Patent ID: 8756392

Claim:
A storage system coupled to a host computer and comprising a plurality of controller units, wherein each of the controller units comprises a plurality of controllers and a storage medium, provides the storage medium as a plurality of logical units to the host computer and stores a tier management information, a CPU load information and a load threshold management information, wherein the tier management information includes an identifier of a logical unit and a tier level of the logical unit, the tier level classifies the logical unit based on its I/O processing capacity, wherein the CPU load information includes an identifier of a CPU, a utilization rate of the CPU, an identifier of the logical unit controlled by the CPU and a utilization rate of the CPU of each logical units, wherein the load threshold management information includes a threshold for a high load of the controller unit and a threshold for a low load of the controller unit, wherein each of the controllers comprises a CPU and is configured to: determine whether or not a load distribution is needed by referring to the load threshold management information; in case that load distribution is needed, determine whether or not a first load distribution can be executed by referring to the CPU load information, where according to the first load distribution, a first logical unit is migrated from a high load CPU to a low load CPU in a same controller unit; in case that the first load distribution can be executed, execute the first load distribution; in case that the first load distribution cannot be executed, determine whether or not a second load distribution can be executed by referring to the CPU load information and the tier management information, where according to the second load distribution, a second logical unit is migrated from high load CPU to a low load CPU in a different controller unit; and in case that the second load distribution can be executed, execute the second load distribution.