Patent ID: 8862849

Claim:
A storage system comprising: a plurality of storage devices including several types of storage devices; a controller providing a plurality of RAID groups which are composed of one type of the storage devices, a plurality of tiers which are composed of the RAID groups, a pool which are composed of the plurality of tiers, and a virtual volume which is provided from the pool; and a memory storing a tier management information including a tier number, a top of tier I/O frequency range and a bottom of tier I/O frequency range, wherein, if an I/O frequency of data of a lower performance tier is greater than a tier I/O frequency border value which is higher than the bottom of the tier I/O frequency range of a higher performance tier, the controller reallocates the data from the lower performance tier to the higher performance tier, and wherein, if an I/O frequency of data of a lower performance tier is greater than the bottom of the tier I/O frequency range of the higher performance tier but lower than the tier I/O frequency border value, the controller does not reallocate the data from the lower performance tier to the higher performance tier.