21 block 12 e RAID 4 block level parity block 8 block 4 block 0 block 13 block

21 block 12 e raid 4 block level parity block 8 block

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block 12 (e) RAID 4 (block-level parity) block 8 block 4 block 0 block 13 block 9 block 5 block 1 block 14 block 10 block 6 block 2 block 15 block 7 block 3 P(12-15) P(8-11) P(4-7) P(0-3) block 12 block 8 block 4 block 0 block 9 block 5 block 1 block 13 block 6 block 2 block 14 block 10 block 3 block 15 P(16-19) P(12-15) P(8-11) P(4-7) block 16 block 17 block 18 block 19 block 11 block 7 (f) RAID 5 (block-level distributed parity) (d) RAID 3 (bit-interleaved parity) b 0 b 1 b 2 b 3 P(b) Figure 11.8 RAID Levels (page 2 of 2) P(0-3) block 11 block 12 (g) RAID 6 (dual redundancy) block 8 block 4 block 0 P(12-15) block 9 block 5 block 1 Q(12-15) P(8-11) block 6 block 2 block 13 P(4-7) block 3 block 14 block 10 Q(4-7) P(0-3) Q(8-11) block 15 block 7 Q(0-3) block 11 22
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Block-Interleaved Distributed-Parity (RAID Level 5) RAID-5 eliminates the parity disk bottleneck by distributing the parity uni- formly over all of the disks. Similarly, it distributes data over all of the disks rather than over all but one. This allows all disks to participate in servicing read operations. RAID-5 has the best small read, large write, performance of any scheme. Small write requests are somewhat inefficient compared with mirroring, due to the read-modify- write operations to update parity. This is the major performance weakness of RAID-5. P+Q redundancy (RAID Level 6) (Finally) RAID-6 performs two different parity calculations, stored in separate blocks on separate disks. Because the parity calculations are different, data can be fully recovered in the event of two disk failures. 23
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