In recent years, attention has focused on the flash memory, a typical nonvolatile semiconductor memory, for storage devices. Because a flash memory does not have a drive portion like a magnetic storage device typified by a HDD (Hard Disk Drive), it features lower power consumption than a HDD. A storage device that uses a memory drive mounted with a plurality of flash memories instead of a HDD has been proposed as an external storage device (refer to Patent Document 1).
Meanwhile, a storage subsystem must be stable, and as such, employs a configuration with duplexed components that enables processing to be carried out even when a malfunction occurs in a portion of the components. For example, in order to heighten data integrity and throughput, RAID (Redundant Array of Inexpensive Disks) technology is used to manage a plurality of storage devices as a single RAID Group, and to redundantly store data. The RAID Group forms one or more logical storage areas at this time. When data is stored in the storage area, redundant data is stored in the storage devices configuring the RAID Group. Storing redundant data in the storage devices makes it possible to recover the data even when one of the storage devices malfunctions.
For example, in a case where a memory drive comprising flash memories is configured as RAID 5, a ECC (Error Correct Code) computed using a plurality of data is stored in an ECC memory drive, and even if one memory drive should malfunction, it is possible to recover the data stored in the failed memory drive by using the remaining data and the ECC.
However, if malfunctions should occur in more than a predetermined number of memory drives, it becomes impossible to recover the data even in a RAID-configured subsystem. Accordingly, a storage subsystem generally makes use of an extra drive in which data has not been stored, called a spare drive.
The storage subsystem employs a configuration in which, in a case where a malfunction occurs in one of the RAID-configured memory drives, the data in the failed memory drive is recovered using the data in the remainder of the RAID-configured memory drives, and is stored in the spare drive.