Linked Index Allocation in Operating System

Linked Index Allocation is a variation of the index allocation method used in operating systems to allocate disk space for files. In this method, instead of using a separate index block to store the pointers to the data blocks, each data block of a file contains a pointer to the next data block in the file. This forms a linked list of data blocks, where each block points to the next block in the sequence, making it easy to retrieve the file's content sequentially.

How Linked Index Allocation Works:

Type of Directories

• Data Blocks and Pointers: Each data block of a file contains not only the actual data but also a pointer to the next block of the file. The last data block of the file contains a special marker (usually a null or end-of-file marker) to indicate that there are no more blocks.

• Accessing File Data: To access the content of the file, the operating system starts at the first data block, reads the data, and follows the pointer to the next block. This process continues until the end of the file is reached (indicated by the special end-of-file marker in the last block).

• Dynamic Allocation:Linked index allocation does not require contiguous storage of file data blocks. As a result, file blocks can be scattered across the disk, and the file can dynamically grow as needed. New blocks are added to the chain by updating the pointer in the last block of the file.

Advantages of Linked Index Allocation

• No External Fragmentation: Like index allocation, linked index allocation avoids external fragmentation because the blocks can be allocated anywhere on the disk, and the file's data can be non-contiguous.

• Efficient File Growth: Files can grow easily because new blocks can be added to the file without requiring contiguous space. The system simply updates the pointer in the last block to point to the new data block.

• Dynamic Allocation: It is well-suited for files that need to expand over time. Since the file data blocks do not need to be contiguous, the system can allocate additional blocks as needed.

• Simple Implementation: The scheme is relatively simple to implement compared to other allocation methods like contiguous allocation or indexed allocation with multiple levels of indexing.

Disadvantages of Linked Index Allocation

• Sequential Access Only: Linked index allocation is optimized for sequential access to file data, as the system must follow the chain of pointers from one block to the next. This makes random access slower because to access a specific block, the system must traverse all the previous blocks in the chain.

• Overhead in Each Block: Each data block must store a pointer to the next block in addition to the file's actual data. This means that each block has a slight overhead, which may waste some space, especially for small files or small block sizes.

• Increased Disk Head Movement: Since the blocks are not contiguous, reading a file might require seeking across the disk. This increases disk head movement and can lead to slower performance when accessing files that are large or have many blocks scattered across the disk.

• Loss of Random Access Efficiency: If you want to read or write a block in the middle of a file, you must start from the first block and follow the pointers sequentially. This makes it inefficient for random access operations.