6.2.2. How Does It Work?
Fedora's RAID levels 4 and 5 use parity information to provide redundancy. Parity is calculated using the exclusive-OR function, as shown in Table 6-4.
Table 6-4. Parity calculation for two drives
| Bit from drive A | Bit from drive B | Parity bit on drive C |
|---|---|---|
| 0 | 0 | 0 |
| 0 | 1 | 1 |
| 1 | 0 | 1 |
| 1 | 1 | 0 |
Notice that the total number of 1 bits in each row is an even number. You can determine the contents of any column based on the values in the other two columns ( A = B XOR C and B = A XOR C ); in this way, the RAID system can determine the content of any one failed drive. This approach will work with any number of drives.
Parity calculations are performed using the CPU's vector instructions (MMX/3DNow/SSE/AltiVec) whenever possible. Even an old 400 MHz Celeron processor can calculate RAID 5 parity at a rate in excess of 2 GB per second.
RAID 6 uses a similar but more advanced error-correcting code (ECC) that takes two bits of data for each row. This code permits recovery from the failure of any two drives, but the calculations run about one-third slower than the parity calculations. In a high-performance context, it may be better to use RAID 5 with a hot spare instead of RAID 6; the protection will be almost as good and the performance will be slightly higher.