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Table Of Contents  CertiGuide to A+ (A+ 4 Real)
 9  Chapter 10: The Small Computer System Interface, SCSI
      9  RAID
           9  RAID Basics

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Error Correcting Code (ECC) and XOR

ECC is a process of generating parity values for error detection and correction during transmission so that data can be reconstructed without retransmission. ECC uses an ECC Parity Algorithm in cache memory. ECC calculations do add some computational overhead, but this is usually only an insignificant 2% - 3% increase in memory access time.119

Exclusive-OR (XOR) is a Boolean logical operation that generates a parity value for each pair of bits in a data word. XOR calculates the RAID-5 parity for data and can be used reconstructing corrupted or missing data. The truth table for XOR is shown in Table 24.


Table 24: XOR Boolean Table

Input Bit 1

Input Bit 2

XOR Output

0

0

0

0

1

1

1

0

1

1

1

0


While simple parity compares all of the bits in a byte, XOR compares every two bits of the byte. The benefit of XOR parity is error detection AND error correction. There is a substantial performance cost when using XOR parity. XOR parity calculations are more intensive than simpler parity calculations, and as a result have a significant impact on host system performance. RAID-5, which uses XOR parity, should only be implemented on a controller that uses hardware-based XOR through its own processor or coprocessor to han-dle the calculations instead of utilizing the host CPU.


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119. http://www-2.cs.cmu.edu/~aberger/pdf/cayley.pdf

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