A storage apparatus connected to a host computer via a network, as a storage device for storing data, includes, for example, multiple magnetic disks. The storage apparatus, with the RAID (Redundant Array of Independent (or Inexpensive) Disks) technology, makes the storage areas of the multiple storage devices redundant and configures RAID groups. Furthermore, the storage apparatus provides, to the host computer, a storage area of the capacity required by the host computer, the storage area being a part of the RAID groups and provided in the form of logical volumes. Generally, examples of logical volumes in storage apparatuses include logical volumes configured from high-performance and expensive storage devices; logical volumes configured from low-performance and inexpensive storage devices; and logical volumes configured having different degrees of redundancy of the RAID configuration, and they are used for different purposes.
Furthermore, the storage apparatus has the Thin Provisioning technology. Thin Provisioning is the technology in which [the storage apparatus], instead of providing a logical volume as a storage area of a fixed capacity to the host computer, provides a virtual logical volume (virtual volume) to the host computer and, in accordance with, for example, data write processing by the host computer to the storage apparatus, allocates, from storage areas (pool) created from multiple logical volumes, storage areas in units of storage area allocation units called pages, to the virtual volume. With the Thin Provisioning technology, some storage apparatuses can dynamically expand the storage capacity to provide to the host computer (see Patent Literature 1).
Here, a page is a storage area created by splitting logical volumes configuring the pool (pool volume) into parts having appropriate capacities according to logical block addresses (LBAs (Logical Block Address)). A page might also be referred to as a segment. Meanwhile, an LBA is the address used, when the host computer reads/writes data from/to the storage apparatus, for designating a data read/write location in the logical volume provided to the host.
A technology of performing efficient operation of a pool configured from pool volume groups of different performance has been proposed (see Patent Literature 2). In this technology, a management server or a storage apparatus monitors the access status (e.g. the number of times of read/write per unit of time (IOPS)) of each page allocated to a virtual volume. With reference to the access statuses of the monitored pages, the pages of high frequencies of use are dynamically migrated to storage areas in a pool volume configured from expensive and highly-reliable storage devices, and the pages of low frequencies of use are dynamically migrated to storage areas in a pool volume configured from low-cost storage devices. With this method, deterioration of performance and reliability of the virtual volume and an increase in the operation cost of the storage apparatus can be prevented.