RAID structure in details

Q.4) Explain RAID structure in details.

Ans.

A combination of disk arrays that implements data stripping and data redundancy is called as redundant arrays of independent disks. It is used to increase performance and improve reliability of the storage system. Data striping increases the performance and data redundancy improves the reliability. Disks are organized in several ways and referred as RAID levels.

Level 0: Non redundant

Data striping is used in level 0 for increase performance but no redundant information is maintained. Striping is done at block level but without any redundancy. Writing performance is best in this level because due to absence of redundant information there is no need to update redundant information.

Level 1: Mirrored

In this level same data is copied on two different disks. This type of redundancy is called as mirroring. It is the most expensive system. Because two copies of same data are available in two different disks, it allows parallel read

Level 1+0: Striping and Mirroring

This level is combination of level 0 and level 1, sometimes called as level 10.

Level 2: Error correcting codes.

This level uses bit-level data stripping in place of block level.It is used with drives with no built in error detection technique. Error-correcting schemes (ECC) store two or more extra bits and it is used for reconstruction of the data if a singlebit is damaged.

Level 3: Bit-Interleaved parity

In this level a single parity bit is used for error correction as well as for detection. Systems have disk controller that detects which disk has failed. Check disks does not contains the information about failed disks. RAID level 3 has a single check disk with parity bit.

Level 4: Block-Interleaved parity

RAID level 4 is similar as RAID level 3 but it has Block-Interleaved parity instead of bit parity.You can access the data independently so read performance is high.

Level 5: Block-Interleaved distributed parity

RAID level 5 distributes the parity block and data on all disks. For each block, one of the disks stores the parity and the others store data.RAID level 5 gives best performance for large read and write.

Level 6: P+Q Redundancy

What happen if more than one disk fails at a time? This level stores extra redundant information to save the data against multiple disk failures. It does not use parity but uses reed-solomon codes for data recovery.