Explain 3 types of RAID Techniques?

➢ There are three RAID techniques

1. striping

2. mirroring

3. parity

Striping -

➢ Striping is a technique to spread data across multiple drives (more than one) to use the drives 

in parallel.

➢ All the read-write heads work simultaneously, allowing more data to be processed in a shorter 

time and increasing performance, compared to reading and writing from a single disk.

➢ Within each disk in a RAID set, a predefined number of contiguously addressable disk 

blocks are defined as a strip.

➢ The set of aligned strips that spans across all the disks within the RAID set is called a stripe.

➢ Fig shows physical and logical representations of a striped RAID set.

➢ Strip size (also called stripe depth) describes the number of blocks in a strip and is the 

maximum amount of data that can be written to or read from a single disk in the set.

➢ All strips in a stripe have the same number of blocks.

✓ Having a smaller strip size means that data is broken into smaller pieces while spread 

across the disks.

➢ Stripe size is a multiple of strip size by the number of data disks in the RAID set.

✓ Eg: In a 5 disk striped RAID set with a strip size of 64 KB, the stripe size is 320KB 

(64KB x 5).

➢ Stripe width refers to the number of data strips in a stripe.

➢ Striped RAID does not provide any data protection unless parity or mirroring is used.

2 Mirroring

➢ Mirroring is a technique whereby the same data is stored on two different disk drives, 

yielding two copies of the data.

➢ If one disk drive failure occurs, the data is intact on the surviving disk drive (see Fig 1.12) 

and the controller continues to service the host’s data requests from the surviving disk of a 

mirrored pair.

➢ When the failed disk is replaced with a new disk, the controller copies the data from the 

surviving disk of the mirrored pair.

➢ This activity is transparent to the host.

➢ Advantages:

✓ complete data redundancy,

✓ mirroring enables fast recovery from disk failure.

✓ data protection

➢ Mirroring is not a substitute for data backup. Mirroring constantly captures changes in the 

data, whereas a backup captures point-in-time images of the data.

➢ Disadvantages:

✓ Mirroring involves duplication of data — the amount of storage capacity needed is twice the amount of data being stored.

✓ Expensive

Parity

➢ Parity is a method to protect striped data from disk drive failure without the cost of 

mirroring.

➢ An additional disk drive is added to hold parity, a mathematical construct that allows recreation of the missing data.

➢ Parity is a redundancy technique that ensures protection of data without maintaining a full 

set of duplicate data.

➢ Calculation of parity is a function of the RAID controller.

➢ Parity information can be stored on separate, dedicated disk drives or distributed across all the 

drives in a RAID set.

➢ Fig shows a parity RAID set.

➢ The first four disks, labeled “Data Disks,” contain the data. The fifth disk, labeled “Parity 

Disk,” stores the parity information, which, in this case, is the sum of the elements in each 

row.

➢ Now, if one of the data disks fails, the missing value can be calculated by subtracting the sum 

of the rest of the elements from the parity value.

➢ Here, computation of parity is represented as an arithmetic sum of the data. However, parity 

calculation is a bitwise XOR operation