RAID (Redundant Array of Independent Disks )

• A RAID joins two or more hard disks so that they make a logical disk.
• There are different reasons why this is done, the most commons ones are:

1)Stopping data loss, when one or more disks of the array fail.

2)Get faster data transfers.

3)Get the ability to change disks while the system keeps running.

4)Join several disks to get more storage capacity; sometimes lots of cheap disks are used, rather than a more expensive one.

• RAID is done by using special hardware or software on the computer.
• The joined hard disks will then look like one hard disk to the user.

•RAID is widely used to protect servers from individual disk failures, critical when your business is at stake.

•If a drive in a RAID array fails, it does not mean that the data contained on that drive is lost, since other drives in the array still contain the data.

•RAID is divided according to levels. The basic level consists of:

1)RAID 0 (Stripping)

• Data blocks are split into stripes based on the adjusted stripe size (for example, 128 KB) and the number of hard disks.

• Each stripe is stored on a separate hard disk

• Significant improvement of the data throughput is achieved using this RAID level, especially with sequential read and write.

• RAID 0 includes no redundancy. When one hard disk fails, all data is lost.

2) RAID 1 (mirroring)

• All data is stored twice on two identical hard disks which is called Disk Mirroring.

• When one hard disk fails, all data is immediately available on the other without any impact on performance and data integrity.

3) RAID 2

• RAID 2 is the only RAID level that can repair errors, the other RAID levels can only detect them.

• When they find that the information needed does not make sense, they will simply rebuild it.

• This is done with calculations, using information on the other disks

• RAID 2 needs at least 10 disks to work.

• Because of its complexity and its need for very expensive and special hardware RAID 2 is no longer used very much.

 

4) RAID 3

• Raid Level 3 is much like RAID Level 0.

• An additional disk is added to store parity information. This is done by bitwise addition of the value of a block on the other disks.

• The parity information is stored on a separate (dedicated) disk. This will cause a problem, because if the parity disk crashes, the parity information is lost.

• RAID Level 3 is usually done with at least 3 disks.

5) RAID 4

• RAID 4 works in the same way as RAID 0.

• The data is striped across the hard disks and the controller calculates redundancy data (parity information) that is stored on a separate hard disk.

• Should one hard disk fail, all data remains fully available.

 

6) RAID 5

• RAID 5 is the most common secure RAID level.

• It is similar to RAID-3 except that data are transferred to disks by independent read and write operations (not in parallel).

• The data chunks that are written are also larger.

• Instead of a dedicated parity disk, parity information is spread across all the drives.

• You need at least 3 disks for a RAID 5 array.