Disk scrubbing versus intra-disk redundancy for high-reliability raid storage systems

Abstract

Two schemes proposed to cope with unrecoverable or latent media errors and enhance the reliability of RAID systems are examined. The first scheme is the established, widely used disk scrubbing scheme, which operates by periodically accessing disk drives to detect media-related unrecoverable errors. These errors are subsequently corrected by rebuilding the sectors affected. The second scheme is the recently proposed intradisk redundancy scheme which uses a further level of redundancy inside each disk, in addition to the RAID redundancy across multiple disks. Analytic results are obtained assuming Poisson arrivals of random I/O requests. Our results demonstrate that the reliability improvement due to disk scrubbing depends on the scrubbing frequency and the workload of the system, and may not reach the reliability level achieved by a simple IPC-based intra-disk redundancy scheme, which is insensitive to the workload. In fact, the IPC-based intra-disk redundancy scheme achieves essentially the same reliability as that of a system operating without unrecoverable sector errors. For heavy workloads, the reliability achieved by the scrubbing scheme can be orders of magnitude less than that of the intra-disk redundancy scheme.