File allocation methods (contiguous, linked, indexed) in Operating Systems - Mini Project: Build & Apply

Understanding File Allocation Methods

📖 Scenario: Imagine you are managing files on a computer disk. Files need to be stored in a way that the computer can find them easily. There are three common methods to allocate space for files: contiguous, linked, and indexed allocation.In this project, you will create simple Python data structures to represent these three file allocation methods and understand how they work.

🎯 Goal: You will build Python representations for the three file allocation methods:Contiguous allocation: files stored in one continuous blockLinked allocation: files stored as linked blocks scattered on diskIndexed allocation: files stored with an index block pointing to data blocksThis will help you visualize and understand how files are organized on disk.

📋 What You'll Learn

Create a dictionary representing files and their allocated blocks for contiguous allocation

Create a helper variable representing the disk size

Create a dictionary representing files and their linked blocks for linked allocation

Create a dictionary representing files with an index block and data blocks for indexed allocation

💡 Why This Matters

🌍 Real World

File allocation methods are used by operating systems to manage how files are stored and accessed on physical disks.

💼 Career

Understanding these methods is important for roles in system administration, operating system development, and storage management.

1

Create contiguous allocation data

Create a dictionary called contiguous_files with these exact entries: 'File1': [0, 1, 2], 'File2': [3, 4], 'File3': [5, 6, 7, 8]. Each list represents continuous disk blocks allocated to the file.

Operating Systems

# Create the contiguous_files dictionary with exact entries
# Your code here

Hint

Use a dictionary with file names as keys and lists of continuous block numbers as values.

2

Add disk size configuration

Create a variable called disk_size and set it to 10 to represent the total number of disk blocks available.

Operating Systems

contiguous_files = {
    'File1': [0, 1, 2],
    'File2': [3, 4],
    'File3': [5, 6, 7, 8]
}

# Create disk_size variable and set it to 10
# Your code here

Hint

Just create a variable named disk_size and assign it the value 10.

3

Create linked allocation data

Create a dictionary called linked_files with these exact entries: 'File1': [0, 3, 5], 'File2': [1, 4], 'File3': [2, 6, 7, 8]. Each list represents linked blocks scattered on disk.

Operating Systems

contiguous_files = {
    'File1': [0, 1, 2],
    'File2': [3, 4],
    'File3': [5, 6, 7, 8]
}

disk_size = 10

# Create linked_files dictionary with exact entries
# Your code here

Hint

Use a dictionary with file names as keys and lists of linked block numbers as values.

4

Create indexed allocation data

Create a dictionary called indexed_files with these exact entries: 'File1': {'index': 9, 'blocks': [0, 1, 2]}, 'File2': {'index': 8, 'blocks': [3, 4]}, 'File3': {'index': 7, 'blocks': [5, 6]}. Each file has an index block and a list of data blocks.

Operating Systems

contiguous_files = {
    'File1': [0, 1, 2],
    'File2': [3, 4],
    'File3': [5, 6, 7, 8]
}

disk_size = 10

linked_files = {
    'File1': [0, 3, 5],
    'File2': [1, 4],
    'File3': [2, 6, 7, 8]
}

# Create indexed_files dictionary with exact entries
# Your code here

Hint

Use a dictionary with file names as keys and dictionaries as values. Each value dictionary has keys 'index' and 'blocks'.

Practice

1. Which file allocation method stores all file blocks in one continuous sequence on disk?

easy

A. Hybrid allocation

B. Linked allocation

C. Indexed allocation

D. Contiguous allocation

Solution

1. Step 1: Understand file storage methods

   Contiguous allocation stores all file blocks one after another in a continuous space on disk.

2. Step 2: Compare with other methods

   Linked allocation stores blocks scattered with pointers; indexed uses an index block. Only contiguous is continuous.

3. Final Answer:

   Contiguous allocation -> Option D

4. Quick Check:

   Continuous blocks = Contiguous allocation [OK]

Hint: Continuous blocks mean contiguous allocation [OK]

Common Mistakes:

• Confusing linked allocation with contiguous
• Thinking indexed allocation stores blocks continuously
• Assuming hybrid is a standard method

2. Which of the following is the correct way to describe linked allocation?

easy

A. File blocks are stored in a continuous sequence

B. Each file block contains a pointer to the next block

C. An index block contains pointers to all file blocks

D. File blocks are stored in a fixed-size table

Solution

1. Step 1: Recall linked allocation structure

   In linked allocation, each file block stores a pointer to the next block, forming a chain.

2. Step 2: Eliminate other options

   Continuous sequence is contiguous; index block is indexed allocation; fixed-size table is unrelated.

3. Final Answer:

   Each file block contains a pointer to the next block -> Option B

4. Quick Check:

   Pointers in blocks = Linked allocation [OK]

Hint: Pointers inside blocks mean linked allocation [OK]

Common Mistakes:

• Mixing up index block with linked pointers
• Thinking linked allocation stores blocks continuously
• Confusing fixed-size table with file allocation

3. Consider a file stored using indexed allocation with an index block containing pointers to data blocks 5, 9, and 12. Which block does the system read first to access the file?

medium

A. The index block

B. Block 5

C. Block 9

D. Block 12

Solution

1. Step 1: Understand indexed allocation access

   The system first reads the index block to find pointers to all data blocks.

2. Step 2: Identify the first block read

   Before accessing data blocks 5, 9, or 12, the index block must be read to know their locations.

3. Final Answer:

   The index block -> Option A

4. Quick Check:

   Index block read first in indexed allocation [OK]

Hint: Index block is always read first in indexed allocation [OK]

Common Mistakes:

• Choosing a data block instead of the index block
• Confusing linked allocation with indexed
• Assuming direct access without index

4. A file system uses linked allocation but the file's blocks are stored non-contiguously without pointers linking them. What is the main problem here?

medium

A. File blocks are stored contiguously, causing slow access

B. File blocks are fragmented but linked correctly

C. Pointers are missing, so file blocks cannot be accessed sequentially

D. Index block is corrupted

Solution

1. Step 1: Understand linked allocation requirements

   Linked allocation requires each block to have a pointer to the next block for sequential access.

2. Step 2: Identify the problem without pointers

   Without pointers, the system cannot follow the chain, so blocks cannot be accessed in order.

3. Final Answer:

   Pointers are missing, so file blocks cannot be accessed sequentially -> Option C

4. Quick Check:

   Missing pointers break linked allocation chain [OK]

Hint: No pointers means linked allocation fails [OK]

Common Mistakes:

• Assuming fragmentation is not a problem
• Confusing contiguous storage with linked allocation
• Thinking index block is involved in linked allocation

5. A file of size 12 blocks is stored using contiguous allocation starting at block 20. If block 25 is corrupted, what is the impact on the file access?

hard

A. The entire file becomes inaccessible due to break in continuity

B. File access continues using pointers from block 25

C. Only block 25 is lost; other blocks are accessible

D. Index block redirects access around block 25

Solution

1. Step 1: Recall contiguous allocation properties

   Files are stored in continuous blocks; any corruption breaks the sequence.

2. Step 2: Analyze impact of corrupted block 25

   Since block 25 is in the middle, the file cannot be read beyond that point, making the entire file inaccessible.

3. Final Answer:

   The entire file becomes inaccessible due to break in continuity -> Option A

4. Quick Check:

   Corruption breaks contiguous file access [OK]

Hint: Contiguous files fail if any block is corrupted [OK]

Common Mistakes:

• Assuming only corrupted block is lost
• Thinking pointers or index block help in contiguous allocation
• Confusing linked or indexed allocation behavior