Contiguous Disk Space Allocation: Understanding its Simplicity and Limitations

Learn about contiguous disk space allocation, a method for storing files in adjacent disk blocks. This guide explains its mechanism, advantages (simplicity), and significant disadvantages (wasted space, difficulty in growing or moving files), highlighting its limitations in modern file systems.

Contiguous Disk Space Allocation

Understanding Contiguous Allocation

Contiguous allocation is a method for allocating space on a hard drive to store files. In contiguous allocation, all the blocks of data for a single file are stored in adjacent (contiguous) locations on the disk. This is a simple approach, but it has some significant limitations in terms of how efficiently the disk space is used and how easily files can be grown or moved.

How Contiguous Allocation Works

When a file is created using contiguous allocation, the operating system finds a contiguous block of disk space large enough to hold the entire file. That block of space is then allocated to the file. All the data for the file is stored within this single block. This is simple but doesn't efficiently use disk space because there must be a large enough contiguous block available at the time the file is created.

Illustrative Example

Imagine a disk with three files. This table shows where each file is stored (contiguous blocks):

Advantages of Contiguous Allocation

• Simple Implementation: Relatively straightforward to implement.
• Excellent Read Performance: Reading a file is fast because all the data is in one place.
• Random Access: Any part of the file can be accessed directly (without needing to follow pointers).

Disadvantages of Contiguous Allocation

• Disk Fragmentation: As files are created and deleted, free space becomes fragmented (scattered), making it difficult to find large, contiguous blocks for new files.
• File Growth: Expanding a file can be difficult because it requires finding an adjacent free space block of sufficient size.