Today's data storage systems are increasingly adopting low-cost disk drives that have higher capacity but lower reliability, leading to more frequent rebuilds and to a higher risk of unrecoverable media errors. We propose a new XOR-based intra-disk redundancy scheme, called interleaved parity check (IPC), to enhance the reliability of RAID systems that incurs only negligible I/O performance degradation. The proposed scheme introduces an additional level of redundancy inside each disk, on top of the RAID redundancy across multiple disks. The RAID parity provides protection against disk failures, while the proposed scheme aims to protect against media-related unrecoverable errors. A comparison between the proposed scheme and traditional redundancy schemes based on Reed-Solomon (RS) codes and single-parity-check (SPC) codes is conducted by analytical means. A new model is developed to capture the effect of correlated unrecoverable sector errors. The probability of an unrecoverable failure associated with these schemes is derived for the new correlated model as well as for the simpler independent error model. Furthermore, we derive closed-form expressions for the mean time to data loss of RAID 5 and RAID 6 systems in the presence of unrecoverable errors and disk failures. We then combine these results for a comprehensive characterization of the reliability of RAID systems that incorporate the proposed intra-disk redundancy scheme. Our results show that in the practical case of correlated errors, the proposed scheme provides the same reliability as the optimum albeit more complex RS coding scheme. Finally, the throughput performance of incorporating the intra-disk redundancy on various RAID systems is evaluated by means of event-driven simulations.
By: Ajay Dholakia, Evangelos Eleftheriou, Xiao-Yu Hu, Ilias Iliadis, Jai Menon and KK Rao
Published in: RZ3652 in 2006
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