A computer system for providing a large-scale data storage service to a host computer has been used.  This computer system is also called a storage system and is known to include a storage apparatus for executing data write and/or read processing on storage devices under its control in response to a command from the host computer, and a management apparatus for managing the storage apparatus. 
The storage apparatus is equipped with a plurality of hard disk drives (HDDs) and manages them as storage devices according to a RAID (Redundant Array of Independent/Inexpensive Disks) system.  Physical storage areas belonging to a plurality of HDDs are provided as logical volumes to the host computer.  The host computer sends a data read/write request to the logical volumes. 
In recent years, the storage capacity of storage apparatuses has been increasing steadily due to changes in the social environment such as an increase in the amount of information.  On the other hand, power-saving techniques for electronic devices have been considered to be very important in order to minimize the influence on the environment as represented by prevention of global warming.  There are two representative means of power-saving techniques for storage apparatuses as follows: prevention of mounting of excessive amount of storage devices (HDDs) by efficient operation of the storage capacity; and reduction of power consumption by stopping unnecessary storage devices. 
An example of the former means is a storage management technique called Thin Provisioning (for example, Japanese Patent Application Laid-Open (Kokai) Publication No.  2005-234820), and an example of the latter means is a storage management technique called MAID (Massive Array of Inactive Disks) (for example, Japanese Patent Application Laid-Open (Kokai) Publication No.  2007-156667 and Japanese Patent No.  4325817). 
The above-mentioned thin provisioning is one of storage virtualization techniques and is characterized in that the capacity efficiency of a storage apparatus is enhanced by forming a pool with the capacity of storage resources and allocating virtual volumes to a host computer, thereby reducing the initial capacity required in a case of logical volumes.  As a result, power consumption can be reduced by reducing the HDDs which should be mounted in the storage apparatus. 
In actual operation of the thin provisioning, a storage apparatus forms a plurality of RAID groups from a plurality of storage devices and the RAID groups are gathered to form a storage pool; and the storage capacity is allocated from the pool in response to access from the host computer to virtual volumes.  When doing so, the storage apparatus allocates a storage area equally to the virtual volumes from the plurality of RAID groups that form the pool. 
On the other hand, the above-mentioned MAID is characterized in that it achieves power-saving by making storage devices, which are not accessed by the host computer and are in a standby state, make the transition to a power-saving state by stopping motors for the storage devices, or limiting power supply to the storage devices.