Patent ID: 8880830

Claim:
A storage system comprising: a first storage device, which is a flash device, whose storage areas are managed as a first tier; a second storage device whose storage areas are managed as a second tier; and a controller, wherein the controller is configured to: provide a virtual volume including a plurality of logical areas to a host, allocate at least one of the storage areas in the first tier or in the second tier to at least one of the logical areas corresponding to an address indicated by a write request from the host to store data of the write request, if the data is stored in the second tier, determine whether or not to relocate the data based on a first access frequency of the data in a predetermined period of a cycle, if the data is stored in the second tier, determine whether or not to relocate the data based on a second access frequency of the data when processing an I/O from the host to the data, and migrate the data determined to be relocated from the second tier to the first tier by changing the allocation to the logical area, wherein the data is determined to be relocated according to at least one of the determination based on the first access frequency and the determination based on the second access frequency to improve the flash hit ratio of the storage system, and wherein a threshold value, for the migration from the second tier to the first tier, utilized in the determination based on the second access frequency is higher than a threshold value utilized in the determination based on the first access frequency.