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198 CHAPTER 6 / EXTERNAL MEMORY

P(b)
b 0 b 1 b 2 b 3

(d) RAID 3 (Bit-interleaved parity)

block 0 block 1 block 2 block 3 P(0-3)
block 4 block 5 block 6 block 7 P(4-7)
block 8 block 9 block 10 block 11 P(8-11)
block 12 block 13 block 14 block 15 P(12-15)

(e) RAID 4 (Block-level parity)

block 0 block 1 block 2 block 3 P(0-3)
block 4 block 5 block 6 P(4-7) block 7
block 8 block 9 P(8-11) block 10 block 11
block 12 P(12-15) block 13 block 14 block 15
P(16-19) block 16 block 17 block 18 block 19

(f) RAID 5 (Block-level distributed parity)

block 0 block 1 block 2 block 3 P(0-3) Q(0-3)
block 4 block 5 block 6 P(4-7) Q(4-7) block 7
block 8 block 9 P(8-11) Q(8-11) block 10 block 11
block 12 P(12-15) Q(12-15) block 13 block 14 block 15

(g) RAID 6 (Dual redundancy)
Figure 6.8 RAID Levels (continued)

as being stored on a logical disk. The logical disk is divided into strips; these strips
may be physical blocks, sectors, or some other unit. The strips are mapped round
robin to consecutive physical disks in the RAID array.A set of logically consecutive
strips that maps exactly one strip to each array member is referred to as a stripe.In
an n-disk array, the first n logical strips are physically stored as the first strip on each
of the n disks,forming the first stripe;the second n strips are distributed as the second