While having more disks increases storage system performance, it also lower overall storage system reliability, because the probability of failure of a disk in disk array is increasing.
Reliability of a disk array can be increased by storing redundant information. If a disk fails, the redundant information is used to reconstruct the data on the failed disk.
One design issue involves here - where to store the redundant information. There are two choices-either store the redundant information on a same number of check disks, or distribute the redundant information uniformly over all disk.
In a RAID system, the disk array is partitioned into reliability group, where a reliability group consists of a set of data disks and a set of check disks. A common redundancy scheme is applied to each group.