Disk Space Allocation Methods in OS (Operating Systems)

Disk Space Allocation Methods in OS

Disk space allocation is a critical aspect of operating systems that determines how files and data are stored on physical storage devices such as hard drives. Effective disk space allocation methods are essential for optimizing storage utilization, enhancing performance, and ensuring data integrity. In this article, we will explore different disk space allocation methods used in modern operating systems and understand their strengths, weaknesses, and applications.


In an operating system, disk space is divided into fixed-size blocks, also known as sectors. These sectors are the smallest unit of data that can be read from or written to the disk. Disk space allocation methods determine how files and data are assigned these sectors and stored on the disk.

Contiguous Allocation

Contiguous Allocation

Contiguous allocation is one of the simplest disk space allocation methods. In this method, files are stored on disk as contiguous blocks of sectors. Each file occupies a continuous range of sectors without any fragmentation. This method offers fast and efficient file access, as the operating system only needs to remember the starting sector of a file. However, it suffers from the drawback of external fragmentation, where free space becomes scattered and discontinuous over time, making it challenging to allocate space for new files.

Linked Allocation

Linked Allocation

Linked allocation overcomes the drawback of external fragmentation present in contiguous allocation. In this method, each file is divided into blocks of sectors, which can be scattered across the disk. Each block contains a pointer to the next block, creating a chain-like structure. The operating system maintains a file allocation table (FAT) or linked list to keep track of these blocks. Linked allocation enables efficient storage utilization, as it can allocate any available free block to a file. However, accessing a specific block within a file becomes slower due to the need to traverse the linked list.

Indexed Allocation

Indexed Allocation

Indexed allocation addresses the limitations of contiguous and linked allocation methods. In this method, each file has an associated index block that contains pointers to all the blocks of the file. The index block acts as a lookup table, enabling quick access to any block of the file. This method eliminates external fragmentation and improves file access time compared to linked allocation. However, it introduces the overhead of maintaining the index blocks, especially for large files with numerous blocks.

Combined Allocation Methods

To leverage the advantages of multiple allocation methods, some operating systems use combined allocation methods. For instance, a common approach is to use contiguous allocation for small files and indexed or linked allocation for large files. This hybrid approach optimizes storage utilization for different file sizes and minimizes fragmentation.

Comparison of Allocation Methods

The choice of disk space allocation method depends on various factors, including file sizes, access patterns, and storage devices. Here’s a comparison of the three main allocation methods:

  • Contiguous allocation:

    • Pros: Fast access, simple implementation
    • Cons: External fragmentation, inefficient storage utilization
  • Linked allocation:

    • Pros: Efficient storage utilization, no external fragmentation
    • Cons: Slow access, traversal of linked list
  • Indexed allocation:

    • Pros: Fast access, no external fragmentation
    • Cons: Index block overhead, additional space requirement

File Fragmentation

File Fragmentation

Regardless of the allocation method used, file fragmentation can occur over time. Fragmentation is the phenomenon where files become scattered across non-contiguous sectors, leading to slower file access. Two types of fragmentation can occur: external fragmentation, as discussed earlier, and internal fragmentation. Internal fragmentation happens when the allocated space within a block is larger than the actual file size, resulting in wasted space.

Which allocation method is best?

The best disk space allocation method depends on the specific needs of the operating system. Contiguous allocation is the simplest and most efficient, but it can lead to fragmentation. Linked allocation is more flexible than contiguous allocation, but it is slower to access files. Indexed allocation is the most flexible and fast, but it is also the most complex to implement.


Efficient disk space allocation is crucial for optimal storage management in operating systems. Various allocation methods, such as contiguous, linked, and indexed allocation, offer different trade-offs in terms of performance, storage utilization, and fragmentation. Operating system designers carefully choose the appropriate allocation method based on the specific requirements of the system and the types of files it needs to handle.

In conclusion, disk space allocation methods play a vital role in ensuring efficient data storage, retrieval, and management. The choice of allocation method should consider factors such as file sizes, access patterns, and system requirements. By implementing the right allocation method, operating systems can optimize storage utilization, enhance performance, and maintain data integrity.

Can an operating system use multiple disk space allocation methods simultaneously?

Yes, some operating systems use combined allocation methods to handle different file sizes effectively. For example, they may use contiguous allocation for small files and indexed or linked allocation for larger files.

Does contiguous allocation suffer from fragmentation?

Yes, contiguous allocation can lead to external fragmentation over time. As files are created, modified, and deleted, free space becomes scattered, making it challenging to allocate contiguous blocks for new files.

What is the advantage of linked allocation over contiguous allocation?

Linked allocation eliminates external fragmentation by allowing files to be stored in non-contiguous blocks. It provides flexible storage utilization by allocating any available free block to a file. However, accessing a specific block within a file requires traversing the linked list, resulting in slower access times.

How does indexed allocation improve file access time?

Indexed allocation uses an index block associated with each file to store pointers to all its blocks. This allows direct access to any block of the file without traversing linked lists. As a result, file access time is faster compared to linked allocation.

What is file fragmentation?

File fragmentation refers to the scattering of file blocks across non-contiguous sectors on a disk. It can lead to slower file access due to the need to access multiple disk locations. Fragmentation can occur both externally, where free space becomes scattered, and internally, where allocated blocks contain wasted space.


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#Disk Space Allocation Methods in OS
#Disk Space Allocation Methods in OS
#Disk Space Allocation Methods in OS
#Disk Space Allocation Methods in OS
#Disk Space Allocation Methods in OS
#Disk Space Allocation Methods in OS
#Disk Space Allocation Methods in OS
#Disk Space Allocation Methods in OS


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