Introduction to Hard Disk Drive

PPT BY VISHAL GARG

CSE 5TH SEMESTER

SHIVALIK COLLEGE OF ENGINEERING

Outcomes

What is Hard Drive Geometry of Hard Drive Hard Disk’s Parts & their Functions Characteristics of Hard Disk How Hard Drives Work Hard Drive Interfaces What is hard disk partition Why make partitions

What is a Hard Drive ?

The hard drive is one of the most important parts of computer because it is used as a long-term storage space for our data.

What that means, is regardless of whether or not the computer is on, or you lose power, the data will still stay stored on this drive keeping it safe.

A hard drive is an integral piece of our computer as operating system and all our data are typically stored there.

In the majority of the situations, if you did not have a working hard drive, or the hard drive malfunctions, you would not be able to boot your computer into the operating system and would get an error.

Image of a Hard Drive

If you opened your computer case and wanted to find your hard drive, it would look similar to the image :

Inside a Hard Drive

If you will open your hard drive, which would immediately void your warranty and potentially damage it, you would see something like the image :

Hard Drive Geometry

Hard disk drives are composed of one or more disks or platters on which data is stored.

The geometry of a hard drive is the organization of data on these platters.

Geometry determines how and where data is stored on the surface of each platter, and thus the maximum storage capacity of the drive.

There are five numerical values that describe geometry:

Heads

Cylinders

Sectors per track

Write precompensation(obsolete, but often seen on older drives)

Landing zone(obsolete, but often seen on older drives)

Heads

Click icon to add picture The number of heads is relative to the total

number of sides of all the platters used to store data (see Figure ).

If a hard disk drive has four platters, it can have up to eight heads.

The maximum number of heads is limited by BIOS to 16.

Some hard disk drive manufacturers use a technology called sector translation.

This allows some hard drives to have more than two heads per platter.

It is possible for a drive to have up to 12 heads but only one platter.

Regardless of the methods used to manufacture a hard drive, the maximum number of heads a hard drive can contain is 16.

Cylinders Data is stored in circular paths on the

surface of each head.

Each path is called a track.

There are hundreds of tracks on the surface of each head. A set of tracks (all of the same diameter) through each head is called a cylinder (see Figure ).

The number of cylinders is a measurement of drive geometry; the number of tracks is not a measurement of drive geometry.

BIOS limitations set the maximum number of cylinders at 1024.

Sectors per Track

A hard disk drive is cut (figuratively) into tens of thousands of small arcs, like a pie.

Each arc is called a sector and holds 512 bytes of data.

A sector is shown in Figure.

The number of sectors is not important and is not part of the geometry; the important value is the number of sectors per track.

BIOS limitations set the number of sectors per track at 63.

Write Precompensation

All sectors store the same number of bytes: 512; however, the sectors toward the outside of the platter are physically longer than those closer to the center.

Early drives experienced difficulty with the varying physical sizes of the sectors.

Therefore, a method of compensation was needed-the write precompensation value defines the cylinder where write precompensation begins.

Note :- The write precompensation value is now obsolete, but is often seen on older drives.

Landing Zone

A landing zone defines an unused cylinder as a "parking place" for the R/W heads.

This is found in older hard disk drives that use stepper motors.

It is important to park the heads on these drives to avoid accidental damage when moving hard disk drives.

Hard Disk’s Parts & their Functions

A hard drive consists of the following: Magnetic platters - Platters are the round plates in the image in previous

slide. Each platter holds a certain amount of information, so a drive with a lot of storage will have more platters than one with less storage. When information is stored and retrieved from the platters it is done so in concentric circles, called tracks, which are further broken down into segments called sectors.

Arm - The arm is the piece sticking out over the platters. The arms will contain read and write heads which are used to read and store the magnetic information onto the platters. Each platter will have its own arm which is used to read and write data off of it.

Motor - The motor is used to spin the platters from 4,500 to 15,000 rotations per minute (RPM). The faster the RPM of a drive, the better performance you will achieve from it.

Characteristics of Hard Disk are:

Seek Time - The amount of time required to move a drive's read/write head to a particular location on the disk. The lower the number, the better.

Access time - The actual amount of time from when a data is requested from a drive and delivered. The lower the number, the better.

RPMS - How fast the platters spin. The higher the number, the faster the drive will be.

Capacity - The amount of storage space available on the drive. Typically most drives purchased today will be 80 Gigabytes or greater. An 80 GB hard drive will on average cost around 60 USD.

Interface - How the hard drive connects to your computer. Hard drive's interface should be supported on your computer before you purchase it.

How hard drives work ?

When the computer wants to retrieve data off of the hard drive, the motor will spin up the platters and the arm will move itself to the appropriate position above the platter where the data is stored.

The heads on the arm will detect the magnetic bits on the platters and convert them into the appropriate data that can be used by the computer. Conversely, when data is sent to the drive, the heads will this time, send magnetic pulses at the platters changing the magnetic properties of the platter, and thus storing your information.

Note : It is important to note, that since the data stored on your hard drive is magnetic, it is not a good idea to play with a magnet near your hard drive.

Hard Drive Interfaces

A hard drive connects to your computer through a specific type of interface. The interface on your hard drive must match the corresponding interface on your motherboard.

Some information about interfaces are explained below. IDE or ATA - This is currently the most common interface used but is quickly

becoming overcome by the newer SATA interface. Hard drives using this type of interface have speeds up to 100 Mbps.

SATA - A newer interface that uses less bulky cables and has speeds starting at 150 Mbps for SATA and 300 Mbps for SATA II. Almost all computer manufacturers have started using SATA drives.

SCSI - This type of interface is typically used in a business environment for servers. Hard Drives designed for a SCSI interface tend to have a faster RPM which therefore provides better performance.

What is a hard disk partition ?

When a hard drive is installed in your computer, it must be partitioned before you can format and use it.

Partitioning a drive is when you divide the total storage of a drive into different pieces. These pieces are called partitions.

Once a partition is created, it can then be formatted so that it can be used on a computer.

When partitions are made, you specify the total amount of storage that you would like to allocate to that partition from the total size of the drive.

Why make partitions ?

There are following reasons of making partitions :

Multiple Filesystems - When you create a partition, then you need to format it before it can be used.

When formatting a partition you tell the operating system what filesystem you would like that partition to be formatted in.

Once you format a partition, and therefore assign the type of filesystem you want to use with it, you can not change that filesystem without formatting the partition again and losing all the data contained on it.

If you require different filesystems on your computer, whether it be for security or for a specific application, you can then make multiple partition and assign one type of filesystem to one of the partitions and another to one of the other partitions.

Why make partitions ?(cont…)

Multiple Operating Systems - Some operating systems use filesystems that are not compatible with other operating systems.

If you want to use two operating systems on the same computer that are not compatible in this way, you could then make two partitions, one for each operating system.

Then a boot loader could be used to switch between them when you start your computer.

Wasted disk Space -When a filesystems store data in a partition, the larger the partition, the greater the chance of wasted space.

By having multiple partitions of smaller size, you reduce the amount of waste that filesystems may create.

Seperate system files from users files - Some components of an operating system require storage space to operate.

For example, in Windows if you have no hard drive space available on the drive that Windows is installed on, you will not be able to print.

By creating a partition solely devoted to the operating system and then another partition dedicated to your own data and programs, there is less of a chance of you using up the space on the operating system partition with your data, and thus not causing problems.