The Essentials of Computer Organization and Architecture by Null Linda & Lobur Julia

Author:Null, Linda & Lobur, Julia
Language: eng
Format: epub
Publisher: Jones & Bartlett Learning
Published: 2014-02-13T16:00:00+00:00

FIGURE 7.28 RAID-2, Bit Interleave Data Striping with a Hamming Code

7.9.4 RAID Level 3

Like RAID-2, RAID-3 stripes (interleaves) data one bit at a time across all of the data drives. Unlike RAID-2, however, RAID-3 uses only one drive to hold a simple parity bit, as shown in Figure 7.29. The parity calculation can be done quickly in hardware using an exclusive OR (XOR) operation on each data bit (shown as bn) as follows (for even parity):

Parity = b0 XOR b1 XOR b2 XOR b3 XOR b4 XOR b5 XOR b6 XOR b7

Equivalently,

Parity = (b0 + b1 + b2 + b3 + b4 + b5 + b6 + b7) mod 2

A failed drive can be reconstructed using the same calculation. For example, assume that drive number 6 fails and is replaced. The data on the other seven data drives and the parity drive are used as follows:

b6 = b0 XOR b1 XOR b2 XOR b3 XOR b4 XOR b5 XOR Parity XOR b7

RAID-3 requires the same duplication and synchronization as RAID-2, but is more economical than either RAID-1 or RAID-2 because it uses only one drive for data protection. RAID-3 has been used in some commercial systems over the years, but it is not well suited for transaction-oriented applications. RAID-3 is most useful for environments where large blocks of data would be read or written, such as with image or video processing.

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