Linked Allocation (Non-contiguous Allocation) in Operating Systems:
Linked allocation is a file storage method where each file is stored in scattered blocks across the memory or disk. Unlike contiguous allocation, the blocks of a file are not stored next to each other but are linked via pointers. Each block of the file contains a pointer to the next block, allowing the system to access the file sequentially, even if the blocks are physically located in different places in memory or on disk.
How Linked Allocation Works:
- When a file is created, the operating system finds free blocks of memory or disk space and stores the file's data in these non-contiguous blocks.
- Each block contains two parts: the actual data of the file and a pointer to the next block (or a special marker if it’s the last block).
- The operating system maintains a list of all the blocks that make up a file, and it follows the pointers to retrieve the data.
Advantages of Linked Allocation:
No External Fragmentation:
- Since files are stored in scattered blocks, linked allocation does not suffer from external fragmentation, unlike contiguous allocation. The system can use any free block, and the file can grow without requiring large contiguous spaces.
Dynamic File Growth:
- Files can grow dynamically by simply allocating new blocks and linking them to the existing file. This is much more flexible than contiguous allocation, where a file might need to be moved to a larger contiguous block if it grows.
Efficient Storage Utilization:
- Linked allocation makes better use of disk space since files can occupy scattered blocks, reducing wasted space compared to contiguous allocation, which requires large enough contiguous free space.
Simple to Implement:
- The structure for linked allocation is simple. The file system only needs to maintain pointers in each block and manage the linked list of blocks for each file.
Disadvantages of Linked Allocation:
Slow Access Time (Sequential Access):
- Since the blocks are scattered, random access to a file can be slow because the system has to follow the pointers sequentially to locate each block. This makes linked allocation inefficient for applications requiring frequent random access to files.
Overhead of Pointers:
- Each block needs to store a pointer to the next block, which increases the storage overhead, especially for large files. This pointer storage can reduce the available space for data in each block.
Extra Disk I/O for Accessing Files:
- Due to the non-contiguous nature of the storage, accessing a file may require multiple disk I/O operations to fetch all its blocks. This can degrade performance, especially when files are large and the blocks are spread far apart.
Increased Complexity in File Management:
- Managing a linked list of blocks for each file adds complexity to the operating system. The system must maintain the chain of pointers, which can become challenging to manage in cases of file fragmentation or corruption.
Use Cases of Linked Allocation:
- File Systems with Large Files: Linked allocation works well for large files where the total file size is unknown or changes over time. For example, databases or log files that grow dynamically can benefit from linked allocation.
- Sequential Access Systems: Linked allocation is suited for systems where files are accessed sequentially, such as in multimedia applications (e.g., video or audio streaming) or backup systems.
- Flexible Disk Space Usage: When disk space is fragmented and non-contiguous, linked allocation allows files to be stored efficiently across available free blocks, minimizing unused space.
Summary:
Linked allocation is a flexible and efficient method for storing files where the file blocks are scattered across memory or disk. While it eliminates external fragmentation and supports dynamic file growth, it suffers from slower random access, additional overhead for pointers, and multiple disk I/O operations. It is best suited for systems with large, growing files and sequential access patterns.
