RAID, Disk Cache.
Q Write short note on RAID disk. --- 6 m
Ans: Magnetic disk has several drawbacks :
i) They have relatively low data transfer rates
ii) Their electromechanical design generates faults . Data transfer rate can be increased by using an array of small disk units. These disks can operate parallel. The data can be distributed in various ways referred to as RAID ( Redundant Array of Independent Disk ) levels 0 to 6.
a) RAID level 0 :- ( Non – Redundant )
b) RAID level 1 :- ( mirror )
c) RAID level 2 :- ( Redundancy through hamming code )
In mirror image, there is waste of memory. To avoid this waste we use hamming code. This code corrects single bit errors.
d) RAID 3 ( Bit – interleaved parity ) Parity disk can be used for reconstruction of data in case of a disk failure.
e) RAID 4 ( Block – level Parity ) Parity strip is stored on a separate disk
f) RAID 5 ( Block – level distributed parity) parity strip is distributed among all disks.
This will solve problem of RAID 4.
g) RAID 6 ( Dual Redundancy )
It provides an extremely high data reliability.
A disk cache is used to reduce read/ write time of a hard disk. It can be a specified portion of RAM ( Random Access Memory ) or it can be included as a part of the hard disk. The disk cache holds data that has recently been read and in some cases, adjacent data areas that are likely to be accessed next write caching is also used in many cases.
Where a program needs to access data from the hard disk, the operating system first check to see if the data is in the cache before reading it from the hard disk. It may be noted that accessing data from RAM is much faster than from a disk
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