File allocation methods (contiguous, linked, indexed) in Operating Systems - Step-by-Step Execution

Concept Flow - File allocation methods (contiguous, linked, indexed)

Start: File to store

↓

Choose allocation method

↓

Contiguous

↓

Find block

↓

Store file

↓

Access file

↓

End

The flow shows starting with a file to store, choosing one of three allocation methods, then following their specific steps to store and access the file.

Execution Sample

Operating Systems

File allocation methods:
1. Contiguous: allocate one continuous block
2. Linked: allocate scattered blocks linked by pointers
3. Indexed: allocate an index block pointing to scattered blocks

This lists the three main file allocation methods and their basic approach to storing file blocks.

Analysis Table

Step	Method	Action	Details	Result
1	Contiguous	Find space	Look for one large free block	If found, allocate it
2	Contiguous	Store file	Write file blocks sequentially	File stored in one continuous area
3	Contiguous	Access file	Read blocks by sequential addresses	Fast access, simple
4	Linked	Find space	Find any free blocks scattered	Allocate blocks anywhere free
5	Linked	Create chain	Link blocks with pointers	Blocks connected via pointers
6	Linked	Store file	Write file blocks in linked blocks	File stored in scattered blocks
7	Linked	Access file	Follow pointers from first block	Slower access, flexible size
8	Indexed	Find space	Find free blocks and one index block	Allocate scattered blocks plus index
9	Indexed	Create index	Store pointers to blocks in index block	Index block holds all pointers
10	Indexed	Store file	Write file blocks anywhere	File stored scattered with index
11	Indexed	Access file	Use index block to find blocks	Fast random access via index
12	-	End	All blocks allocated and linked/indexed	File stored and accessible

💡 All methods complete when file blocks are allocated and accessible by their method

State Tracker

Variable	Start	After Contiguous	After Linked	After Indexed	Final
Free space	Large free blocks scattered	One large block allocated	Multiple scattered blocks allocated	Multiple scattered blocks + index block allocated	Reduced free space accordingly
File blocks	Not stored	Stored sequentially	Stored scattered with pointers	Stored scattered with index pointers	File fully stored
Access method	None	Sequential read	Pointer following	Index lookup	File accessible

Key Insights - 3 Insights

Why does contiguous allocation require a large continuous free space?

Because as shown in execution_table row 1, contiguous allocation looks for one large free block to store the entire file sequentially. If such a block is not available, the file cannot be stored contiguously.

How does linked allocation handle scattered free space?

As seen in rows 4-7, linked allocation finds any free blocks scattered anywhere and links them with pointers, allowing the file to be stored in non-contiguous blocks.

What advantage does indexed allocation have over linked allocation when accessing file blocks?

According to rows 8-11, indexed allocation uses an index block to directly point to all file blocks, enabling faster random access compared to following pointers one by one in linked allocation.

Visual Quiz - 3 Questions

Look at the execution_table. At which step does the linked allocation method create pointers between blocks?

A Step 9

B Step 5

C Step 2

D Step 1

💡 Hint

Check the 'Action' and 'Method' columns for linked allocation steps in the execution_table.

According to variable_tracker, what happens to 'Free space' after indexed allocation?

A One large block is allocated

B No blocks are allocated

C Multiple scattered blocks plus an index block are allocated

D Only scattered blocks are allocated without index

💡 Hint

Look at the 'Free space' row under 'After Indexed' in variable_tracker.

If contiguous allocation cannot find a large enough block, what is the likely outcome?

A File storage fails or another method is needed

B File is stored scattered with pointers

C File is stored with an index block

D File is stored sequentially anyway

💡 Hint

Refer to execution_table row 1 and key_moments about contiguous allocation requirements.

Concept Snapshot

File allocation methods:
- Contiguous: store file in one continuous block; fast access but needs large free space
- Linked: store file in scattered blocks linked by pointers; flexible but slower access
- Indexed: use an index block pointing to scattered blocks; fast random access
Each method balances storage efficiency and access speed differently.

Full Transcript

File allocation methods determine how files are stored on disk. Contiguous allocation stores all file blocks in one continuous space, requiring a large free block but allowing fast sequential access. Linked allocation stores file blocks scattered anywhere on disk, linking them with pointers, which allows flexible storage but slower access as pointers are followed. Indexed allocation uses a special index block that holds pointers to all file blocks, enabling fast random access even if blocks are scattered. The execution flow starts with choosing a method, then allocating blocks accordingly, and finally accessing the file using the method's approach. Variables like free space and file blocks change as allocation proceeds. Key points include contiguous needing large free space, linked allowing scattered storage via pointers, and indexed providing fast access via an index block.

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