Parity is a function of RAID in which an XOR result of at least two striped blocks is calculated and stored on an additional block. This requires at least three drives. Two of the drives will contain striped block data, the remaining drive will contain a parity block.
How parity works
Assume we have three hard drives using 8 bit parity blocks. Drive one and drive two will contained the striped data, drive three the parity. If the bits from Drive 1 and Drive 2 are the same then the parity will be 0, if different, the parity will be 1. Doing this we can recalculate the bits from any of the three drives if one where to fail.
| Disk 1: | 1 | 0 | 1 | 1 | 0 | 0 | 1 | 1 |
| Disk 2: | 0 | 0 | 1 | 0 | 1 | 1 | 0 | 1 |
| Parity: | 1 | 0 | 0 | 1 | 1 | 1 | 1 | 0 |