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Redundancy, Error Detection and Error Correction
Parity:
Parity is a simple technique used in memories and hard drives.
For each byte (8 bits) we need to use one extra bit - Parity Bit
Parity bit is always set in such a way that the total number of bits (including parity) set to 1 must be Odd
| 11000001 | 0 |
| 11010010 | 1 |
| 10001001 | 0 |
RAID
Redundant Array of Inexpensive Disks
Is a technique that allows to recover information if one entire disk fails.
To make example simpler assume we have 9 disks and each bit is stored in a different disk:
| Parity | ||||||||
| 1 | 1 | 0 | 0 | 0 | 0 | 0 | 1 | 0 |
| 1 | 1 | 0 | 1 | 0 | 0 | 1 | 0 | 1 |
| 1 | 0 | 0 | 0 | 1 | 0 | 0 | 1 | 0 |
If we know that one disk fails, we just use the extra (last) to determine the missing bit
Example: What is the value of the missing bit?
| 1 | 1 | 0 | ? | 1 | 0 | 1 | 1 | 1 |
| 0 | 0 | 1 | ? | 0 | 0 | 1 | 0 | 0 |
| 1 | 1 | 1 | ? | 0 | 0 | 0 | 0 | 0 |
| 0 | 0 | 0 | ? | 0 | 0 | 0 | 0 | 0 |