LEVELS OF RAID / LEVEL OF REDUNDANCY

LEVELS OF RAID / LEVEL OF REDUNDANCY 

In  the following discussion, we discuss the different RAID levels, where we are considering some sample that would just fit on four disks. It means that,without any RAID technology, our storage system would contain exactly four data disks, BUT,depending on the RAID level chosen in the RAID technology, our storage system would vary from zero disk to four disk.

1. Level 0 : Non- redundant :  A RAID Level 0 system uses data stripping to increase the maximum bandwidth available. No redundant information is maintained. While being the solution with the     lowest cost, reliability is a problem, since the MTTF decreases linearly with the number of disk       drives in the array. RAID level 0 has the best write performance of all RAID levels, because             absence of redundant information implies that no redundant information needs to be updated! RAID level 0 does not have the best read performance of all raid levels,  since systems with redundancy have a choice of scheduling disk accesses. 

     In our example, the RAID Level 0 solution consists of only four data disks. Independent of the number of data disks, the effective space utilization for a RAID Level 0 system is always 100 percent.

2. Level 1 : Mirrored : A RAID Level 1 system is the most expensive solution . Instead of having one copy of the data, two identical copies of the data on two different disks are maintained. This type of redundancy is often called mirroring. Every write of a disk block involves a write on both disks. These writes may not be performed simultaneously, since a global system failure (Ex : due to power outrage) could occur while writing the blocks and then leave both copies in an inconsistent state. Therefore, we always write a block on one disk first and then write the other copy on the mirror disk.
       In our example, we need for data and four check disks with mirrored data for a RAID Level 1 implementation. the effective space utilization is 50 percent independent of the number of data disks.

3. level 0+1 : Stripping and Mirroring : RAID Level 0+1 : Stripping sometimes also referred to as Raid Level 10 - combines stripping and mirroring - Thus, as in RAID Level 1, read requests of the size of a disk block can be scheduled both  to a disk or its mirror image. In addition, read requests of the size of several contiguous blocks benefit from the aggregated bandwidth of all disks. the cost for writes is an analogous to RAID Level 1. 

           In our example, four data disk require four check disks and effective space utilization is always 50 percent.

4. Level 2 : Error- Correcting Codes : In RAID Level 2 the stripping unit is a single bit. The redundancy scheme used in hamming code. in our example with four data disks, only three check disks are needed. In general, the number of check disks grows logarithmically with the number of data disks. Stripping at the bit level has the implication that in a disk array with D data disks, the smallest unit of transfer for a read is a set of D blocks. Thus, Level  2 is good for workloads with many large requests since for each request the aggregated bandwidth of all data disks is used. But RAID Level 2 is bad for small requests of the size of an individual block for the same reason.

For a RAID Level 2 implementation with for data disks, there check disks are needed. Thus in our example the effective space utilization is about 57 percent. The effective space utilization increase with the number of data disks. For example, in a setup with 10 data disks, four check disks are needed and the effective space utilization is 71 percent. in a setup with 25 data disks, five check disks are required and the effective space utilization grows to 83 percent.

5. Level 3 : bit - Interleaved parity :  While the redundancy scheme used in RAID Level 2 improve in terms of cost upon RAID Level 1, it keeps more redundant information than is necessary. Hamming Code, as used in RAID Level 2, has advantage of being able to identify which disk has failed. But disk controllers can easily detect which disk has failed. Thus, the check disks do not need to contain information to identify the failed disk.information to recover the lost data is sufficient. Instead of using several disks with to store hamming Code, RAID Level 3 has a single check disk with parity information. Thus, the reliability overhead for RAID Level 3 is a single disk, the lowest overhead possible.

The performance characteristics of RAID Level 2 and RAID Level 3 are very similar. RAID Level 3 can also process only are Input / Output at a time, the minimum transfer unit is D blocks, and a write requires a read - modify - write cycle.

6. level 4 : Block - Interleaved Parity : RAID Level 4 has a stripping unit of a disk block, instead of a single bit as in RAID level 3, Block - level stripping has the advantage that read requests of the size of a disk block can be served entirely by the disk where the requested block resides. Large read requests of several disk blocks can still utilize the aggregated bandwidth of the D disks.

The write of a single block still requires a read - modify - write cycle, but only one data disk and the check disk are involved. The parity on the check disk can be updated without reading all D disk blocks because the new parity can be obtained by noticing the differences between the old data block and the new data block and them applying the difference to the parity block on the check disk :

New parity = (Old Data XOR New Data) XOR (Old parity).

RAID Level 3 and 4 configurations with four data disks require just a single check disk. Thus, in our example, the effective space utilization is 80 percent. The effective space utilization increases with the number of data disks, since always only one check disk is necessary.

7. Level 5 : Block - Interleaved Distributed Parity : RAID Level 5 improves upon Level 4 by distributing the parity blocks uniformly over all disks, instead of storing them as a single check disk. This distribution has two advantages. First, several write requests can potentially be processed in parallel, since the bottleneck of a unique check disk has been eliminated. second, read request have a higher level of parallelism. since the data is distributed over all disks, read request involve all disks, whereas in systems with a dedicated check disk never participate in reads.
A RAID Level 5 system has the best performance of all RAID levels with redundancy for small and large read and large write requests. Small writes still require a read - modify - write cycle and are thus less efficient than in RAID Level 1.
In our example, the corresponding RAID Level 5 system has 5 disks overall and thus the effective space utilization is the same as in RAID level 3 and 4.

8. Level 6 : P + Q Redundancy : A RAID Level 6 system uses Read - Solomon codes to be able to recover from upto two simultaneous disk failure. RAID Level 6 requires to check disks, but it also uniformly distributes redundant information at block level as in RAID Level 5 - Thus, the performance characteristics per small and large read requests and for large write requests are analogs to RAID Level 5. For small writes, the read - modify - write procedure involves 6 instead for four disks as compared to RAID Level 5, since two blocks with redundant information need to the updated.
   For a RAID Level 6 system with storage capacity equal to four data disks, six disks are required. Thus, in our example, the effective space utilization is 66 percent.