RAID apparatus and access control method therefor which balances the use of the disk units

A RAID apparatus comprising:

• a plurality of physical disk units storing a plurality of copies of each of logical volumes;
• and
• a disk controller accessing any of the physical disk units which stores a designated logical volume to thereby access said designated logical volume,
• said disk controller including:
• a plurality of device adapters for accessing said physical disk units in accordance with a request, said plurality of device adapters connected to each physical disk unit;
• a memory storing a logical volume structure table storing statuses of the logical volumes and indicating said physical disk units of each logical volume, and a disk management table storing a number of operations of each physical disk unit and statuses of said connected device adapters, for each physical disk unit;
• and
• control means for referring to said statuses of the logical volumes in said logical volume structure table for the designated logical volume and judging whether a status of said designated logical volume indicates abnormal, comparing with each other the numbers of operations stored in said disk management table that correspond to the plurality of physical disk units which store said designated logical volume when said designated logical volume status is judged normal, selecting a single minimum waiting physical disk unit on which said designated logical volume is allocated according to the comparison of the numbers of operations in said disk management table, judging whether said device adapter connected to said selected single minimum waiting physical disk is normal from referring to said statuses of said connected device adapters in said disk management table, and outputting a request to said judged normal device adapter.