Hard disk drive

The hard disk drive (HDD) was invented by IBM in 1950s. Initially the HDD is known as Winchester  Disk. As we know very well that a hard disk is non volatile storage device, which stores the data even when the power is switched off. Although the hard disk drive is much faster than the floppy disk drive or cartridge drive, but its principle is same as that of the floppy disk drive or cartridge drive.

In a hard disk drive the data is stored on a magnetic media or surface layered onto hard disk platters. These platters are coated with magnetic particles. These magnetic particles are iron oxide particles. These platters are coated on both sides.

A hard disk drive consist of 3 to 10 magnetic disks called platters. These platters are mounted on a motor, which spin the platter at a very high speed. The rotation speed of a disk platters is up to 3600 rpm, which is 10 to 20 times faster than a floppy disk drive. The distance between the two adjacent platter is ½ inch. All the disks platters move simultaneously in same direction with the same speed. The entire arrangement is placed in a cover. The entire unit means platters or heads are called hard disk assembly (HAD).

The storage capacity range from 10 MB to several gigabytes.

Data is stored on to the disk in the form of files. A file is simply a named collection of bytes. When a program running on a computer request  a file, the hard disk retrieves its bytes and sends them to the CPU one at a time. There are the two ways  to measure the performance of a hard disk:

1. Data rate: The data rate is the number of bytes per second that the drive can deliver to the CPU. It is also known as transmission rate. the rate lie between 5 to 40 MBPS.
2. Seek time: It is the amount of time between when a CPU requests a file and when the first byte of the file is sent to the CPU. 10 to 20 millisecond is common.

Tracks and sectors

All hard disk drives write data on to the platters in concentric rings, called tracks. The spacing of the tracks and number of tracks on each platter surface depend upon the capacity of the drive. Today’s hard disk drives contain 1000 to 2000 tracks on each platter surface.

Each track on each platter surface is further divided into the pie shaped wedges, called sectors. Each sector stores 512 bytes of data. The number of sectors on a track can vary depending upon the density of the disk. Each sector is numbered, starting from 0. Each sector is divided in to two parts called

Address area and data area. The address area is used to identify a sector whereas the data area stores 512 bytes of data. The address area also contain the CRC information.

Hard disk drive components

Some of the important component of HDD are

• Platters: One of the most important component of the hard disk drive is platter. A platter is made up of rigid material like aluminum or glass ceramic, having coated with magnetic material on both sides. The hard disk write data on this platter. The number of platters in a hard disk drive varies from three to ten.
• Read / write head: Like FDD, the HDD also contain a read/write head. The head is responsible for writing data to and reading it from the magnetic material applied to the platter surfaces. For each platter surface there is one read/write head means if there are 3 platters means 6 surfaces, that means there are six read/write head.

Fixed head are not used in today’s hard disk. The moving head is mounted on a actuator arm. None of the head move independently, when the arm moves all the heads move back and forth. Where as in case of fixed head, there is a read/write head for each and every track.  The head is made up of iron oxide core and it is wrapped with electromagnetic coils.

• Head slider : The read/write head assembly of hard disk drive rides on arm called the head slider.
• Head actuator : Head actuator arm are light weight, rigid, triangular pieces of a metal alloy which holds the  head slider and the head and it move the head back and forth across the platter.
• Spindle motor: The spindle motor rotates the platters. The spindle motor rotate the platters at a constant speed so that there is no impact of vibration or noise can carried out, which disturb the position of the platters in relation to the head. Moreover the spindle motor is responsible for maximum part of power consumption of the hard drive.
• Circuit board or logic circuit: It contain  the electronic circuit. This board is mounted below the chassis and contain all circuits that is responsible for the communication with the system, movement of head, spinning of platters and so on.
• Cables and connectors: A HDD also contains the 2 cables and connectors.

Power cable provide two types of power to the HDD ; +5 V for logic circuit and +12 V for rotating the drive motor.

Operation on Hard Disk Drive

There are two types of operations performed on the HDD.

1. Read operation          2.    Write operation

Read operation: For reading data from the HD, CPU generate a read request/command and passes it to the drive controller along with the address of required data location. When the HDD controller get this address and command the head start working and it come closer to surface while the disk is spinning. When the required location is reached, it is confirmed by CRC codes and then the head generates a voltage pulse when it passes over the location. The drive controller amplifies the pulses and decode them to reproduce the original data.

Write operation: The write operation of the hard drive is very much similar to the read operation. The system send the write command and sends it through system bus to drive controller. The address of the required data location is also transmitted with the write command. When the write command is received by the HDD, it start spinning. Then the actuator arm moves and locate the required track. Once the required track is found the head waits for required sector to come below it, when the sector is reached below it, the head moves and stays on the sector. It encode the original data in to the bits and generate the magnetic or voltage pulses to write it on the desired sector.

The SMART Technology

More over the HDD also possesses the feature  called SMART (Self Monitoring Analysis & Reporting Technology). By this technology the system measure the performance of the HDD and system facing problem using a technique called Predictive Failure Analysis.

Key features of HDD:

1. Performance: The performance of a system is largely depends upon the performance of a HDD. How better the HDD work? How fast it load the PC Boot up program and other program and how’s its performance in case of the multitasking system? Such as graphics work, editing sound and video or working with databases.
2. Storage Capacity: Better be the performance of the system if the storage capacity is large or big. Because the large size disk store better features than the low space devices.
3. Software support: the hard disk should be fast and such that it provide the needy space to the large software and load them efficiently.
4. Reliability: A HDD must be reliable in case of delivering of the data, reading of data and writing of the data. It does not loss the data during the read and write operation.
5. Latency: It is the amount of time that it takes for the platter to spin, bringing the sector to the right position. The faster the platter spin, lower is the latency.
6. Access time: A drive’s access time represent the total delay between the beginning of a read and write operation and the time when the drive actually begins reading and writing.
7. Track switch time: Also known as Cylinder Switch Time or Track to Track seek time. This is the time taken by the actuator arm to move the head from one track to the next adjacent track.
8. Areal Density (AD): It is the measurement of the overall efficiency of a magnetic storage media. Greater the areal density, more data the drive can store in the same amount of the space. This is possible because manufacturer have increased both the number of bits stored in a single track and the number of tracks on each platter surface. The two factors are known as drive’s areal density. o To compute areal density multiply the bits per inch (BPI) to the track per inch (TPI). BPI is the number of bits stored in one inch length of a single track, and the TPI is number of track in one inch length from the centre of the platter to its outer edges. The areal density measures in bits per square inch (BPSI).

Some Advanced Technology

1. Cylinder Skewing: The primary purpose of the cylinder skewing is to minimize the movement of the head. When one track ends the heads move to next track and start reading it from the starting position of each track is same. But in case of cylinder skewing the starting position of each track is different from the adjacent one. By this feature the head immediately read the next track when the previous tracks ends.
2. Zoned bit recording or Sector sparing: In the starting days, the each track contain the same number of sectors. It lead to the fact that sectors on the outer tracks had lower density and the sectors on the inner track had higher density. To overcome this problem Zoned Bit Recording (ZBR) method was introduced. Under this method the total tracks are divided in to the zones. The tracks in a particular zone contain the same number of sectors. So that there is a stability between to two types of densities.

Note : When a hard disk is manufactured, a certain amount of pressure is maintained in the hard disk so that it works properly. Such pressure lift the heads at a proper height from the platter surface. That is why unlike FDD, the head of the hard disk drive does not touch the surface. But if the pressure reduces, then it is not able to maintain the proper height between the head and the surface and the head crash. Head crash can be caused by electronic failure, a sudden power failure, wear and tear, poor manufactured disks etc.

In the most desktop and server drives, when powering down, the heads are moved to the landing zone, an area of the disk usually near its inner diameter, where no data stored. This area is known ad Contact Start and Stop (CSS) zone.