What is Hard Disk

What is Hard Disk - Boot Disk - External Hard Drive - Hard Drives - Hard Drive- Hard Disk - Network Storage - Platter

Today, hard disk is most commonly used storage device in personal computers and laptop computers. Most application programs and operating systems require hard disk for installation and operation. A hard disk, also called a hard disk drive. It contains one or more metal platters mounted on a central spindle, like a stack of rigid diskettes. A hard disk is actually, a stack of platters. Each platter is coated on both sides with magnetic material. Both surfaces of each disk or platter are used to store information except for the top and bottom platters.

The hard disk and drive is a single unit. It includes the hard disk, the motor that rotates the platters. Each platter has two read/write heads, one for each side. The hard disk has also arms that move the read/write heads to the proper location on the platter to read or write data. The entire unit is enclosed in an airtight, sealed case. The hard disk is not portable. It is permanently fixed with the system unit. It is because hard disk is also referred to as fixed disk.

A disk or plotter has a flat circular shape. The surface of the disk is logically divided into circular tracks, which are subdivided into sectors. The set of tracks of different platters that are at a particular head position is referred to as cylinder.

However, the basic storage unit is sector. All sectors contained the same number of bytes. Usually, in a single sector 512 bytes are stored. The total capacity of a disk can be calculated by multiplying together, the following:

• Number of cylinders, which is the same as the number of tracks per surface.
• Number of read/write heads.
• Number of sectors per track.
• Number of bytes per sector.  

Today, the storage capacity of hard disk in PCs is from 40 GB to 160 GB (or more). The storage capacity of hard disk is determined from the number of platters it contains and their density. A plotter is made of aluminum, glass, or ceramic and is coated with an alloy material that allows storing data magnetically on the disk’s surface. Like floppy disk, hard disk is formatted before to store data on it. The operating system makes logical tracks on both sides of each platter and divides each track into sectors. Each platter of hard disk has a very high density. A large number of tracks are made on each surface of the platter.

Today, the storage capacity of hard disk in PCs is from 40 GB to 160 GB (or more). The storage capacity of hard disk is determined from the number of platters it contains and their density. A plotter is made of aluminum, glass, or ceramic and is coated with an alloy material that allows storing data magnetically on the disk’s surface. Like floppy disk, hard disk is formatted before to store data on it. The operating system makes logical tracks on both sides of each platter and divides each track into sectors. Each platter of hard disk has a very high density. A large number of tracks are made on each surface of the platter.

Hard Disk Performance :

The performance of the hard disk depends on the following factors.

1. Seek Time

Seek time is the time required to move a read/write head to a particular cylinder or track. Seek time is variable, because it will take more time to reach track 300 from track 1 than to reach track 100. Therefore, seek time is expressed as average seek time.

2. Rotational Latency

Once a cylinder or track is selected, the disk controller waits until the read/write head reaches to an appropriate sector. The time required to reach the head at the beginning of an appropriate sector is called rotational latency or rotational delay.

3. Transfer Time

Transfer time is the time required to transfer data, read from the disk into main memory. Alternatively, it is the time required to write data on the disk from main memory.

4. Assess Time

Access time is the sum of seek time, rotational delay and transfer time.

Mathematically it is written as;

Access time = Average Seek Time + Rotational Latency + Transfer Time

Once the read/write head is in position, the read or write operation is performed. Typically, a disk can transfer several megabytes of data per second. Therefore, the value of seek time and rotational latency is in milliseconds.