📘 PAPER 5 – OPERATING SYSTEM (UNIT 3 – FILE SYSTEM & I/O MANAGEMENT) university of allahabad

🔴 UNIT 3 – FILE SYSTEM & I/O MANAGEMENT

1️⃣ File System – Introduction

✅ What is a File?

A file is a collection of related information stored on secondary storage.

✅ File System

The file system is a method used by the OS to:

Store files
Retrieve files
Organize data efficiently

2️⃣ File Attributes

Each file has the following attributes:

Attribute	Description
Name	File name
Type	File extension
Size	File size
Location	Disk location
Protection	Access rights
Time & Date	Creation/modification time

3️⃣ File Operations

Common file operations:

✔ Create
✔ Open
✔ Read
✔ Write
✔ Close
✔ Delete

4️⃣ File Access Methods

🔹 1. Sequential Access

Data accessed sequentially
Example: Text file

✔ Simple
❌ Slow for random access

🔹 2. Direct Access

Access any record directly
Example: Database files

✔ Fast
✔ Efficient

🔹 3. Indexed Access

Uses index to locate data
Combination of sequential & direct

5️⃣ Directory Structure

Types of Directory Structures:

🔹 1. Single Level Directory

All files in one directory
❌ Name conflict

🔹 2. Two-Level Directory

Separate directories for each user

🔹 3. Tree Structure Directory

Hierarchical structure
Most commonly used

🔹 4. Acyclic Graph Directory

Shared files
No cycles

6️⃣ File Allocation Methods

🔹 1. Contiguous Allocation

Files stored in continuous memory

✔ Fast access
❌ External fragmentation

🔹 2. Linked Allocation

File blocks linked using pointers

✔ No fragmentation
❌ Slow access

🔹 3. Indexed Allocation

Index block stores pointers

✔ Fast access
✔ No fragmentation

7️⃣ Free Space Management

Methods used to manage free disk space:

✔ Bit vector
✔ Linked list
✔ Grouping
✔ Counting

8️⃣ Disk Scheduling

Purpose:

Reduce disk access time

🔹 Disk Scheduling Algorithms

1️⃣ FCFS (First Come First Serve)

✔ Simple
❌ High seek time

2️⃣ SSTF (Shortest Seek Time First)

✔ Better than FCFS
❌ Starvation possible

3️⃣ SCAN (Elevator Algorithm)

✔ Moves in one direction
✔ Reduces starvation

4️⃣ C-SCAN

✔ Uniform wait time
✔ Used in modern OS

9️⃣ I/O Management

I/O System Responsibilities:

✔ Device communication
✔ Error handling
✔ Buffering
✔ Spooling

🔹 I/O Techniques

1. Programmed I/O

CPU controls I/O
❌ Slow

2. Interrupt Driven I/O

CPU works during I/O
✔ Better performance

3. Direct Memory Access (DMA)

Data transferred directly to memory
CPU not involved

✔ Fastest
✔ Efficient

🔟 Protection & Security

🔹 Protection

Ensures controlled access to resources.

🔹 Security

Prevents unauthorized access.

Protection Mechanisms:

✔ Passwords
✔ Access control lists
✔ Encryption

📌 EXAM IMPORTANT QUESTIONS (UNIT 3)

✔ Explain file allocation methods
✔ Disk scheduling algorithms
✔ Explain DMA
✔ File access methods
✔ Directory structure
✔ Difference between FCFS and SSTF