RAID levels

Disks are potential bottlenecks for system performance and storage system reliability. If the disk fails, the data is lost. A  disk array is used  to increase performance and reliability through data striping and redundancy. Instead of having a single copy of data, redundant information is maintained and carefully organized so that in the case of a disk failure, it can be used to reconstruct the contents of the failed disk. These redundant array of independent disk organizations are referred as RAID levels and each level represents a tradeoff between reliability and performance.
In Data Striping, the data is segmented into equal-size partitions that are distributed over multiple disks. The size of the partition is called the striping unit. The partitions are usually distributed using a round robin mechanism.
In Redundancy, if the mean time to failure of a single disk is about a few years, it is smaller for a disk array. Hence check disks and parity schemes are involved to improve reliability. The check disk contains information that can be used to recover from failure of any one disk in the array.This group of data disks and check disks together constitute reliability groups.
Here is a list of the RAID levels:
Level 0: Non-redundant: A RAID level 0 system uses data striping to increase maximum bandwidth available.  No redundnant information is maintained.  This is usually the least expensive solution.
Level 1: Mirrored: Instead of having one copy of the data, two copies of the data are maintained. This type of redundancy is often called mirroring. Every write on a disk block involves write on both disk.This allows parallel reads between disk blocks but is usually the most expensive solution.
Level 0+1: Striping and Mirroring: Like in level 1, read requests can be scheduled to both the disk and its mirror image and bandwidth for contiguous blocks is improved from the aggregation of all the disks.
Level 2: Error-correcting codes: In RAID level 2, the striping unit is a single bit. The redundancy scheme used is the Hamming code. The number of check disks grows logarithmically with the number of data disks.
Level 3: Bit-Interleaved Parity: Redundancy schema in RAID level 2 is better in terms of cost than RAID level 1 but it keeps more redundant information than is necessary. Instead of using several disks to store hamming code that informs which disk has failed, we rely on that information from the disk controller and use a single check disk with parity information which is the lowest overhead possible.
Level 4: Block-Interleaved parity. RAID level 4 has a striping unit of a disk block, instead of a single bit  as in RAID level 3.  Block-level striping has the advantage that read requests the size of a disk block can be served entirely by the disk where the requested block resides.  The write of a single block  still requires a read-modify-write cycle, but only one data disk  and the check disk are involved and the difference between the old data block and the new data block is noted.
Level 5: Block Interleaved distributed parity: This level improves upon the previous level by distributing the parity blocks uniformly over all disks,  instead of sorting them on a single check disk. This has two advantages. First, several write requests  potentially be processed in parallel, since the bottleneck of  a unique check is removed. Second, read requests have a higher degree of parallelism. This level usually has the best performance.
Level 6: P+Q redundancy: Recovery from the failure of a single disk is usually not sufficient  in very large disk arrays. First, a second disk might fail before replacement and second the probability of a second disk failing is not negligible. A RAID level 6 system uses Reed-Solomon codes to be able to recover from two simultaneous disk failures.