2. Introduction
Mass storage refers to systems meant to store large amount of data. There are three mass storage basic
structures. They are :
1. Magnetic Disks
2. Solid-state Disks
3. Magnetic Tapes
1. Magnetic Disks
● A storage device that can be assumed as the shape of Gramophone record (type of analog storage
medium) .
● It provides the bulk of secondary storage for modern computer systems.
3.
4. ● Each disk platter has a flat circular shape like CD. Common platter diameters range from 1.8 to 5.25
inches.
● Bits are saved in the magnetized surface in marks along concentric circles known as tracks.
● The tracks are frequently divided into areas known as sectors. Sector is the basic unit of storage.
● The collection of all tracks that are the same distance from the edge of the platter called cylinder.
● The number of cylinder in the disk drive equals the number of tracks in each platter.
● The arm assembly which moves all heads simultaneously from one cylinder to another.
Access Time
The access time of a record on a disk includes three components such as :
1. Seek time / Random access time
2. Rotational delay / Latency time
3. Data transfer time
1. Seek time / Random access time
● The time required to arrange the read / write head at the desired track called seek time.
● For example , Suppose that the read / write head is on track 2 and the record to be read is on track 5 , then
the read / write head must move from track 2 to track 5.
● The average seeks time on a modern disk is 8 to 12 ms.
5. 2. Rotational delay / Latency time
● The time required to position the read / write head on a specific sector when the head has already been
placed on the desired track called rotational delay / latency time.
● The rotational delay is based on the speed of rotation of the disk.
● On the average latency will be half of one revolution time.
● The average latency time on modern disks is 4.2 to 6.7 ms.
3. Data transfer time
● Data transfer time is the actual time needed to send the data.
In the disk head files on an extremely thin cushion of air , the head will make contact with the disk surface.
Although the disk platters are coated with a thin protective layer , sometimes it will damage the magnetic
surface. This accident called a head crash.
