Reliability and Performance of Mirrored Disk Organizations

Abstract

Disk mirroring or redundant array of independent disk (RAID) level 1 is a popular paradigm to achieve fault-tolerance and a higher disk access bandwidth for read requests. We consider four RAID1 organizations: basic mirroring (BM), group rotate declustering (GRD), interleaved declustering (ID) and chained declustering (CD). The last three organizations provide a more balanced disk load than BM when a single disk fails, but are more susceptible to data loss than BM when additional disks fail. We compare the four organizations from the viewpoint of: (i) reliability [we quote results from [Thomasian, A. and Blaum, M. (2006) Reliability analysis of mirrored disks. IEEE Trans. Comput., 55, 1640–1644.]] (ii) performability, (iii) performance. In (ii) and (iii), we postulate discrete requests to small randomly placed blocks. For (ii), we compute the mean number of disk requests processed to the point where data loss occurs. For the sake of tractability in (iii), the response time is obtained assuming Poisson arrivals and a first come first serve policy. The ranking from the viewpoint of reliability and performability is: BM, CD, GRD, ID (with two clusters). BM and CD provide the worst performance, ID has a better performance than BM and CD, but is outperformed by GRD. These results are also shown using an asymptotic expansion method. Areas of further research are also discussed, which include the applicability of the mirroring techniques to storage bricks.