RAID LEVEL 4

RAID 4

•Minimum 3 disks required•Data is spread across all the drives in blocks •Dedicated parity drive

•If one drive in the array fails then concept of parity data is used (Exclusive Or)

•Write Operation

Read Operation

Advantages & Disadvantages

•Good performance for read•Poor performance for write operation•Bottleneck•Its used is uncommon

RAID LEVEL 5

RAID 5

•RAID 5 is organized in a similar fashion to RAID 4.The difference is that RAID 5•distributes the parity strips across all disks.•RAID 5 comprises block-level striping with distributed parity. It requires that all drives but one be present to operate. Upon failure of a single drive, subsequent reads can •be calculated from the distributed parity such that no data is lost. RAID 5 requires at least three disks.

Following are the key points to remember for RAID level 5.Minimum 3 disks.Good performance ( as blocks are striped ).Good redundancy ( distributed parity ).

Working• First we need to remind you XOR definition:

• XOR function result is equal 1 if both arguments are different.

• XOR (0, 1) = 1

• XOR (1, 0) = 1

• XOR function output is equal 0 if both arguments are same.

• XOR (0, 0) = 0

• XOR (1, 1) = 0

• Now let us assume we have 3 drives with the following bits:

• | 101 | 010 | 011 |

• And we calculate XOR of those data and place it on 4th drive

• XOR (101, 010, 011) = 100 (XOR (101,010) = 111 and then XOR (111, 011) = 100

• So the data on the four drives looks like this below:

• | 101 | 010 | 011 | 100 |

• Now let’s see how the XOR MAGIC works. Let’s assume the second drive has failed. When we calculate XOR all the remaining data will be present from the missing drive.

• | 101 | 010 | 011 | 100 |

• XOR (101, 011, 100) = 010

• You can check the missing other drives and XOR of the remaining data will always give you exactly the data of your missing drive.

• | 101 | 010 | 011 | 100 |

• XOR (101, 010, 100) = 011

• What works for 3 bits and 4 drives only, works for any number of bits and any number of drives. Real RAID 5 has the most common stripe size of 64k (65536 * 8 = 524288 bits )

•So the real XOR engine only needs to deal with 524288 bits and not 3 bits as in our exercise. This is why the RAID 5 needs a very efficient XOR engine in order to calculate it fast.•So when adding one drive for parity you will be able to rebuild the missing data in case of any drive failure.

Advantages•Cost effective - only 1 extra disk is required.•Read data transactions are very fast while write data transaction are somewhat slower (due to the parity that has to be calculated).

Disadvantages• Individual block data transfer rate same as a single disk.•Like RAID 3, this is complex technology.

RAID LEVEL 6

RAID 6

Very similar to RAID 5, but adds an additional parity block.Raid 6 creates two parity blocks for each data block.Can handle two disk failureThis RAID configuration is complex to implement in a RAID controller, as it has to calculate two parity data for each data block

RAID 6

Raid 6 is of extremely high help in systems having a very high capacity and when the rebuilding of a RAID 5 volume takes a long time or when there is significant chance of failure of another drive before the rebuild process is completed.

RAID 6 increases the data reliability by using the two parity stripes. These stripes allow for the failure of the two disks which can occur within the RAID disk drive set. Major application of RAID 6 is in SATA, IDE and the SCSI environments.

Independent Data Disks with Two Independent Parity Schemes

Advantages of RAID 6•Raid 6 is highly reliable compared to the other raid levels.•It is the most suited raid level for small as well as moderate size database systems. •Raid 6 provides protection against multiple bad block failures while non-degraded.•It allows for the failure of two disks simultaneously with no data loss.

Disadvantages of RAID 6

•The raid 6 controller design is very complex compared to the other level controllers.•The overhead of the controller for computing parity addresses is high.•RAID 6 needs N+2 drives in order to implement, the reason being the dual parity scheme.•Write performance can be the same as that for RAID 5.•Two drives are used for parity calculations.

Applications recommended for RAID 6

•Web and email servers.•Database servers.•Intranet servers.•Excellent fault tolerance.•File and application servers.

Thank You!