Disk Scheduling Algorithms

(Click to go File Systems and Storage Management)

Used to optimize read/write operations:

Disk scheduling is the method used by operating systems to decide the order in which disk I/O operations are performed. This is important because disk operations are time-consuming, and efficient scheduling can reduce the time it takes to complete all I/O tasks.

FCFS, (First Come First Serve)

SSTF (Shortest Seek Time First)

SCAN (Elevator Algorithm)

C-SCAN

LOOK

First Come, First Serve (FCFS):

• The simplest algorithm where the I/O requests are handled in the order in which they arrive.
• It can lead to long waiting times for some requests, especially if a request at the beginning of the queue is far from the current disk position.

Shortest Seek Time First (SSTF):

·         The disk scheduler selects the request that is closest to the current head position.

·         While this minimizes the seek time, it can cause starvation for requests that are far away from the current position.

SCAN (Elevator Algorithm):

·         The disk arm moves in one direction (left or right) and processes all requests in that direction until it reaches the end.

·         After reaching the end, it reverses direction and processes requests in the opposite direction.

·         This reduces the overall seek time compared to FCFS, but may cause delays for requests near the ends of the disk.

C-SCAN (Circular SCAN):

·         Similar to SCAN but, once the disk head reaches the end, it immediately returns to the beginning (without servicing any requests in the reverse direction).

·         This ensures that all requests are processed in a more uniform manner.

LOOK and C-LOOK:

·         LOOK is a variant of SCAN where the disk arm only moves as far as the last request in the current direction, rather than going all the way to the end of the disk.

·         C-LOOK is similar to C-SCAN but reverses direction only when there are no more requests in the current direction.

Comparison of Disk Scheduling Algorithms

Algorithm	Advantages	Disadvantages
FCFS	Simple to implement, easy to understand.	Can lead to inefficient use of the disk (long wait times).
SSTF	More efficient than FCFS in terms of seek time.	Can cause starvation for distant requests.
SCAN	Fairer, reduces total seek time than FCFS.	Can be inefficient for systems with many requests at the extremes.
C-SCAN	More uniform distribution of seek time.	Still suffers from high overhead when there are many requests.
LOOK	More efficient than SCAN (doesn't go to the extremes).	Similar to SCAN but can still cause delays.

Conclusion

Both directory structure and disk scheduling are crucial components in file systems and overall system performance. Directory structures ensure that files are organized logically, while disk scheduling algorithms ensure that disk I/O operations are completed efficiently. Balancing both can result in an optimized user experience, faster file access, and improved system responsiveness.

 

File System Implementation in Linux:

Linux file system implementation explains how files are stored, organized, and managed internally by the OS. It is a key concept in Operating Systems.

Mounting & Unmounting File systems

Mounting & Unmounting File Systems (Linux)

1. Mounting (Definition)

Mounting is the process of attaching a file system (like a hard disk, USB, or partition) to a directory (called a mount point) so that its files become accessible.

In Linux, everything is accessed through directories, so devices must be mounted first.

2. Mount Point

A directory where the file system is attached

Common mount points:

/mnt (temporary mount)

/media (for removable devices like USB)

3. Mount Command

Syntax:

mount -t <file_system_type> <device_name> <mount_point>

Example:

sudo mount -t ext4 /dev/sdb1 /mnt

ext4 → File system type

/dev/sdb1 → Device (USB/partition)

/mnt → Mount point

4. Check Mounted File Systems

mount

or

df -h

5. Unmounting (Definition)

Unmounting is the process of safely detaching a file system from its mount point.

This ensures no data is lost or corrupted.

6. Unmount Command

Syntax:

umount <device_name or mount_point>

Example:

sudo umount /mnt

or

sudo umount /dev/sdb1

7. Important Notes

Always unmount before removing USB drives

If you get “device busy” error:

Close open files

Use:

lsof | grep /mnt

8. Automatic Mounting

Done using the file:

/etc/fstab

Helps mount file systems automatically at system startup

 

Key Points

Mount → Attach file system

Unmount → Detach file system

Required before accessing storage devices

Prevents data loss

Short Answer

Mounting is the process of attaching a file system to a directory to access its contents, while unmounting safely removes it. Linux uses mount and umount commands, and /etc/fstab is used for automatic mounting.

 

Understanding File Permissions

File permissions in Linux control who can access a file and what actions they can perform. This is essential for security and system management.

1. Types of Users

There are three categories:

Owner (u) → The creator of the file

Group (g) → Users in the same group

Others (o) → Everyone else

2. Types of Permissions

Each file has three basic permissions:

Read (r) → View file contents

Write (w) → Modify file

Execute (x) → Run file (like a program/script)

3. Permission Representation

Example:

-rwxr-xr--

Breakdown:

- → File type (- = file, d = directory)

rwx → Owner permissions

r-x → Group permissions

r-- → Others permissions

4. Numeric (Octal) Representation

Each permission has a number:

r = 4

w = 2

x = 1

Add values:

Permission	Value
rwx	7
rw-	6
r-x	5
r--	4

Example:

chmod 754 file.txt

Owner → 7 (rwx)

Group → 5 (r-x)

Others → 4 (r--)

5. Changing Permissions

Using Symbolic Method:

chmod u+x file.txt   # Add execute to owner
chmod g-w file.txt   # Remove write from group
chmod o=r file.txt   # Set read-only for others

Using Numeric Method:

chmod 644 file.txt

6. Changing Ownership

chown user file.txt
chgrp group file.txt

7. Directory Permissions

r → List files

w → Create/delete files

x → Enter directory

8. Special Permissions (Advanced)

SUID (Set User ID) → Run file as owner

SGID (Set Group ID) → Run with group privileges

Sticky Bit → Only owner can delete files (used in /tmp)

Key Points

Permissions ensure system security

Represented as rwx or numbers

Controlled using chmod, chown

Different rules for files and directories

Short Exam Answer

File permissions in Linux define access rights for owner, group, and others using read, write, and execute permissions. They can be represented symbolically or numerically and modified using commands like chmod and chown.

 

(chmod, chown, ls -l)

File permissions in Linux control who can access a file and what actions they can perform. This is essential for security and system management.

1. Types of Users

There are three categories:

Owner (u) → The creator of the file

Group (g) → Users in the same group

Others (o) → Everyone else

2. Types of Permissions

Each file has three basic permissions:

Read (r) → View file contents

Write (w) → Modify file

Execute (x) → Run file (like a program/script)

3. Permission Representation

Example:

-rwxr-xr--

Breakdown:

- → File type (- = file, d = directory)

rwx → Owner permissions

r-x → Group permissions

r-- → Others permissions

4. Numeric (Octal) Representation

Each permission has a number:

r = 4

w = 2

x = 1

Add values:

Permission	Value
rwx	7
rw-	6
r-x	5
r--	4

Example:

chmod 754 file.txt

Owner → 7 (rwx)

Group → 5 (r-x)

Others → 4 (r--)

5. Changing Permissions

Using Symbolic Method:

chmod u+x file.txt   # Add execute to owner
chmod g-w file.txt   # Remove write from group
chmod o=r file.txt   # Set read-only for others

Using Numeric Method:

chmod 644 file.txt

6. Changing Ownership

chown user file.txt
chgrp group file.txt

7. Directory Permissions

r → List files

w → Create/delete files

x → Enter directory

8. Special Permissions (Advanced)

SUID (Set User ID) → Run file as owner

SGID (Set Group ID) → Run with group privileges

Sticky Bit → Only owner can delete files (used in /tmp)

Key Points

Permissions ensure system security

Represented as rwx or numbers

Controlled using chmod, chown

Different rules for files and directories

Short Exam Answer

File permissions in Linux define access rights for owner, group, and others using read, write, and execute permissions. They can be represented symbolically or numerically and modified using commands like chmod and chown.