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General Methodology : The System Scheme

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III. GENERAL METHODOLOGY A. EVENODD Parity Scheme Even odd scheme consist of simple exclusive-OR computation, A major advantage in this technique is that, it only requires parity hardware, which is already present in the RAID-5 controller, hence it is easy to implement on the RAID-5, without any hardware change. Earlier for the optimal redundant storage, it makes use of Reed-Solomon(RS) scheme. But even-odd is better because the exclusive-OR required here is 50% of the RS scheme. When a disk fails, by doing exclusive-ORing we can retrieve data on the surviving disks and writing this into a spare disk. The mean time to data loss (MTTDL) of such a system is proportional to the square of the disk mean time between failures (MTBF) and inversely proportional to the square of the number of disks and the mean time to reconstruct (MTTR) the failed disk. Initially Reed-Solomon (RS) scheme is used for the redundancy purpose, but it contains many drawbacks: when the error correcting capability of the code is broken, there is infinite error propagation. A solution was found and later generalized for multiple disk arrays. Even-Odd technique use encoding procedure that is based on exclusive-OR operation and does not involve recursive computations. Even Odd requires about twice as many exclusive-OR operations compared to simple parity scheme but this is optimal since we have two redundant disks. Even-Odd is more efficient than Reed-Solomon codes. In addition to having an optimal number of

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