In computers, parity (from the Latin paritas, meaning equal or equivalent) is a technique that checks whether data has been lost or written over when it is moved from one place in storage to another or when it is transmitted between computers.
How parity works
Because data transmission is not an entirely error-free process, data is not always received in the same way as it was transmitted. A parity bit adds checksums into data that enable the target device to determine whether the data was received correctly.
An additional binary digit, the parity bit, is added to a group of bits that are moved together. This bit, sometimes referred to as a check bit, is used only to identify whether the moved bits arrived successfully.
Parity and raid
The concept of parity is also used in redundant array of independent disks (RAID) protection. RAID devices use enhanced forms of parity checking such as vertical and horizontal parity. Some RAID groups -- such as RAID 4 or RAID 5 -- have one or more disk drives that contain parity information that allows them to rebuild data if a drive failure occurs. For example, double-parity RAID (also known as RAID 6), stripes data across a set of at least four drives at a block level, like RAID 5, but then writes a second set of parity data across all the drives.
This approach guards against data loss in up to two failed drives. Drawbacks to double-parity RAID include the use of a complex controller, the cost of two extra drives for implementation and slower write transactions due to the extra parity set.
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.
How does this happen? If the RAID group is using even parity, it can figure out what was on the failed drive by adding up the bits on the remaining drives. If the data on the remaining drives adds up to an odd number, the information on the failed drive must have been a one to maintain even parity. If the data on the remaining drives adds up to an even number, the data on the failed drive must have been a zero.