L-7.6: Indexed File Allocation in Operating System

Summary

In this video, the concept of indexed allocation in computer storage is explained, highlighting its advantages and disadvantages. Indexed allocation involves dividing files into blocks and storing them across different locations on a disk, with an index block that points to the data's location. This allows for random access, eliminating external fragmentation. However, it introduces overhead due to pointers and the potential need for multi-level indexing with large files. The video also touches on real-world applications, such as how Unix uses indexed allocation through inodes. A clear comparison is made with contiguous and linked list allocation techniques.

Takeaways

• 😀 Indexed allocation is a method of file storage where each file has an index block pointing to its data blocks on disk.
• 😀 The index block works like an index in a book, providing the location of each data block for a file.
• 😀 Files are divided into blocks, and each block can be stored at any location on the disk, depending on availability.
• 😀 An index block is created for each file, allowing for easy retrieval of data without having to search sequentially like in linked list allocation.
• 😀 Indexed allocation supports direct or random access, allowing quick data retrieval from any block without searching other blocks first.
• 😀 Unlike contiguous allocation, indexed allocation avoids external fragmentation since data can be scattered across the disk.
• 😀 Every file has its own index block, similar to how each book has its own index, even if multiple files exist.
• 😀 The disadvantage of indexed allocation is pointer overhead, as each index block consumes storage for pointers instead of data.
• 😀 The multi-level index problem occurs when the index block is too large to fit into a single block, requiring multiple index blocks.
• 😀 Unix file system uses an inode structure to implement indexed allocation, where the 'i' represents the index of data blocks.
• 😀 Despite its advantages, indexed allocation can create overhead and storage inefficiency in cases where files are either very small or very large.

Q & A

What is indexed allocation in file storage systems?

Indexed allocation is a file storage method where an index block is used to store the addresses of the data blocks of a file. The index block allows quick access to data blocks scattered across the disk without the need for sequential traversal.

How does indexed allocation differ from contiguous allocation?

In contiguous allocation, data blocks are stored next to each other in contiguous locations, which can lead to external fragmentation. In indexed allocation, data blocks can be stored anywhere on the disk, with an index block providing the location of each block, avoiding external fragmentation.

What is the purpose of the index block in indexed allocation?

The index block stores the physical locations of the data blocks of a file. It acts as a reference guide, allowing the system to quickly access the data blocks based on their locations stored in the index.

What is the advantage of direct access in indexed allocation?

Direct access, or random access, allows the system to retrieve specific data blocks without needing to search through other blocks sequentially. By using the index block, the system can jump directly to the desired data block, improving retrieval efficiency.

What does it mean that indexed allocation avoids external fragmentation?

Indexed allocation avoids external fragmentation because data blocks can be stored anywhere on the disk, rather than needing to be stored contiguously. This flexibility ensures that space on the disk is utilized more effectively, without leaving gaps between data blocks.

What is the disadvantage of pointer overhead in indexed allocation?

Pointer overhead occurs because each file requires an index block to store pointers to the data blocks. These pointers consume storage space, and when files are large, multiple index blocks may be needed, further increasing the overhead.

What is the multi-level index problem in indexed allocation?

The multi-level index problem arises when a file is very large, causing the index block to grow beyond the capacity of a single block. This results in the need for multiple index blocks, creating additional complexity and overhead in managing file data.

How does indexed allocation allow for efficient data retrieval?

Indexed allocation enables efficient data retrieval by providing direct access to any data block via the index block. Instead of searching through blocks sequentially, the system can quickly jump to the location of the desired block, improving retrieval speed.

What are the real-world applications of indexed allocation?

A real-world example of indexed allocation is the **Unix file system**, which uses a structure called **inode** to store the index of file data locations. The inode enables efficient management of file data using indexed allocation.

What are the potential storage inefficiencies in indexed allocation?

One inefficiency is the unused space in index blocks when files are small, as a full block may be reserved just to store a few pointers. Additionally, large files require multiple index blocks, which increases storage overhead and may lead to further indexing complexity.