ASSIGNMENT
ASSIGNMENT
Q.8 Explain file allocation method in detail
Answer :-
File allocation methods are techniques used by operating systems to manage how files are stored on disk storage systems.
These methods determine the layout and organization of file data on physical or logical storage media, impacting performance, access speed, and storage efficiency.
Below is a detailed explanation of the common file allocation methods:
1. Contiguous Allocation :- In this method, each file occupies a set of consecutive blocks on the disk.
Key Features:
Allocation: The starting block and the length of the file are recorded in the file allocation table.
Access: Direct and sequential access are fast because all the file’s blocks are located next to each other.
Advantages:
Simple to implement.
High performance for sequential file access.
Disadvantages:
External fragmentation: Free blocks are scattered across the disk, making it hard to allocate large files.
File size limits: Expanding a file may require relocation if adjacent blocks are unavailable.
Example:-
A file of size 4 blocks starts at block 5. The file occupies blocks 5, 6, 7, and 8.
2. Linked Allocation :- In this method, each file is represented as a linked list of disk blocks.
Key Features:
- Allocation: Each block contains data and a pointer to the next block. The last block contains a null pointer.
- Access: Sequential access is straightforward, but direct access requires traversing the list.
Advantages:
- No external fragmentation.
- Files can grow dynamically as needed.
Disadvantages:
- Increased overhead due to storage of pointers.
- Performance degradation for random access.
- Susceptible to pointer corruption.
Example:
- If a file occupies blocks 2, 6, 9, and 12, block 2 contains a pointer to block 6, block 6 points to block 9, and so on.
3. Indexed Allocation :- In this method, each file has an index block, which contains pointers to all the file’s data blocks.
Key Features:
- Allocation: A separate index block is created for each file. The index block stores the addresses of the file’s data blocks.
- Access: Direct access is efficient as the block addresses are stored in the index block.
Advantages:
- No external fragmentation.
- Efficient for direct access.
Disadvantages:
- Overhead of maintaining an index block.
- Limited file size unless multi-level indexing is used.
Example:
- For a file, the index block at block 3 might point to data blocks 7, 15, and 20.
4. Clustered Allocation :- This is a hybrid approach combining contiguous and linked allocation. Files are divided into clusters (groups of contiguous blocks) that are linked together.
Key Features:
- Reduces fragmentation compared to purely contiguous allocation.
- Improves performance for sequential access compared to purely linked allocation.
- Overhead is lower than linked allocation due to fewer pointers.