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What is RAID? Explain types of RAID in detail.

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27.What is RAID? Explain types of RAID in detail.
Ans.
1. RAID is an acronym for Redundant Array of Independent Disks, Used for maintaining copy of the data.

 2. The information of system’s RAID configuration is stored in ‘/etc/traidtab’.

 3. There are two types of RAID – Hardware RAID and Software RAID.

 • Hardware RAID – in Hardware RAID, the disks have their own RAID controller with built in software that handles RAID disk setup.
The controller is a card in one of the system’s expansion slots.
It may be built onto the system board.
The O/S does not control the RAID level used, it is controlled by the hardware RAID controller.

 • Software RAID – there is no RAID controller card. The O/S is used to set up a logical array and the O/S controls the RAID level used by the system.

 4. Six RAID levels are commonly used.
The three most commonly used RAID levels are

 • RAID level 0 – stripping
This level requires at least two disks and uses a method called striping that writes data across both drives.
There is no redundancy provided by this level of RAID, since the loss of either drive makes it impossible to recover the data.
This level does give a speed increase in writing to the disks.

 • RAID level 1 – Mirroring This level requires at least two disks and uses a method called mirroring.
With mirroring, the data is written to both of the drives. So, each drive is an exact mirror of the other one and if one fails the other still holds all the data.
There are two variants to level 1 with one variant using a single disk controller that writes to both disks.
The other variant uses two disk controllers, one for each disk.
This variant of RAID level 1 is known as duplexing.

 • RAID level 2 – Error Checking and Correction
Requires minimum 3 disks. RAID 2 adds error checking and correcting checksums to RAID-1.
ECC is Error correcting code in which each data signal confirms to specific rules of construction so that departures from this construction in the received signal can generally be automatically detected and corrected. 

 • RAID level 3 – Byte-Level Striping with Parity Disk
Requires minimum 3 disks, It does striping, like RAID-0, but at a very small granularity.
It also adds parity disks which helps in error detection and recovery.

 • RAID level 4 – Block-Level Striping with Parity disk
Requires minimum 3 disks, It attempts to add error checking and recovery to RAID-3 by doing block level striping with the addition of a single parity disk.

 • RAID level 5 – Block-Level Striping with Distributed Parity
It requires at least three disks and uses striping to write the data across the two disks.
But unlike 1, this level uses the third disk to hold parity information that can be used to reconstruct the data from either, but not both, of the two disks after a single disk failure.


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