Disk Scheduling, File Management & Distributed Systems

1. Disk Scheduling

Disk scheduling is the technique used by the operating system to decide the order in which disk read and write requests are executed. Since multiple processes may request access to the hard disk at the same time, an efficient scheduling algorithm helps reduce waiting time, improve response time and increase overall system performance.

Objectives of Disk Scheduling

• Reduce seek time.
• Reduce rotational delay.
• Increase disk throughput.
• Improve response time.
• Provide fairness to all requests.

2. FCFS (First Come First Serve)

FCFS is the simplest disk scheduling algorithm. Requests are served in the same order in which they arrive.

Working

Advantages

• Simple to implement.
• No starvation.
• Fair scheduling.

Disadvantages

• High seek time.
• Poor average response time.
• Low throughput.

3. SSTF (Shortest Seek Time First)

SSTF selects the disk request that is closest to the current head position. It reduces total head movement compared to FCFS.

Advantages

• Lower seek time.
• Better performance than FCFS.
• Higher throughput.

Disadvantages

• Starvation is possible.
• Long waiting requests may be ignored.

4. SCAN Algorithm

The SCAN algorithm moves the disk head in one direction, servicing all requests until it reaches the end of the disk. It then reverses direction and services requests on the return path.

Advantages

• Better throughput.
• Less starvation.
• Efficient for heavy workloads.

Disadvantages

• Some requests wait longer.
• More head movement than LOOK.

5. C-SCAN (Circular SCAN)

C-SCAN moves the head only in one direction. After reaching the last track, the head returns directly to the first track without servicing requests during the return journey.

Advantages

• Uniform waiting time.
• Fair scheduling.
• Suitable for multi-user systems.

Disadvantages

• Additional head movement.
• Return path is unused.

6. LOOK Algorithm

LOOK is an improved version of SCAN. Instead of moving to the last cylinder, the disk head reverses as soon as there are no more requests in the current direction.

Advantages

• Less seek time than SCAN.
• Improved efficiency.
• Better response time.

7. C-LOOK Algorithm

C-LOOK is the circular version of LOOK. The disk head services requests in one direction only and jumps directly to the first pending request instead of moving to the beginning of the disk.

Advantages

• Uniform waiting time.
• Reduced unnecessary movement.
• Better performance than C-SCAN.

8. Comparison of Disk Scheduling Algorithms

9. File Concepts

A File is a collection of related data stored on secondary storage such as a hard disk or SSD. Files are used to permanently store programs, documents, images, videos, databases and other information. The operating system manages files so that users can easily create, access, modify and delete data whenever required.

Characteristics of a File

• Every file has a unique name.
• Files store related information.
• Data is stored permanently.
• Each file occupies disk space.
• Files can be shared among users.

Common File Attributes

Basic File Operations

10. File Manager

A File Manager is a component of the operating system responsible for managing files and directories. It acts as an interface between users and the file system by providing facilities for creating, deleting, copying, moving and organizing files.

Functions of File Manager

• Create new files and folders.
• Delete unwanted files.
• Copy and move files.
• Rename files.
• Maintain directory structure.
• Allocate disk space.
• Provide file protection.
• Support file sharing.

Advantages of File Manager

• Easy file organization.
• Fast file searching.
• Better disk utilization.
• Supports security and permissions.
• Simplifies file operations.

Disadvantages

• Consumes system resources.
• Large file systems become complex.
• Improper permissions may create security risks.

11. File Organization

File Organization refers to the method used to arrange and store records inside a file. Proper organization improves data retrieval speed, storage efficiency and system performance.

Types of File Organization

Sequential File Organization

Records are stored one after another in a fixed order. Searching is performed sequentially from the beginning.

Direct File Organization

Records are accessed directly using their address or key value without searching the entire file.

Indexed File Organization

An index table stores pointers to records. Searching becomes much faster because the operating system first searches the index.

Advantages of Proper File Organization

• Faster searching.
• Efficient storage utilization.
• Easy maintenance.
• Improved system performance.
• Supports large databases.

Disadvantages

• Indexed files require extra storage.
• Updating indexes increases overhead.
• Complex implementation for large systems.

12. File Access Methods

A file access method defines how data is read from or written to a file. The operating system provides different access methods depending on the application requirements. Choosing the correct access method improves file searching speed and overall system performance.

Types of File Access Methods

1. Sequential Access

Sequential Access reads records from the beginning of the file until the required record is found. This method is simple but slower for large files.

Advantages

• Simple implementation.
• Suitable for sequential processing.
• Low storage overhead.

Disadvantages

• Slow searching.
• Not suitable for large databases.

2. Direct (Random) Access

Direct Access allows the operating system to jump directly to any record without reading previous records. It is widely used in modern database systems.

Advantages

• Very fast searching.
• Quick data modification.
• Efficient for databases.

Disadvantages

• More complex implementation.
• Requires address calculation.

3. Indexed Access

Indexed Access maintains a separate index table that stores pointers to records. The operating system first searches the index and then accesses the required record directly.

Advantages

• Fast searching.
• Efficient for large files.
• Supports both sequential and direct access.

Disadvantages

• Requires additional storage.
• Index maintenance increases overhead.

13. File Allocation Methods

File Allocation Method determines how disk blocks are assigned to a file. Proper allocation improves storage efficiency and file access speed.

Objectives

• Efficient disk utilization.
• Fast file access.
• Reduce fragmentation.
• Easy file expansion.

Types of Allocation Methods

1. Contiguous Allocation

In Contiguous Allocation, all blocks of a file are stored in consecutive memory locations.

Advantages

• Very fast access.
• Simple implementation.
• Supports sequential and direct access.

Disadvantages

• External fragmentation.
• Difficult file expansion.

2. Linked Allocation

In Linked Allocation, each disk block contains a pointer to the next block. Blocks may be scattered anywhere on the disk.

Advantages

• No external fragmentation.
• Easy file growth.

Disadvantages

• Slow random access.
• Pointer storage overhead.

3. Indexed Allocation

Indexed Allocation stores addresses of all file blocks in a separate index block. The operating system uses this index to locate data quickly.

Advantages

• Fast random access.
• No external fragmentation.
• Easy file expansion.

Disadvantages

• Extra index block required.
• Consumes additional storage.

Comparison of Allocation Methods

14. Free Space Management

Whenever a new file is created, the operating system must locate empty disk blocks where the file can be stored. The technique used to identify and manage these unused disk blocks is called Free Space Management. It helps improve storage efficiency and prevents wastage of disk space.

Objectives

• Efficient utilization of disk space.
• Fast allocation of free blocks.
• Reduce fragmentation.
• Improve file storage performance.

Methods of Free Space Management

Bit Map Method

Advantages

• Easy implementation.
• Fast searching.
• Suitable for modern operating systems.

Disadvantages

• Consumes additional memory.
• Large disks require large bitmaps.

15. Directory Systems

A Directory is a special file that stores information about other files and folders. It helps users organize, locate and manage files efficiently.

Functions of Directory

• Stores file names.
• Maintains file locations.
• Supports searching.
• Provides file organization.
• Supports file sharing.

Types of Directory Structure

Advantages

• Easy file organization.
• Fast searching.
• Supports hierarchical storage.
• Easy backup and maintenance.

Disadvantages

• Complex implementation for large systems.
• Permission management becomes difficult.

16. File Protection

File Protection is the mechanism used by the operating system to prevent unauthorized access, modification or deletion of files. It ensures that only authorized users can perform specific operations on files.

Need of File Protection

• Prevent unauthorized access.
• Protect confidential information.
• Maintain data integrity.
• Provide secure file sharing.
• Avoid accidental deletion.

Types of File Permissions

Protection Mechanisms

• Password Protection.
• Access Control List (ACL).
• File Ownership.
• User Groups.
• Permission Bits.
• Encryption.

Advantages

• Improves system security.
• Protects sensitive information.
• Supports secure file sharing.
• Prevents unauthorized modification.

Disadvantages

• Improper permissions may block valid users.
• Complex management in large systems.

17. File Organization & Access Mechanism

File Organization defines how records are physically stored inside a file, whereas the File Access Mechanism describes how these records are retrieved and modified by the operating system. A proper combination of organization and access method improves storage efficiency and system performance.

Working Process

Advantages

• Fast data retrieval.
• Better storage utilization.
• Easy modification of records.
• Improved system performance.

18. File Sharing Implementation Issues

Modern operating systems allow multiple users to access the same file simultaneously. While file sharing improves collaboration, it also creates synchronization, consistency and security challenges.

Major File Sharing Issues

• Concurrent access by multiple users.
• Data inconsistency.
• Race condition.
• Permission conflicts.
• Deadlock while accessing shared files.
• Security risks.

Advantages of File Sharing

• Easy collaboration.
• Improves productivity.
• Reduces duplicate files.
• Supports network environments.

Disadvantages

• Security threats.
• Synchronization issues.
• Access conflicts.

19. File Management in Linux

Linux follows a hierarchical file system where every file and directory starts from the Root Directory (/). Everything in Linux is treated as a file, including devices and hardware resources.

Important Linux Directories

Common Linux File Commands

20. Introduction to Distributed Systems

A Distributed System is a collection of multiple independent computers connected through a network that work together as a single system. Users experience the system as one computer even though processing is performed by multiple machines.

Characteristics

• Resource Sharing.
• Transparency.
• Scalability.
• Fault Tolerance.
• High Availability.

Advantages

• Fast processing.
• High reliability.
• Easy resource sharing.
• Better scalability.
• Cost effective.

Disadvantages

• Complex design.
• Network dependency.
• Security challenges.
• Difficult synchronization.

📊 Unit 5 PYQ Analysis

Unit 5 is very important because disk scheduling numericals and file management theory questions are frequently asked in RGPV exams.

⭐ Important Exam Questions

1. Explain disk scheduling and its objectives.
2. Explain FCFS, SSTF, SCAN and C-SCAN disk scheduling.
3. Compare LOOK and C-LOOK algorithms.
4. Explain file concepts and file attributes.
5. Explain functions of File Manager.
6. Explain Sequential, Direct and Indexed access methods.
7. Explain Contiguous, Linked and Indexed allocation methods.
8. Explain Free Space Management methods.
9. Explain Directory Systems with types.
10. Explain File Protection and file permissions.
11. Explain File Management in Linux.
12. Explain Distributed Systems with advantages and disadvantages.

🎤 Viva Questions

• What is disk scheduling?
• What is seek time?
• What is file?
• What is file manager?
• What is sequential access?
• What is indexed allocation?
• What is free space management?
• What is directory system?
• What is file protection?
• What is distributed system?

⚡ Quick Revision Sheet

❓ Frequently Asked Questions

Is Unit 5 important for RGPV exams?

Yes. Unit 5 is important because disk scheduling numericals and file management theory questions are commonly asked.

Which topic is most important in Unit 5?

Disk Scheduling, File Allocation Methods, Directory Systems and File Protection are the most important topics.

Can numerical questions come from Unit 5?

Yes. Disk scheduling algorithms like FCFS, SSTF, SCAN, C-SCAN, LOOK and C-LOOK can come as numerical questions.

📚 Related Operating System Units