Mass Storage System

Mass Storage System – This topic is about physical memory which use as secondary memory.

• Magnetic Disks

A magnetic disk is a storage device which uses a magnetization process to access, write and rewrite data. It is shielded with a magnetic coating and stores data in the form of spots, tracks and sectors. zip disks, hard disks, and floppy disks are common examples of magnetic disks.

• Magnetic Tapes

In the early times magnetic tapes were used for secondary storage before introduction of hard disk drivers but today are used mostly for backups. To access a particular spot on a magnetic tape can be slow in process but once writing or reading begins access speeds are comparable to disk deriver. The tape driver has capacity range from 20-200 GB and compression can double that range.

• Disk Structure

In present computer system, most of the secondary storage is in the form of magnetic disks. Therefore, the structure of a magnetic disk is compulsory to recognize how the data in the disk is retrieved by the computer. A magnetic disk involves several platters. Each platter is distributed into circular shaped tracks. The size of the tracks adjacent to the center is fewer than the size of the tracks farther from the center. Respectively track is further separated into sectors, as shown above in the figure. Tracks of the identical distance from center form a cylinder. A read/write head is used to read and write data from a sector of the magnetic disk.

The speed of the disk is measured by two ways:

1.       Transfer rate:  This is the speed at which data moves from disk to computer.

2.       Random access time: The sum of the rotational latency and seek-time. 

Seek-time is basically the time taken by the arm to move to the requisite track. 

Rotational latency is described as the time taken by the arm to reach the requisite sector in the track.

Although the disk is organized as sectors and tracks physically, the data is logically organized and addressed as an array of blocks with fixed size. A block can be 512 or 1024 bytes in its size. Each and every logical block is mapped sequentially with a sector on the disk. In this manner, every sector in the disk will have a logical address.

• Disk Attachment

Disk drives may be attached either directly to a network or specific host according to the following two ways:

1. Host attached storage
2. Network attached storage
3. Storage are network

• Disk Scheduling

The scheduling schemes used for disk scheduling are:

1. FCFS scheduling
2. SSTF scheduling
3. SCAN scheduling
4. C-SCAN scheduling
5. LOOK scheduling
6. Selection of a disk scheduling algorithm.

Direct Memory Access