strip 2 c RAID 2 redundancy through Hamming code b b 1 b 2 b 3 f b f 1 b f 2 b

Strip 2 c raid 2 redundancy through hamming code b b

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strip 2 (c) RAID 2 (redundancy through Hamming code) b 0 b 1 b 2 b 3 f 0 (b) f 1 (b) f 2 (b) strip 15 strip 11 strip 7 strip 3 Figure 11.8 RAID Levels (page 1 of 2) 18
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Mirrored (RAID Level 1) The traditional solution, called mirroring or shadowing , uses twice as many disks as a non-redundant disk array. Whenever data is written to a disk, the same data is also written to a redundant disk in parallel, so that there are always two copies of the infor- mation. When data is read from a disk, it is retrieved from the disk with the shorter queuing, seek and rotational delays. Recovery from a disk failure is simple: the data may still be accessed from the second drive. Mirroring is frequently used in database applications where availability and transaction time are more important than storage efficiency. RAID-1 can achieve high I/O request rates if most requests are reads, which can be satisfied from either drive: up to twice the rate of RAID-0. 19
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Parallel Access (RAID Level 2) Memory systems have provided recovery from failed components with much less cost than mirroring by using Hamming codes . Hamming codes contain parity for distinct overlapping subsets of components. In one version of this scheme, four disks require three redundant disks, one less than mirroring. On a single read request, all disks are accessed simultaneously. The re- quested data and the error-correcting code are presented to the disk-controller, which can determine if there was an error, and correct it automatically (and hopefully report a problem with one of the disks). On a single write, all data disks and parity disks must be accessed. Since the number of redundant disks is proportional to the log 2 of the total number of the disks on the system, storage efficiency increases as the number of data disks increases. With 32 data disks, a RAID 2 system would require 7 additional disks for a Hamming-code ECC. Not considered very economical. 20
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Bit-Interleaved Parity (RAID Level 3) Unlike memory component failures, disk controllers can easily identify which disk has failed. Thus, one can use a single parity disk rather than a set of parity disks to recover lost information. In a bit-interleaved, parity disk array, data is conceptually interleaved bit-wise over the data disks, and a single parity disk is added to tolerate any single disk failure. Each read request accesses all data disks and each write request accesses all data disks and the parity disk. In the event of a disk failure, data reconstruction is simple. Consider an array of five drives, four data drives D 0, D 1, D 2, and D 3, and a parity drive D 4. The parity for bit k is calculated with: D 4( k ) = D 0( k ) xorD 1( k ) xorD 2( k ) xorD 3( k ) Suppose that D 1 fails. We can add D 4( k ) xorD 1( k ) to both sides, giving: D 1( k ) = D 4( k ) xorD 3( k ) xorD 2( k ) xorD 0( k ) The parity disk contains only parity and no data, and cannot participate in reads, resulting in slightly lower read performance than for redundancy schemes that distribute the parity and data over all disks. Bit-interleaved, parity disk arrays are frequently used in applications that require high bandwidth but not high I/O rates.
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