Raid stands for Redundant Array of Inexpensive Disk. It combines multiple small disks into a single logical store unit. Its performance is more than that of Single Large expensive drive (SLED).

For Raid

MTBF of Array = MTBF of individual drive/ Total no of drive in array

Therefore in Raid Mean time between failures (MTBF) is less as compared to SLED.Raid helps in achieving redundancy, low latency, higher bandwidth for reading and writing. It also helps recovery at the time of hard-disk crash.

Raid offers different level. They are as fallows:

1. Raid - 0 It has more improved performance and data storage capacity but is not redundant. Therefore failure of any disk results in data lost. It is also called "striping".

2. Raid - 1 It provides redundancy by writing all data in two disks. The speed of accessing data becomes faster. But the array capacity is equal to the capacity of one member disk. Therefore it is more expensive. This also referred as "Mirroring".

3. Raid -2 It uses Hamming error correction code correction.

4. Raid -3 It strips data at byte level across several drives with parity stored on one drive.

5. Raid -4 It stripes data at block level from several drives with parity stored on one drive. The speed of reading data is fast enough but for writing it is slow. Since writing requires the parity data be updated each time.

6. Raid-5 It distributes the parity among all drives. If the drive fails need replacement but the array is not destroyed by the single disk failure.

7. Raid-6 It is stripped with dual parity. If two drives fail the array continues the operation. It provides more safety of data.

There are two types of raid solution in Linux:

1. Software Raid Solution: It offers cheaper raid solution not only for expensive disk controller cards but also works with cheaper IDE disks as well as SCSI disks. There are two Linux software RAID implementation: MD (multi-disk) driver and the EVMS driver.

2. Hardware Raid Solution The hardware-based system manages the RAID subsystem independently from the host and presents to the host only a single disk per RAID array.

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