RAID (Redundant Array of Independent Disk), Lecture notes of Operating Systems

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RAID (REDUNDANT ARRAY OF

INDEPENDENT DISK)

->unit-

RAID:

1. RAID refers to redundancy array of the independent disk.
2. It is a technology which is used to connect multiple secondary storage

devices for increased performance, data redundancy or both.

3. It gives you the ability to survive one or more drive failure depending

upon the RAID level used.

4. It consists of an array of disks in which multiple disks are connected to

achieve different goals.

RAID-0:

RAID 0 splits data into smaller groups and stores them in different

disks (striping)

RAID 0 provides unparalleled read/write speeds but lacks

redundancy, resulting in the loss of all data within the array if one of

the drives fails.

To implement RAID 0, at least two drives are required

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RAID-0:

ADVANTAGE:

• In this level, throughput is increased because multiple data requests

probably not on the same disk.

• This level full utilizes the disk space and provides high performance.
• It requires minimum 2 drives.

DISADVANTAGE:

• It doesn't contain any error detection mechanism.
• The RAID 0 is not a true RAID because it is not fault-tolerance.
• In this level, failure of either disk results in complete data loss in

respective array.

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RAID-1:

ADVANTAGE:

• The main advantage of RAID 1 is fault tolerance. In this level, if

one disk fails, then the other automatically takes over.

• In this level, the array will function even if any one of the drives

fails.

DISADVANTAGE:

In this level, one extra drive is required per drive for

mirroring, so the expense is higher.

RAID-2:

RAID 2 consists of bit-level striping using hamming code parity.

In this level, each data bit in a word is recorded on a separate disk and

ECC code of data words is stored on different set disks.

Due to its high cost and complex structure, this level is not

commercially used. This same performance can be achieved by RAID 3

at a lower cost.

ADVANTAGE:

It uses the hamming code for error detection.

DISADVANTAGE:

It requires an additional drive for error detection.

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RAID-3:

ADVANTAGE:

• In this level, data is regenerated using parity drive.
• It contains high data transfer rates.
• In this level, data is accessed in parallel.

DISADVANTAGE:

It required an additional drive for parity.

• It gives a slow performance for operating on small sized files.

RAID-4:

RAID 4 consists of block-level stripping with a parity disk. Instead of

duplicating data, the RAID 4 adopts a parity-based approach.

Level 3 and level 4 both are required atleast three disks to implement

RAID.

parity can be calculated using an XOR function

If the data bits are 0,0,0,1 then the parity bits is XOR(0,1,0,0) = 1. If the

parity bits are 0,0,1,1 then the parity bit is XOR(0,0,1,1)= 0.

That means, even number of one results in parity 0 and an odd

number of one results in parity 1.

EXAMPLE:

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RAID-5:

ADVANTAGE:

• This level is cost effective and provides high performance.
• In this level, parity is distributed across the disks in an array.
• It is used to make the random write performance better.

DISADVANTAGE:

In this level, disk failure recovery takes longer time as parity

has to be calculated from all available drives.

• This level cannot survive in concurrent drive failure.

RAID-6:

This level is an extension of RAID 5. It contains block-level stripping

with 2 parity bits.

In RAID 6, you can survive 2 concurrent disk failures.

Suppose you are using RAID 5, and RAID 1. When your disks fail, you

need to replace the failed disk because if simultaneously another disk

fails then you won't be able to recover any of the data, so in this case

RAID 6 plays its part where you can survive two concurrent disk failures

before you run out of options.

RAID 16

ANY

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