File allocation methods (contiguous, linked, indexed) in Operating Systems - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the contiguous file allocation method?

Contiguous file allocation stores all the blocks of a file in consecutive locations on the disk. This makes reading the file fast because the disk head moves less.

beginner

How does linked file allocation work?

In linked allocation, each file is a linked list of disk blocks. Each block contains a pointer to the next block. This allows files to be stored anywhere on the disk but can be slower to access.

intermediate

What is the main advantage of indexed file allocation?

Indexed allocation uses an index block that contains pointers to all the file's blocks. This allows direct access to any block, improving speed and flexibility.

intermediate

What is a common problem with contiguous allocation?

Contiguous allocation can cause external fragmentation because files need continuous space, making it hard to find large enough free blocks as disk fills up.

intermediate

Why might linked allocation be slower than indexed allocation?

Linked allocation requires following pointers from one block to the next, which means reading multiple disk blocks sequentially. Indexed allocation allows direct access to any block via the index, speeding up access.

Which file allocation method stores file blocks in consecutive disk locations?

A Contiguous allocation

B Linked allocation

C Indexed allocation

D None of the above

Contiguous allocation stores all file blocks one after another in consecutive disk locations.

In linked allocation, what does each block contain?

A The file size

B An index of all blocks

C The entire file data

D A pointer to the next block

Each block contains a pointer to the next block, forming a linked list.

Which allocation method uses an index block to store pointers to file blocks?

A Indexed allocation

B Linked allocation

C Contiguous allocation

D Sequential allocation

Indexed allocation uses a special index block that holds pointers to all the file's blocks.

What is a major disadvantage of contiguous allocation?

A Slow access to file blocks

B External fragmentation

C Complex index management

D Pointers use extra space

Contiguous allocation can cause external fragmentation because files need continuous space.

Why is indexed allocation faster than linked allocation for random access?

A Because it stores blocks consecutively

B Because it uses pointers in each block

C Because it uses an index block for direct access

D Because it compresses file data

Indexed allocation allows direct access to any block via the index block, speeding up random access.

Explain the differences between contiguous, linked, and indexed file allocation methods.

Think about how files are stored and accessed on disk.

Describe a real-life situation where linked file allocation might be preferred over contiguous allocation.

Consider flexibility versus speed.

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