Understanding Magnetic Disk According to Experts

Magnetic disk is the first Direct Access Storage Device (DASD) made by the computer industry. Magnetic Disk is a circular disk made of a certain material (metal or plastic) with a surface coated with a material that can be magnetized. The read/write mechanism used is called a head, which is a conducting coil during reading and writing operations, the head is stationary while the disk moves underneath usually that hangs above the surface and is suspended on an air cushion, except on floppy disks where the disk head touch to the surface.

In a magnetic disk there are two methods of layout data on the disk, namely Constant Angular Velocity and Multiple Soned Recording. Disks are organized (the surface of the disk is divided) in the form of concentric rings called tracks or lines that separate the tracks as shown below. Each track is separated by gap, the function of the gap is to prevent or reduce reading or writing errors caused by a head slip or due to magnetic field interference.

Blocks of data are stored in disk-sized blocks called sectors. Tracks are usually filled with several sectors, generally 10 to 100 sectors per track, for more details see the following picture:

Image Blocks of data on a magnetic disk.

Examples of magnetic disks are hard disks and floppy disks.

Physical Characters on Magnetic Disk

Disk Pack is a type of storage device on a magnetic disk, which consists of several stacks of aluminum disks. In a pack / stack generally consists of 11 disks. Each platter is 14 inches in diameter (8 inches on a mini disk) and resembles a vinyl record. The surface is coated with a metal-oxide film that contains magnetization as in magnetic tape.

The many tracks on a platter exhibit storage characteristics on the surface layer, disk drive capacity and access mechanism. Discs have 200 – 800 tracks per surface (the number of tracks on a disc is fixed). A disc pack of 11 discs has 20 surfaces for storing data.

Both sides of each disc are used to store data, except for the top and bottom surfaces which are not used to store data, because that is where dirt/dust is more easily exposed than the inner surface. Also the arm on the outer surface can only access half the data.

For access, disk packs are arranged on a disk drive which includes a controller, access arm, read/write head and a mechanism for pack rotation. There are disk drives that are made built-in with disk packs, so these disk packs cannot be moved which are called non-removable. While disk packs that can be moved are called removable.

The disk controller handles code changes of record addressing, including proper drive selection and code changes of required disk pack data positions on the drive. The controller also manages buffer storage to handle error detection, error correction and control of read/write head activity.

The array of disks on a disk pack rotates continuously at a rotational speed of 3600 per minute. Unlike on tape, disk rotation does not stop between the disks on the device.

The disadvantage is that in a situation where the read/write head collides with the record storage surface on the disk, this is known as a head crash.

Components on Magnetic Disk

The hard disk consists of several important components. The main component is a plate (platter) which functions as a data store. This plate is a solid disk that is round and flat, both sides of the surface are coated with a special material so that it has magnetic patterns. a shaft called a spindle.

Image of Components on a magnetic disk.

1. Spindle

The hard disk consists of a spindle which is the center of rotation of the pieces of magnetic disk data storage. This spindle rotates quickly, therefore it must use high quality bearings.

In the past, hard disks used ball bearings, but now hard disks use fluid bearings. With fluid bearings, frictional forces and noise levels can be minimized. This spindle determines the rotation of the hard disk. The faster the hard disk rpm rotation, the faster the data transfer.

2. Magnetic Disc (Magnetic Disk)

On this magnetic disk, data is stored on the hard disk. The magnetic disc is in the form of a thin plate with a shape like a CD-R. In the hard disk there are several magnetic disks.

The first hard disk was made, consisting of 50 magnetic discs measuring 0.6 meters and rotating at a speed of 1,200 rpm. Currently the hard disk rotation speed has reached 10,000rpm with data transfer reaching 3.0 Gbps.

3. Read-write Head

Read-write Head is a data collector from a magnetic disk. This head floats a thin distance from the magnetic disk. In the past, the head was in direct contact with the magnetic disc, resulting in wear and tear on the surface due to friction. Now between the head and magnetic disks have been spaced so that the life of the hard disk is longer.

Read-write heads are made of materials that are constantly evolving, starting from Ferrite heads, MIG (Metal-In-Gap) heads, TF (Thin Film) Heads, (Anisotropic) Magnetoresistive (MR/AMR) Heads, GMR (Giant Magnetoresistive) Heads and now used are CMR (Colossal Magnetoresistive) Heads.

4. Enclosure

The enclosure is the outer covering of the hard disk. Enclosure serves to protect all parts of the hard disk from being exposed to dust, moisture and other things that can cause data damage.

In the enclosure there is a breath filter that makes the hard disk not airtight, this aims to dissipate the heat that is inside the hard disk due to the spindle rotation process and the Read-write head reading.

5. Interfacing Module

The interfacing module is a set of electronic circuits that control the inner workings of the hard disk, process data from the head and produce data that is ready to be read by the next process. The interfacing module that used to be widely used was the IDE (Integrated Drive Electronics) system with the ATA system which had a 40 pin connection.

How Magnetic Disks Work

1. Data Representation and Addressing

Data on disk is also blocked like data on magnetic tape. Calling a block is the amount of data accessed on a storage device. Data from the disk is moved to a buffer on the computer’s main storage for access by a program. The ability to access directly on disk means that records are not always accessed sequentially. There are 2 basic techniques for addressing data stored on disk, namely:

Cylinder Method;

Addressing based on cylinder number, surface number and record number. All tracks of the disc pack form a cylinder. So if a disk pack has 200 tracks per surface, it has 200 cylinders.

The surface number portion of the record addressing represents the cylindrical surface of the record being stored. If there are 11 discs, then the surface number is from 0 – 19 (1 – 20). Addressing of the record number indicates where the record is located on the track indicated by the cylinder number and the surface number.

Sector Method

Each track of the pack is divided into sectors. Each sector is a storage area for a fixed number of characters. Addressing records is based on sector number, track number and surface number. The sector number assigned by the disk controller indicates which track to access and the address of the record located on which track.

Each track on each platter has the same storage capacity, even though the track diameter is different. Capacity uniformity is achieved by adjusting the appropriate density of the data representation for each track size. Another advantage of the uniform capacity approach is that files can be placed on disk without changing the location of the sector number (track or cylinder) in the file.

2. Movable-Head Disk Access

Movable-head disk drives have a read/write head for each record storage surface. The mechanical system used by positioning the access-arm is such that the read/write head of the addressing surface points to the track. All access-arms on the device are moved simultaneously but only the active head will point to the surface.

3. How to Access Records Stored on Disk Pack

The disk controller changes the code assigned to the address of the record and specifies which track on the device the record is located on. The access arm is moved, so that the read/write head position is on the right cylinder.

This read/write head points to the active track. Then the disk will spin until it points to a record at the location of the read / write head. Then the data will be read and transferred through the channel requested by the program in the computer.

ACCESS TIME = SEEK TIME (change arm to cylinder)

+ HEAD ACTIVATION TIME (track selection)

+ ROTATIONAL DELAY (record selection)

+ TRANSFER TIME

Seek Time

Is the time it takes to move the read / write head on the disk to the correct cylinder position.

Head Activation Time

Is the time it takes to move the read/write head on the disk to the correct track position.

Rotational Delay (Lateney)

Is the time it takes to rotate the disc to the right record position.

Transfer Time

Is a time that shows the speed of rotation and the amount of data transferred.

1. Fixed – Head Disk Access

A disk that has a read/write head for each track on each storage surface, whose access mechanism cannot be moved from cylinder to cylinder.

ACCESS TIME = HEAD-ACTIVATION TIME + ROTATIONAL DELAY + TRANSFER TIME

The large number of read / write heads causes the price of fixed-head disk drives to be more expensive than movable-head disk drives. A disk that uses a fixed-head disk drive has a smaller capacity and density than a disk that uses a movable-head disk drive.

5. File Organization and Access Methods on Magnetic Disk

To form a file on a magnetic disk can be done sequentially, index-sequential or direct. Meanwhile, to retrieve data from a file stored on disk, it can be done directly using the direct access method or sequential access method (sequentially).

Advantages and Disadvantages of Magnetic Disk

Superiority :

  1. Data storage on this media is nonvolatile, meaning that the stored data will not be lost when the computer is turned off.
  2. The data on this media can be read, erased and rewritten.
  3. This medium is easy to use.

Weakness :

  1. The main enemies of magnetic media such as floppy disks and hard disks are mildew and rust. Because of this fungus and rust, the durability or life of this medium is short.
  2. This magnetic media is a striped shape (track, sector), so the speed and storage capacity is low when compared to optical media.

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