1. A storage system for storage management in a tiered storage environment, the storage system comprising: a plurality of storage volumes in a pool which are divided into a plurality of tiers having different tier levels, the tiers being organized according to a tier configuration rule, the plurality of storage volumes provided by a plurality of physical storage devices in the storage system; a controller controlling the plurality of physical storage devices, the controller including a processor and a memory, the controller changing tier configurations of the tiers of storage volumes when the tier configuration rule is changed, the tier configurations including the tier levels; wherein the controller allocates the pool to a plurality of virtual volumes based on a change of tier levels against the physical storage devices which occurs when the pool does not meet the tier configuration rule that was in effect; wherein the pool includes a plurality of thin provisioning pools; and wherein the controller receives an input to allocate two or more of the thin provisioning pools to one virtual volume and merges the two or more thin provisioning pools into a consolidated thin provisioning pool to be allocated to the one virtual volume.