Rebuild processing in RAID5 with emphasis on the supplementary parity augmentation method[37]

Abstract

The rotated parity RAID5 disk array tolerates single disk failures by continuing operation by on-demand reconstruction of data blocks of the failed disk, until the systematic reconstruction of the contents of the failed disk is completed by the rebuild process on a spare disk. Supplementary Parity Augmentation (SPA), unlike the pyramid code, which has two parities covering half of the arrays disks each, extends RAID5's P parity with an additional S parity, which covers half of the disks. The extra load with respect to RAID5 of updating the S parity by one half of the disks is compensated by the more efficient on demand reconstructtion and rebuild processing when a disk fails. Although SPA has the same disk space redundancy level as RAID6, unlike RAID6 it can only deal with roughly half of all possible double disk failure cases for eight disks. For rebuild processing SPA reads half of the disks required by RAID5 and this leads to a higher Mean Time to Data Loss than RAID5, since fewer Latent Sector Errors are encountered. We review performance and reliability modeling of RAID5 arrays to provide insights into SPA's performance and reliability, which cannot be gained from numerical results alone. SPA is outperformed by the Intra-Disk Redundancy schemes combined with RAID5, which results in RAID6's reliability and RAID5 performance.,