Differentiate between the access mechanism of a magnetic disk to that of CD ROM? Why do you need a number of disks (RAID)? Define different levels of RAID?

RAID is an acronym first defined by David A. Patterson, Garth A. Gibson, and Randy Katz at the University of California, Berkeley in 1987 to describe a redundant array of inexpensive disks,[1] a technology that allowed computer users to achieve high levels of storage reliability from low-cost and less reliable PC-class disk-drive components, via the technique of arranging the devices into arrays for redundancy. There are various combinations of these approaches giving different trade-offs of protection against data loss, capacity, and speed. RAID levels 0, 1, and 5 are the most commonly found, and cover most requirements.

 RAID 0
RAID 0 (striped disks) distributes data across multiple disks in a way that gives improved speed at any given instant. If one disk fails, however, all of the data on the array will be lost, as there is neither parity nor mirroring. In this regard, RAID 0 is somewhat of a misnomer, in that RAID 0 is non-redundant. A RAID 0 array requires a minimum of two drives. A RAID 0 configuration can be applied to a single drive provided that the RAID controller is hardware and not software (i.e. OS-based arrays) and allows for such configuration. This allows a single drive to be added to a controller already containing another RAID configuration when the user does not wish to add the additional drive to the existing array. In this case, the controller would be set up as RAID only (as opposed to SCSI only (no RAID)), which requires that each individual drive be a part of some sort of RAID array.
 

 RAID 1
RAID 1 mirrors the contents of the disks, making a form of 1:1 ratio realtime backup. The contents of each disk in the array are identical to that of every other disk in the array. A RAID 1 array requires a minimum of two drives. Although RAID 1's writing process copies the data identically to all drives, a RAID 1 mirror would not be suitable as a permanent backup solution for businesses, since RAID architecture by design allows for certain failures to take place (e.g. vandalism or accidental file deletion). However for home or other applications, where vandalism is very unlikely and accidental file deletion is put up with, RAID 1 offers a good backup solution.

RAID 3/4
RAID 3 or 4 (striped disks with dedicated parity) combines three or more disks in a way that protects data against loss of any one disk. Fault tolerance is achieved by adding an extra disk to the array, which is dedicated to storing parity information; the overall capacity of the array is reduced by one disk. A RAID 3 or 4 array requires a minimum of three drives: two to hold striped data, and a third for parity. With the minimum three drives needed for RAID 3, the storage efficiency is 66 percent. With six drives, the storage efficiency is 87 percent. The main disadvantage is poor performance for multiple, simultaneous, and independent read/write operations.

RAID 5
Striped set with distributed parity or interleave parity requiring 3 or more disks. Distributed parity requires all drives but one to be present to operate; drive failure requires replacement, but the array is not destroyed by a single drive failure. Upon drive failure, any subsequent reads can be calculated from the distributed parity such that the drive failure is masked from the end user. The array will have data loss in the event of a second drive failure and is vulnerable until the data that was on the failed drive is rebuilt onto a replacement drive. A
single drive failure in the set will result in reduced performance of the entire set until the failed drive has been replaced and rebuilt.