Block storage device having hierarchical disks with different access frequencies

A storage system comprising:

• a first block storage device accessed by a first host computer, and a second block storage device accessed by a second host computer, the first block storage device and the second block storage device sharing an object storage device having a content addressable storage (CAS) function of providing a cloud service,
• wherein
• the first block storage device includes:
• a first hierarchical logical disk composed of a plurality of first logical blocks and created by using storage areas of plural kinds of first physical devices and a storage area of the object storage device, with the object storage device as a lowest hierarchy and the plural kinds of first physical devices having different access response performances as a plurality of hierarchies above the lowest hierarchy;
• a first deduplication unit configured to, when access frequency of a first logical block stored in a hierarchy one level above the lowest hierarchy is reduced to fall below a threshold, do a search of the object storage device for a logical block having a hash of the first logical block as a content address;
• and
• a first hierarchy changing unit configured to, when the search by the first deduplication unit fails, perform transmission of the first logical block to the object storage device for storing the first logical block in the object storage device and thereby change a hierarchy to store the first logical block to the lowest hierarchy, whereas when the search by the first deduplication unit succeeds, omit the transmission and change a hierarchy to store the first logical block to the lowest hierarchy, and
• the second block storage device includes:
• a second hierarchical logical disk composed of a plurality of second logical blocks and created by using storage areas of plural kinds of second physical devices and a storage area of the object storage device, with the object storage device as a lowest hierarchy and the plural kinds of second physical devices having different access response performances as a plurality of hierarchies above the lowest hierarchy;
• a second deduplication unit configured to, when access frequency of a second logical block stored in a hierarchy one level above the lowest hierarchy is reduced to fall below the threshold, do a search of the object storage device for a logical block having a hash of the second logical block as a content address;
• and
• a second hierarchy changing unit configured to, when the search by the second deduplication unit fails, perform transmission of the second logical block to the object storage device for storing the second logical block in the object storage device and thereby change a hierarchy to store the second logical block to the lowest hierarchy, whereas when the search by the second deduplication unit succeeds, omit the transmission and change a hierarchy to store the second logical block to the lowest hierarchy.