Unit 5: I/O Management and Disk Scheduling

I/O Management and Disk Scheduling

I/O Management is the function of the Operating System that controls and coordinates all input and output operations between the computer and external devices.

I/O Devices

I/O devices are hardware components that allow the system to communicate with the external world.

Classification of I/O Devices

Type	Examples
Input Devices	Keyboard, Mouse, Scanner
Output Devices	Monitor, Printer
Storage Devices	Hard Disk, SSD
Communication Devices	Modem, Network Card

Device Characteristics

Characteristic	Description
Data Transfer Rate	Speed of data transfer
Unit of Transfer	Byte or block
Access Method	Sequential or random
I/O Control	Polling, Interrupts, DMA

I/O Subsystems

I/O Subsystem is the OS layer that manages I/O devices efficiently and uniformly.

Functions of I/O Subsystem

Function	Explanation
Device abstraction	Hides hardware details
Scheduling	Orders I/O requests
Buffering	Smooth data transfer
Error handling	Detects and handles errors
Device allocation	Assigns devices

Components of I/O Subsystem

Component	Role
Device Drivers	Interface between OS and device
Interrupt Handlers	Respond to device signals
I/O Scheduler	Manages request queue
Buffers & Caches	Temporary storage

I/O Buffering

Buffering uses main memory as temporary storage during I/O operations to handle speed mismatches.

Why Buffering is Needed

• CPU faster than I/O devices
• Avoid data loss
• Improve system throughput

Types of Buffering

Type	Description
Single Buffering	One buffer used
Double Buffering	Two buffers alternate
Circular Buffering	Multiple buffers in cycle

Advantages

• Overlapping of I/O and CPU execution
• Reduced waiting time
• Smooth data transfer

Disk Storage

Disk Structure Components

Component	Description
Platter	Circular disk surface
Track	Concentric circles
Sector	Smallest storage unit
Cylinder	Same track on all platters
Read/Write Head	Accesses data

Disk Access Time

Disk Access Time = Seek Time + Rotational Latency + Transfer Time

Term	Meaning
Seek Time	Head movement time
Rotational Latency	Wait for sector
Transfer Time	Actual data transfer

Disk Scheduling

Disk scheduling decides the order of disk I/O requests to minimize seek time.

Disk Scheduling Algorithms

1. FCFS (First Come First Serve)

Feature	Explanation
Order	Arrival order
Advantage	Simple
Disadvantage	High seek time

2. SSTF (Shortest Seek Time First)

Feature	Explanation
Selection	Nearest request
Advantage	Reduced seek time
Disadvantage	Starvation

3. SCAN (Elevator Algorithm)

Feature	Explanation
Movement	Back and forth
Advantage	Fair
Disadvantage	End delay

4. C-SCAN

Feature	Explanation
Direction	One direction only
Advantage	Uniform wait
Disadvantage	Extra movement

5. LOOK / C-LOOK

Feature	Explanation
Improvement	Stops at last request
Advantage	Efficient

Comparison Table

Algorithm	Starvation	Performance
FCFS	No	Poor
SSTF	Yes	Good
SCAN	No	Better
C-SCAN	No	Best

RAID (Redundant Array of Independent Disks)

RAID combines multiple disks to improve performance, reliability, or both.

RAID Levels

Level	Technique	Benefit
RAID 0	Striping	High speed
RAID 1	Mirroring	High reliability
RAID 2	Hamming code	Error correction
RAID 3	Byte-level striping + parity
RAID 4	Block-level striping + parity
RAID 5	Distributed parity	Balanced
RAID 6	Dual parity	Fault tolerance

Popular RAID Comparison

RAID	Min Disks	Fault Tolerance
RAID 0	2	None
RAID 1	2	1 disk
RAID 5	3	1 disk
RAID 6	4	2 disks

Advantages of RAID

• Improved performance
• Data redundancy
• Fault tolerance
• High availability

Disadvantages

• Costly
• Complex setup
• Storage overhead

Quick Revision Table

Topic	Key Focus
I/O Devices	Input–Output hardware
I/O Subsystem	OS management layer
Buffering	Speed mismatch handling
Disk Scheduling	Seek time reduction
RAID	Performance & reliability

FILE SYSTEM

A File System is a method used by the Operating System to store, organize, manage, and protect data on storage devices such as hard disks and SSDs.

File Concept

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

Files provide a logical view of data storage to users and programs.

File Attributes

Attribute	Description
Name	Human-readable identifier
Identifier	Unique file number
Type	Text, binary, executable
Location	Disk address
Size	File length
Protection	Access permissions
Time & Date	Creation, modification
Owner	User information

File Operations

Operation	Description
Create	Create new file
Open	Access file
Read	Read data
Write	Modify data
Close	Release file
Delete	Remove file

File Organization and Access Mechanism

File organization defines how files are physically stored on disk.

Types of File Organization

Type	Description	Use Case
Sequential	Records stored in order	Logs, tapes
Indexed	Index table for fast access	Databases
Hashed	Hash function maps keys	Large files

File Access Mechanisms

Access Method	Explanation
Sequential Access	Data accessed in order
Direct (Random) Access	Access any block directly
Indexed Access	Uses index for quick access

Comparison Table

Feature	Sequential	Direct	Indexed
Speed	Slow	Fast	Very fast
Flexibility	Low	High	High

File Directories

A directory is a special file that stores information about other files.

Directory Structures

Type	Description
Single-Level	One directory for all files
Two-Level	Separate directory per user
Tree-Structured	Hierarchical structure
Acyclic Graph	Allows shared files
General Graph	Allows cycles

Directory Operations

Operation	Description
Search	Find a file
Create	Add file
Delete	Remove file
Rename	Change name
Traverse	Move through structure

File Sharing

File sharing allows multiple users or processes to access the same file.

Types of File Sharing

Type	Explanation
Local Sharing	Within same system
Remote Sharing	Over network

File Locking

Lock Type	Purpose
Shared Lock	Multiple readers
Exclusive Lock	Single writer

Consistency Problems

• Simultaneous write conflicts
• Data corruption

Solution: File locking and access control.

File System Implementation Issues

1. File Allocation Methods

Method	Description	Issue
Contiguous	Continuous blocks	External fragmentation
Linked	Linked blocks	Slow access
Indexed	Index block	Extra overhead

2. Free Space Management

Method	Description
Bit Map	Bit per block
Linked List	Linked free blocks
Grouping	Groups of free blocks
Counting	Extents

3. Directory Implementation

• Linear list
• Hash table

4. Disk I/O Performance

• Block size
• Cache usage
• Read-ahead

File System Protection and Security

Need for Protection

• Prevent unauthorized access
• Ensure data integrity
• Maintain privacy

Protection Mechanisms

1. Access Control

Access Type	Meaning
Read	View content
Write	Modify content
Execute	Run file

2. Access Control List (ACL)

User	Permissions
Owner	RWX
Group	RW
Others	R

3. Capability List

• Users possess capabilities
• More secure than ACL

4. Encryption

• Protects data from theft
• Used in modern file systems

5. Authentication

• Passwords
• Biometrics
• Digital certificates

Comparison: ACL vs Capability List

Feature	ACL	Capability
Stored With	File	User
Security	Medium	High
Flexibility	High	Medium

Exam-Friendly Summary

Topic	Key Point
File Concept	Named data collection
File Access	Sequential, Direct, Indexed
Directory	File organization
File Sharing	Multi-user access
Implementation	Allocation & free space
Protection	Access control & security