virtual volume being configured of multiple virtual pages to each of which an actual page in an allocation source tier is allocated in accordance with a write request, the allocation source tier being included in one of the pools associated to the virtual volume;
• and
• a controller which, if an I/O frequency to a virtual page is higher/lower than a threshold in a reallocation process, migrates a data element stored in a first source actual page allocated to the virtual page to a first destination actual page in a first destination tier included in the one of the pools, and allocates the first destination actual page to the virtual page instead of the first source actual page;
• wherein when the controller migrates actual pages allocated to the virtual volume from the one of the pools to another of the pools, the controller migrates a data element stored in a second source actual page in a second source tier included in the one of the pools to a second destination actual page in a second destination tier included in the another of the pools and allocates the second destination actual page to the virtual volume instead of the second source actual page, the second source tier having the same tier type as that of the second destination tier.