RAID Levels – Redundant Array of Independent Disks

Lists of Descriptive Questions Answers and Short Study Notes On Operating System – RAID – Redundant Array of Independent Disks

  • (1) Explain Different RAID levels (RAID – Redundant Array of Independent Disks)

 

Question-1 Explain Different RAID levels (RAID – Redundant Array of Independent Disks)

  • Parallel processing is being used more and more to speed up CPU performance,parallel I/O can be a good idea.
  • The basic idea behind RAID is to install a box full of disks next to the computer, replace the disk controller card with RAID controller.
  • All RAID have the property that the data are distributed over drives, to allow parallel operation.

 

RAID level 0

raid-level-0

  • It consists of viewing the virtual single disk, as being divided up into strips of k sectors, and distributing data over multiple drives as shown in the figure is called striping.
  • The RAID level 0 organization writes consecutive stripes over the drives in round-robin fashion.
  • Command to read data block, consisting of four consecutive strips, RAID controller will break, this command up into four separate commands, one for each of the four disks and have them to operate in parallel.
  • Thus we have parallel I/O without the software knowing about it.
  • One disadvantage of RAID level 0 is that the reliability is potentially worse than having SLED (Single Large Expensive Disk).

 

 

RAID level 1 is a true RAID.

raid-level-1

  • It duplicates all the disks, so in figure there are four primary disks and four backup disks.
  • On a write operation every strip is written twice.
  • On a read operation either copy can be used, distributing the load over more drives.
  • Write performance is no better than for a single drive.
  • Read performance is twice as good.
  • Fault tolerance is excellent: if a drive crashes, a copy is simply used instead.
  • Recovery consists of simply installing a new drive and copying the entire backup drive to it.

 

RAID level 2

raid-level-2

  • RAID level 2 works on word, possibly even a byte basis.
  • Imagine splitting each byte into a pair of 4-bit nibbles, then adding Hamming code to each one to form a 7-bit word, of which bit 1, 2 and 4 were parity bits, as shown in figure below.
  • In this RAID level 2 each of seven drives needs synchronized in terms of arm position and rotational position, and then it would be possible to write the 7-bit Hamming coded word over the seven drives, one bit per drive.
  • Here, losing one drive did not cause problem, which can be handled by Hamming code on the fly.
  • But, on the down side this scheme requires all the drives to be rotationally synchronized and it also asks a lot of controller, since it must do a Hamming checksum every bit time.

 

RAID level 3

raid-level-3

  • RAID level 3 is simplified version of RAID level 2, as shown in the above figure.
  • Here single parity bit is computed for each data word and written to a parity drive.
  • As in RAID level 2 the drives must be exactly synchronized.
  • In case of drive crashing, it provides 1-bit error correction since position of each bad bit is known.

 

RAID level 4

raid-level-4

  • RAID level 4 works with strips and not individual word with parity.
  • They do not require synchronization of drives.
  • As shown in the figure RAID level 4 is like RAID level 0, with strip-for-strip parity written onto an extra drive, for example, if each strip is k bytes long, all strips are EXCLUSIVE ORed together, resulting in a parity strip k bytes long.
  • If a drive crashes, the lost bytes can be recomputed from the parity drive by reading the entire set of drives.
  • This design protects against the loss of a drive but performs poorly for small updates, if one sector is changed, it is necessary to read all the drives in order to recalculate the parity.
  • It creates heavy load on parity drive.

 

RAID level 5

raid-level-5

  • As with RAID level 4, there is a heavy load in the parity drive, it may become bottleneck.
  • This bottleneck can be eliminated in RAID level 5 by distributing the parity bits uniformly over all the drives, round robin fashion, as shown in the above figure.
  • In the event of drive crash, reconstructing the contents of the failed drive is complex process.