What is a parity unit?

A parity drive is a storage device used as part of a computer system that contains parity data for redundancy and backup purposes. This is typically part of a Redundant Array of Independent Disks (RAID), in which one or more disk drives are connected to act as a single system. When data is stored on these devices, parity information can be created for later use in case one of the disks fails. A parity drive is not necessarily part of every RAID configuration, but it does allow for simple and effective data recovery.

A parity drive serves as a backup in case something happens to an internal hard drive.

The basic function of a parity drive is to provide additional storage of “parity bits”, which are pieces of data used to back up primary drives to a disk array. An array is a computer configuration where multiple disks, such as two or more hard drives, are connected and used as a single storage system. While several different methods are used for this, a RAID is among the most common ways. There are several types of RAIDS and more complex “tiers” often include the use of a parity unit to provide effective backup and information redundancy.

A parity drive works through the use of parity bits stored in it. The simplest example of how parity bits work is on a RAID or other system that uses three drives in total. Two of the drives would be used as real data storage disks, while the third would function as a parity drive. Whenever data is saved to the RAID, each piece of information is split in half, with one part going to one drive and the other part going to the second.

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Computer data consists of bits, which are binary pieces of data represented by one or zero. Whenever information is stored on a system with a parity unit, one bit from each storage unit is added to the other. If the result is an even number, a zero-valued parity bit is saved in the parity device, while an odd result creates the value one. This can be used if one of the storage drives fails, to recreate the missing data to restore what was lost.

For example, a “1” on one device and a “0” on the other would generate a “1” to be stored in the parity unit, since this is an odd value when added. If the storage drive with data “0” becomes corrupted, it can be replaced with a new, empty disk. The system can then examine the existing data, find the remaining “1” in the datastore, compare it to the “1” in the parity device, and recognize that a “0” needs to be recreated to restore the lost data. This is redundancy and allows an array to efficiently recover data even if part of the original system is lost.

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