what is parity in raid

Parity is a technique used in redundant array of independent disks (RAID) protection to detect errors in a storage system. Parity computations are used in RAID drive arrays for fault tolerance by calculating the data in two drives and storing the results on a third. The parity is computed by XORing a bit from drive 1 with a bit from drive 2 and storing the result on drive 3. In certain RAID configurations, such as RAID 5, there is vertical and horizontal parity. This means that a disk or set of disks contain parity information that allows them to rebuild data in the event of a drive failure. When data is written to a RAID group, it will always have the correct parity, as it will have gone through various error-checking algorithms. That way, if a drive in the RAID group fails, the system uses the information on the remaining disks along with the parity information to rebuild the data on the failed drive to a spare drive. RAID 5 parity uses a mathematical function called “XOR” to calculate parity, which allows it to reconstruct data when one drive fails. RAID 5 can protect against a single drive failure, whereas RAID 6 can protect against two drive failures.