Index Allocation in Operating System

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Index Allocation is a method used in file systems to allocate space for files on storage devices like hard drives. Instead of storing the file data directly in contiguous blocks, index allocation uses a separate index block (or array) that holds the pointers to the data blocks.

This approach provides an efficient way to manage file storage, especially for files that may not fit into contiguous blocks.

How Index Allocation Works

In index allocation, each file has an index block, which is a block that stores the addresses (pointers) of the data blocks where the actual content of the file is stored.

The file itself is not stored in contiguous blocks, but rather, the index block helps the system find the blocks scattered across the storage device. This method enables flexibility in managing files that grow or shrink in size. Heres how it works step by step −

• Index Block: When a file is created, the file system allocates an index block for the file. This index block contains pointers (addresses) to the data blocks that hold the actual content of the file.

• File Data Storage: The data blocks that store the actual content of the file may not be contiguous. Instead, they are scattered across the disk, and their locations are stored in the index block.

• Accessing Data: When a process needs to read the file, it uses the index block to find the locations of the data blocks. The index block points to each data block in the sequence in which they should be read.

• Multiple Levels of Indexing: For larger files, the system may use multiple levels of index blocks

Types of Index Allocation

Following are the types of index allocation −

• Single-Level Index Allocation: In this scheme, one index block stores the addresses of all data blocks in the file. The index block is usually fixed in size and points to a fixed number of data blocks. Example: A file with 5 data blocks will have an index block containing 5 pointers, each pointing to a data block.

• Multilevel Index Allocation: For large files, a multi-level index approach is used. In this case, an index block points to other index blocks (e.g., double or triple-level index), which in turn point to the data blocks. This allows files to be larger than what can be handled by a single index block.

Advantages of Index Allocation

Following are the advantages of the index allocation −

• Flexibility: Files can grow and shrink dynamically since the data blocks do not need to be contiguous. New blocks can be allocated anywhere on the disk, and the index block simply gets updated to point to the new locations.

• Efficient Storage: Unlike contiguous allocation, which may lead to fragmentation, index allocation avoids fragmentation since the file's data blocks can be scattered across the disk.

• Random Access: Index allocation allows for efficient random access to file data. By accessing the index block, the operating system can directly locate any block of the file without needing to read through the entire file sequentially.

• No External Fragmentation: Since the data blocks can be scattered across the disk, index allocation avoids external fragmentation, which occurs when there is not enough contiguous space to store a file.