# Magnetic Disk in Computer Architecture

## Magnetic Disk in Computer Architecture-

In computer architecture,

• Magnetic disk is a storage device that is used to write, rewrite and access data.
• It uses a magnetization process.

## Architecture-

• The entire disk is divided into platters.
• Each platter consists of concentric circles called as tracks.
• These tracks are further divided into sectors which are the smallest divisions in the disk.

• A cylinder is formed by combining the tracks at a given radius of a disk pack.

• There exists a mechanical arm called as Read / Write head.
• It is used to read from and write to the disk.
• Head has to reach at a particular track and then wait for the rotation of the platter.
• The rotation causes the required sector of the track to come under the head.
• Each platter has 2 surfaces- top and bottom and both the surfaces are used to store the data.
• Each surface has its own read / write head.

## Disk Performance Parameters-

The time taken by the disk to complete an I/O request is called as disk service time or disk access time.

Components that contribute to the service time are-

1. Seek time
2. Rotational latency
3. Data transfer rate
5. Queuing delay

## 1. Seek Time-

• The time taken by the read / write head to reach the desired track is called as seek time.
• It is the component which contributes the largest percentage of the disk service time.
• The lower the seek time, the faster the I/O operation.

## Specifications

Seek time specifications include-

1. Full stroke
2. Average
3. Track to Track

### 1. Full Stroke-

• It is the time taken by the read / write head to move across the entire width of the disk from the innermost track to the outermost track

### 2. Average-

• It is the average time taken by the read / write head to move from one random track to another.

 Average seek time = 1 / 3 x Full stroke

### 3. Track to Track-

• It is the time taken by the read-write head to move between the adjacent tracks.

## 2. Rotational Latency-

• The time taken by the desired sector to come under the read / write head is called as rotational latency.
• It depends on the rotation speed of the spindle.

 Average rotational latency = 1 / 2 x Time taken for full rotation

## 3. Data Transfer Rate-

• The amount of data that passes under the read / write head in a given amount of time is called as data transfer rate.
• The time taken to transfer the data is called as transfer time.

It depends on the following factors-

1. Number of bytes to be transferred
2. Rotation speed of the disk
3. Density of the track
4. Speed of the electronics that connects the disk to the computer

## 4. Controller Overhead-

• The overhead imposed by the disk controller is called as controller overhead.
• Disk controller is a device that manages the disk.

## 5. Queuing Delay-

• The time spent waiting for the disk to become free is called as queuing delay.

## NOTE-

 All the tracks of a disk have the same storage capacity.

## Storage Density-

• All the tracks of a disk have the same storage capacity.
• This is because each track has different storage density.
• Storage density decreases as we from one track to another track away from the center.

Thus,

• Innermost track has maximum storage density.
• Outermost track has minimum storage density.

## Important Formulas-

### 1. Disk Access Time-

Disk access time is calculated as-

 Disk access time = Seek time + Rotational delay + Transfer time + Controller overhead + Queuing delay

### 2. Average Disk Access Time-

Average disk access time is calculated as-

 Average disk access time = Average seek time + Average rotational delay + Transfer time + Controller overhead + Queuing delay

### 3. Average Seek Time-

Average seek time is calculated as-

 Average seek time = 1 / 3 x Time taken for one full stroke

Alternatively,

If time taken by the head to move from one track to adjacent track = t units and there are total k tracks, then-

Average seek time

= { Time taken to move from track 1 to track 1 + Time taken to move from track 1 to last track } / 2

= { 0 + (k-1)t } / 2

= (k-1)t / 2

### 4. Average Rotational Latency-

Average rotational latency is calculated as-

 Average rotational latency = 1 / 2 x Time taken for one full rotation

Average rotational latency may also be referred as-

• Average rotational delay
• Average latency
• Average delay

### 5. Capacity Of Disk Pack-

Capacity of a disk pack is calculated as-

 Capacity of a disk pack = Total number of surfaces x Number of tracks per surface x Number of sectors per track x Storage capacity of one sector

### 6. Formatting Overhead-

Formatting overhead is calculated as-

 Formatting overhead = Number of sectors x Overhead per sector

### 7. Formatted Disk Space-

Formatted disk space also called as usable disk space is the disk space excluding formatting overhead.

It is calculated as-

 Formatted disk space = Total disk space or capacity – Formatting overhead

### 8. Recording Density Or Storage Density-

Recording density or Storage density is calculated as-

 Storage density of a track = Capacity of the track / Circumference of the track

From here, we can infer-

Storage density of a track ∝ 1 / Circumference of the track

### 9. Track Capacity-

Capacity of a track is calculated as-

 Capacity of a track = Recording density of the track x Circumference of the track

### 10. Data Transfer Rate-

Data transfer rate is calculated as-

 Data transfer rate = Number of heads x Bytes that can be read in one full rotation x Number of rotations in one second

OR

 Data transfer rate = Number of heads x Capacity of one track x Number of rotations in one second

### 11. Tracks Per Surface-

Total number of tracks per surface is calculated as-

 Total number of tracks per surface = (Outer radius – Inner radius) / Inter track gap

## Points to Remember-

• The entire disk space is not usable for storage because some space is wasted in formatting.
• When rotational latency is not given, use average rotational latency for solving numerical problems.
• When seek time is not given, use average seek time for solving numerical problems.
• It is wrong to say that as we move from one track to another away from the center, the capacity increases.
• All the tracks have same storage capacity.

To gain better understanding about magnetic disk-

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Summary
Article Name
Magnetic Disk in Computer Architecture
Description
Magnetic Disk is a storage device. Disk performance parameters- Seek time, Rotational Latency, Data Transfer Rate. Disk Formulas- Seek time Formula, Rotational Latency Formula etc.
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Publisher Name
Gate Vidyalay
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