1. Start From Physical Reality
Unlike memory, a hard disk drive (HDD) is a mechanical storage device. Data is not accessed electronically at fixed speeds; instead, physical movement is involved.
Inside a traditional hard disk, data is stored magnetically on rotating disks called platters, while a mechanical arm moves a read/write head to access specific locations.
Because physical movement is required, disk access is significantly slower than RAM access.
Why Understanding Disk Structure Matters
The operating system must understand how disks are organized in order to:
Store files efficiently
Retrieve data quickly
Schedule disk requests intelligently
Minimize access delays
Key Problem
The disk contains millions of storage locations.
How can the operating system locate and access the required data efficiently?
Understanding disk structure is the first step.
Key Insight
Unlike RAM, disk performance is heavily influenced by physical movement.
2. Basic Components of a Hard Disk
A hard disk is composed of several physical components working together.
2.1 Platters
Platters are circular magnetic disks on which data is stored.
Characteristics:
Made of aluminum, glass, or ceramic
Coated with magnetic material
Rotate continuously at high speed
Common rotational speeds:
| Type | RPM |
|---|---|
| Standard HDD | 5400 RPM |
| Performance HDD | 7200 RPM |
| Enterprise HDD | 10000–15000 RPM |
Structure
----------------
| |
| Platter |
| |
----------------
Modern disks contain multiple platters stacked vertically.
Why Multiple Platters?
More platters provide:
Higher storage capacity
Better utilization of physical space
Key Insight
Actual data is stored magnetically on platter surfaces.
2.2 Read/Write Heads
Each platter surface has a dedicated read/write head.
Purpose:
Read stored magnetic data
Write new data
Important Observation
Heads do not touch the platter.
Instead:
Head
↓
Tiny Air Gap
↓
Platter Surface
This prevents physical damage during rotation.
Head Movement
Heads move together across platters using an actuator arm.
Head
↓
---------------------
/ \
| Platter |
\_____________________/
Key Insight
Data access requires moving the head to the correct location.
2.3 Tracks
Data on a platter is organized into concentric circles called tracks.
Outer Track
____________
/ \
| _________ |
| | | |
| | Track | |
| |_________| |
\____________/
Each track stores a sequence of bits.
Characteristics:
Circular structure
Located at different radii
Numbered from outer to inner tracks
Key Insight
A track is the basic circular path where data is stored.
2.4 Sectors
Each track is divided into smaller pieces called sectors.
Track
___________________
| \ | / | \ | / |
| \ | / | \ | / |
|--- Sectors -------|
| / | \ | / | \ |
| / | \ | / | \ |
-------------------
A sector is the smallest physical storage unit on disk.
Typical sector sizes:
| Sector Type | Size |
|---|---|
| Traditional | 512 Bytes |
| Advanced Format | 4 KB |
Why Sectors Exist
Dividing tracks into sectors allows:
Efficient addressing
Easier error detection
Faster access
Key Insight
Data is ultimately stored inside sectors.
2.5 Cylinders
This is one of the most important concepts in disk organization.
A cylinder consists of all tracks located at the same radius across all platters.
Example:
Platter 1 → Track 20
Platter 2 → Track 20
Platter 3 → Track 20
Together they form:
Cylinder 20
Visualization
Side View
Track 5
|
|
Track 5
|
|
Track 5
= Cylinder 5
Why Cylinders Matter
Moving between tracks in the same cylinder requires:
No Head Movement
Only head switching.
This is much faster.
Key Insight
A cylinder is a vertical collection of tracks at the same radius.
3. Complete Disk Organization
The hierarchy of disk storage is:
Disk
↓
Platters
↓
Tracks
↓
Sectors
↓
Data
Complete View
Disk
└── Platters
└── Tracks
└── Sectors
└── Data
Key Insight
Every piece of stored data ultimately resides inside a sector on a track of a platter.
4. How Data is Accessed
Suppose the operating system needs data stored in a particular sector.
The disk cannot instantly provide it.
Several steps are required.
Step 1: Seek Operation
The read/write head must move to the correct track.
Current Track = 20
Required Track = 150
Head movement occurs.
This time is called:
Seek Time
Definition
Time required to move the disk head to the target track.
Example
Track 20 → Track 150
Key Insight
Seek time is usually the largest component of disk access time.
Step 2: Rotational Latency
Even after reaching the correct track:
The required sector may not be under the head.
The platter must rotate until it arrives.
Example
Head
↓
Sector Not Yet Arrived
Disk continues rotating.
Eventually:
Head
↓
Required Sector Arrives
Definition
Time spent waiting for the desired sector to rotate beneath the head.
Key Insight
The disk may already be on the correct track but still be waiting for the correct sector.
Step 3: Data Transfer
Once the sector reaches the head:
Data transfer begins.
Transfer Time
Time required to read or write the requested data.
Example
Sector → Buffer
Key Insight
Transfer time is usually smaller than seek time and rotational latency.
5. Disk Access Time
The total time required to retrieve data is called disk access time.
Conceptual Formula
Disk Access Time
= Seek Time
+ Rotational Latency
+ Transfer Time
Example
| Component | Time |
|---|---|
| Seek Time | 8 ms |
| Rotational Latency | 4 ms |
| Transfer Time | 1 ms |
Total:
Access Time
= 8 + 4 + 1
= 13 ms
Important Observation
Most delay comes from mechanical movement.
Key Insight
Reducing head movement is the primary goal of disk optimization.
6. Disk Addressing
The operating system must identify where data resides.
There are two major approaches.
6.1 Physical Addressing
Historically, disks used:
Cylinder
Head
Sector
Address format:
(Cylinder, Head, Sector)
Example:
(120, 3, 25)
Meaning:
Cylinder 120
Head 3
Sector 25
Limitation
Too dependent on physical disk geometry.
6.2 Logical Block Addressing (LBA)
Modern systems use LBA.
Instead of exposing cylinders and sectors:
The disk appears as a simple sequence of numbered blocks.
Example
Block 0
Block 1
Block 2
Block 3
and so on.
OS View
0 → 1 → 2 → 3 → 4 → 5 ...
Disk Controller View
Internally converts:
LBA
↓
Physical Location
Advantages
Simpler
Hardware independent
Easier management
Key Insight
Modern operating systems see disks as linear arrays of logical blocks.
7. Why Disk Scheduling is Needed
A disk often receives multiple requests simultaneously.
Example queue:
98
183
37
122
14
124
65
67
If processed randomly:
98 → 14 → 183 → 37 → 124
The head moves excessively.
Result
Large seek times
Poor performance
Increased waiting
Goal of Disk Scheduling
Minimize:
Head Movement
which reduces:
Seek Time
and improves:
Disk Throughput
Key Insight
Disk scheduling exists because seek time dominates disk access cost.
8. Summary
| Component | Purpose |
|---|---|
| Platter | Magnetic storage surface |
| Track | Circular storage path |
| Sector | Smallest physical storage unit |
| Cylinder | Same-radius tracks across platters |
| Head | Reads/Writes data |
| Seek Time | Move head to track |
| Rotational Latency | Wait for sector |
| Transfer Time | Read/write data |
| LBA | Logical block numbering |
| Disk Scheduling | Reduce head movement |
Final Insight
A hard disk is fundamentally a mechanical device composed of platters, tracks, sectors, cylinders, and read/write heads. Accessing data requires physical head movement and platter rotation, making disk operations much slower than memory operations. Since seek time dominates access cost, operating systems use logical block addressing and disk scheduling algorithms to minimize head movement and maximize storage performance.