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Issue No.12 - December (2006 vol.55)
pp: 1640-1644
Disk mirroring or RAID level 1 (RAID1) is a popular paradigm to achieve fault tolerance and a higher disk access bandwidth for read requests. We consider four RAID1 organizations: basic mirroring, group rotate declustering, interleaved declustering, and chained declustering, where the last three organizations attain a more balanced load than basic mirroring when disk failures occur. We first obtain the number of configurations, A(n, i), which do not result in data loss when i out of n disks have failed. The probability of no data loss in this case is A(n,i)/{n \choose i}. The reliability of each RAID1 organization is the summation over 1 \leq i \leq n/2 of A(n, i) r^{n-i}(1-r)^{i}, where r denotes the reliability of each disk. A closed-form expression for A(n,i) is obtained easily for the first three organizations. We present a relatively simple derivation of the expression for A(n,i) for the chained declustering method, which includes a correctness proof. We also discuss the routing of read requests to balance disk loads, especially when there are disk failures, to maximize the attainable throughput.
Disk mirroring, RAID level 1, reliability modeling, interleaved declustering, chained declustering, group rotate declustering.
Alexander Thomasian, Mario Blaum, "Mirrored Disk Organization Reliability Analysis", IEEE Transactions on Computers, vol.55, no. 12, pp. 1640-1644, December 2006, doi:10.1109/TC.2006.201
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