Redundant Array of Independent Disk (RAID) combines multiple small, inexpensive disk drives into an array of disk drives which yields performance more than that of a Single Large Expensive Drive (SLED). RAID is also called Redundant Array of Inexpensive Disks.
Storing the same data in different disk increases the fault-tolerance.
The array of Mean Time Between Failure (MTBF) = MTBF of an individual drive, which is divided by the number of drives in the array. Because of this reason, the MTBF of an array of drives are too low for many application requirements.
The various types of RAID are explained below −
RAID Level-0 is not redundant. Since no redundant information is stored, performance is very good, but the failure of any disk in the array results in data loss. A single record is divided into strips typically 512 bytes and is stored across all disks. The record can be accessed quickly by reading all disks at the same time, called as striping.
RAID Level-1 provides redundancy by writing all data into two or more drives. The performance is faster on reads and slower on writes compared to a single drive. If anyone drives fails, no data is lost. This method is called mirroring.
RAID Level-2 is used for Hamming error correction codes and is used with drives which do not have built-in error detection.
RAID Level-3 stripes data at a byte level across several drives, with parity stored on one drive. Byte-level stripping hardware supports efficient use.
RAID Level-4 that stripes data at a block level across several drives, with parity stored on one drive. Parity information allows recovery from the failure of any single drive. The performance of the level-4 array is good for reads.
Writes, however, require that parity data be updated each time. Because only one drive in the array stores redundant data. The cost per megabyte is low.
RAID Level-5 is similar to level 4, but distributes parity among the drives. This can speed up small writes in the multiprocessing system. The performance for reads is lower than a level-4 array. The cost per megabyte is the same as level-4.
Given below is the summary of all the types of RAID −
|RAID-0||It is the fastest and most efficient array type but offers no fault-tolerance.|
|RAID-1||It is the array of choice for a critical, fault tolerant environment.|
|RAID-2||It is used today because ECC is embedded in almost all modern disk drives.|
|RAID-3||It is used in single environments which access long sequential records to speed up data transfer.|
|RAID-4||It offers no advantages over RAID-5 and does not support multiple simultaneous write operations.|
|RAID-5||It is the best choice in a multi-user environment. However, at least three drives are required for the RAID-5 array.|