Disks are potential bottleneck for system performances and storage system reliability.
The disk access time is relatively higher than the time required to access data from main memory and performs CPU operation. Also the disk drive contains some mechanical parts and it involves mechanical movement, so the failure rate is also high.
The disk performance has been improving continuously, microprocessor performance has improved much more rapidly.
A disk array is an arrangement of several disk, organized to increase performance and improve reliability of the resulting storage system. Performance is increased through data striping. Reliability is improved through redundancy.
Disk arrays that implement a combination of data striping and redundancy are called Redundant Arrays of Independent Disks (RAID).