Introduction to RAID : RAID Components and Features : Parity

Parity

Parity generates a set of redundancy data from two or more parent data sets. The redundancy data can be used to reconstruct one of the parent data sets in the event of a drive failure. Parity data does not fully duplicate the parent data sets, but parity generation can slow the write process. In RAID, this method is applied to entire drives or stripes across all of the drives in a drive group. The types of parity are described in the following table.

Table 3. Types of Parity

Parity Type

Description

Dedicated

The parity data on two or more drives is stored on an additional disk.

Distributed

The parity data is distributed across more than one drive in the system.

RAID 5 combines distributed parity with disk striping. If a single drive fails, it can be rebuilt from the parity and the data on the remaining drives. An example of a RAID 5 drive group is shown in the following figure. RAID 5 uses parity to provide redundancy for one drive failure without duplicating the contents of entire drives. RAID 6 also uses distributed parity and disk striping, but adds a second set of parity data so that it can survive up to two drive failures.

Figure 9. Example of Distributed Parity (RAID 5)