As an alternative to replication, modern data storage systems employ advanced erasure codes to protect data from storage node failures because of their ability to provide high data reliability at high storage efficiency. In contrast to previous studies, we consider the practical case where the length of codewords in an erasure coded system is much smaller than the number of storage nodes in the system. In this case, there exists a large number of possible ways in which different codewords can be stored across the nodes of the system. For replication-based systems, it is well-known that the mean time to data loss is significantly affected by the choice of placement of replicas. In particular, the declustered replica placement scheme provides significantly higher reliability than other placement schemes. In this paper, these results are extended to erasure coded systems and it is shown that a declustered placement of codewords can significantly improve system reliability. A detailed reliability analysis is presented that accounts for the rebuild times involved, the amounts of partially rebuilt data when additional nodes fail during rebuild, and the fact that modern systems utilize an intelligent rebuild process by rebuilding the most critical codewords first.
A shorted version of this report has been published in: "Quantitative Evaluation of Systems", Proc. 10th Int'l Conf. on Quantitative Evaluation of SysTems "QEST 2013," Buenos Aires, Argentina, Lecture Notes in Computer Science vol. 8054 (springer 2013) 241-257.
By: V. Venkatesan, I. Iliadis
Published in: RZ3827 in 2012
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