Disk Scrubbing Versus Intradisk Redundancy for RAID Storage Systems

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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. A new model is developed to evaluate the extent to which disk scrubbing reduces the unrecoverable sector errors. The probability of encountering unrecoverable sector errors is derived analytically under very general conditions regarding the workload characteristics and for a broad spectrum of disk scrubbing schemes, which includes the deterministic and random scrubbing schemes. We show that the deterministic scrubbing scheme is the most efficient scrubbing scheme. We also derive closed-form expressions for the percentage of unrecoverable sector errors that the scrubbing scheme detects and corrects, the throughput performance, and the minimum scrubbing period achievable. 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 interleaved parity-check (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. Finally, the I/O and throughput performances are evaluated by means of analysis and event-driven simulation.

By: I. Iliadis, R. Haas, X.-Y. Hu, E. Eleftheriou

Published in: ACM Transactions on Storage, volume 7, (no 2), pages Article 5 in 2011

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