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1 Journal of Polish Safety and Reliability Association

1 2015

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On the average file unavailability for specific storage disk arrangements in Cloud systems

Besson M., Tanguy Ch., Grall A.

Journal of Polish Safety and Reliability Association

2015

Vol. 6, No. 1

23--30

Cloud computing is a growing field since data storage is becoming ever more decentralized. Providers of Cloud solutions want to insure the safety and availability of their customers’ data. In order to increase these performance indices, several storage policies have been implemented: replication, erasure codes, etc. A few of them rely on randomized procedures. In this paper, we focus on the influence of a specific storage policy on the availability of a given file. Taking only disk failures into account, we provide a general formula for the average file unavailability U ̅, which is a generalization of the well-known k-out-of-n problem, to which it reduces when disks are identical. We then calculate U ̅ for several configurations when disks have different reliabilities, and show that the disk arrangement has a major impact on the result. We also provide an approximation which could be helpful for more complex arrangements.