Using shared parity disks to improve the reliability of RAID arrays

We propose to increase the reliability of RAID level 5 arrays used for storing archival data. First, we identify groups of two or three identical RAID arrays. Second, we add to each group a shared parity disk containing the diagonal parities of their arrays. We show that the new organization can tolerate all double disk failures and between 75 and 89 percent of triple disk failures without incurring any data loss. As a result, the additional parity disk increases the mean time to data loss of the arrays in the group it protects by at least 14,000 percent.