Patent ID: 7949827

Claim:
A storage system having: a host system; a plurality of disk drives for storing data transmitted from the host system via a network; and a memory apparatus having a plurality of groups to maintain the reliability of data in the disk drives and managing pool volumes in the groups using logical volumes and virtual volumes associated with the pool volumes, comprising: an access information table storing access information, which is information about the access counts for the pieces of data stored in the disk drives; a judgment unit for, when migrating data in the pool volumes to equalize the data access counts, judging which data migration—data migration in units of pages or data migration in units of volumes—is most appropriate based on the information in the access information table; a data migration unit for performing, based on the judgment made by the judgment unit, data migration in units of pages or data migration in units of volumes so that the data access counts are equalized among the groups; and a control unit for controlling the judgment unit and the data migration unit so that the judgment and data migration is repeated until the data access counts are equalized; the judgment unit includes: a first distribution map creation portion for creating a group access count distribution map with reference to the access information table; a first judgment portion for judging whether there is any group that should be subject to the equalization according to the type of the group access count distribution map created by the first distribution map creation unit; a second distribution map creation portion for, when the first judgment portion judges that there is a group(s) that should be subject to the equalization, creating a logical volume access count distribution map for each of the group having the largest access count and the group having the smallest access count, with reference to the access information table; a second judgment portion for judging whether there is any logical volume that should be subject to the equalization according to the types of the logical volume access count distribution maps created by the second distribution map creation unit; and a third distribution map creation portion for, when the second judgment portion judges that there is a logical volume(s) that should be subject to the equalization, creating a page access count distribution map for each of the logical volume having the largest access count and the logical volume having the smallest access count, with reference to the access information table; wherein there are two types of distribution maps the third distribution map creation portion creates, which are a first type indicating that the difference between the page access counts is large and a second type indicating that the difference is small; and when the two distribution maps the third distribution map creation portion creates are both of the first type, the data migration unit migrates data in units of pages.