Background:

    RAID is a type of disk array which follows a formalized definition of how data is stored and retrieved. A disk array is simply a set of two or more identical disk drives interfaced to a host computer in such a manner as to appear as a single disk unit to that host. The disk array's hardware (or software) manages the data distribution such that the host computer is unaware that data is actually being written to and/or read from multiple disk drives. RAID technology expands upon this simple disk array approach to define several methods to provide data redundancy and higher performance.

    Unlike discrete disk drives connected directly to a host computer system, RAID systems spread data across all of the disk drives within the RAID system. Since parity data is also stored, the failure of any single drive will not result in the loss of data. Rather, a new drive can be installed and the data reconstituted from the remaining functioning drives, thus allowing RAID systems to be fault tolerant. The benefit of this feature is that if any single disk in the RAID array fails, the system continues to function without down time or loss of any data. This is possible because the redundant data or error correction information is stored separately from the data within the RAID array. Furthermore, the redundant data or error correction information can be used to reconstruct any data that was stored on a failed disk. This redundancy can be further extended to other components within the system to further increase data availability.

What is Parity Data?

    Parity is additional information stored with the original data to ensure that the original data is recoverable in the event of an error. Consider the following example. Suppose I ask you to remember (or store) the following four numbers for me - 1, 8, 3, 4 and you dutifully write the numbers down. The following week I need the numbers and ask you to retrieve them. As you read them off of the paper, you find that the fourth number is illegible and you cannot decipher it. Part of the data has now become unrecoverable - a disastrous situation for a computer. Suppose now that when I gave you the original four numbers, you also wrote down a fifth number (parity data), 16, which is the sum of the four. This time, if the fourth number were illegible, you could simply subtract 12 (the sum of the three known numbers) from 16 (the fifth number) to obtain 4 (the missing number). Thus, the data has been recovered. RAID systems store parity information in much the same manner.