File Allocation Methods


File allocation methods refer to the strategies employed by computer operating systems for the efficient distribution of storage space on disks or other storage media. Their main objective is to optimize the utilization of available space and minimize fragmentation, which can impede file access and decrease the overall performance of the system. There are several different file allocation methods that are commonly used, each with its own strengths and weaknesses.

Contiguous File Allocation

In this method, files are stored in a continuous block of free space on the disk meaning that all the data for a particular file is stored in one continuous section of the disk. When a file is created, the operating system searches for a contiguous block of free space large enough to accommodate the file. If such a block is found, the file is stored in that block, and the operating system keeps track of the starting address and the size of the block.

The advantage of contiguous file allocation is that it provides fast access to files, as the operating system only needs to remember the starting address of the file. When a user requests access to a file, the operating system can quickly locate the file's starting address and read the entire file sequentially. This method is particularly useful for large files, such as video or audio files, which can be accessed more quickly when stored in contiguous blocks.

However, contiguous file allocation has some limitations. One significant disadvantage is that it can lead to fragmentation when files are deleted or when new files are created. If a file is deleted, the space it occupied becomes free, but that space may not be contiguous with the remaining free space on the disk. This can result in gaps or fragments of free space scattered throughout the disk, making it difficult for the operating system to find contiguous blocks of free space for new files.

Linked File Allocation

In this method, files are stored in non-contiguous blocks of free space on the disk, and each block is linked to the next block using a pointer. When a file is created, the operating system searches for a series of free blocks that are large enough to store the file, and it links them together using pointers. Each block contains the address of the next block in the file, allowing the operating system to access the entire file by following the chain of pointers.

The advantage of linked file allocation is that it can accommodate files of any size, as the file can be stored in multiple non-contiguous blocks. This method also avoids fragmentation, as files can be stored in any available free space on the disk, without the need to find a contiguous block of free space.

However, linked file allocation has some limitations. One significant disadvantage is that it can result in slower access times to files, as the operating system needs to follow the chain of pointers to access the entire file. This method may also require more disk space, as each block contains a pointer to the next block in the file. Additionally, if a pointer becomes damaged or lost, it can result in the loss of the entire file, as the operating system cannot access the entire chain of blocks.

Indexed File Allocation

To address some of the limitations, operating systems can use a variation of linked file allocation called indexed file allocation. In indexed file allocation, files are stored in noncontiguous blocks, but instead of linking each block together, the operating system creates an index block that contains a list of pointers to each block in the file. When a file is created, the operating system searches for a series of free blocks that are large enough to store the file and creates an index block that contains pointers to each of those blocks. Each block of the file is then stored in a separate block on the disk.

The advantage of indexed file allocation is that it provides fast access to files, as the operating system only needs to read the index block to locate the file's blocks. This method also avoids fragmentation, as files can be stored in any available free space on the disk, without the need to find a contiguous block of free space. Indexed file allocation also reduces the risk of data loss, as the index block can be duplicated to provide redundancy.

However, indexed file allocation has some limitations. One significant disadvantage is that it can result in wasted disk space, as the index block can take up a significant amount of space on the disk. This method also requires more disk space than linked file allocation, as each block of the file is stored separately on the disk.

There are several types of indexed file allocation methods used in computer operating systems, each with its own strengths and weaknesses −

  • Single-Level Index − This method is the simplest form of indexed file allocation. In this method, a single index block is created for each file, and it contains pointers to the blocks that make up the file. This method is useful for small files but can become inefficient for larger files as the index block can take up a significant amount of space.

  • Multi-Level Index − This method is an improvement over the single-level index method. In this method, multiple index blocks are used to store the pointers to the blocks that make up the file. The first level index block contains pointers to the second level index blocks, and so on. This method is useful for large files as it reduces the size of each index block and allows for faster access to the file.

  • Combined Index − This method combines the benefits of both contiguous and indexed file allocation methods. In this method, a portion of the file is stored contiguously, and the rest is stored using indexed file allocation. The contiguous portion of the file is accessed quickly, while the indexed portion can accommodate files of any size.

  • Linked Index − This method is similar to linked file allocation, but instead of linking blocks of the file together, an index block is created that contains pointers to the next index block. Each index block contains pointers to the data blocks that make up the file. This method is useful for large files, but it can result in slower access times to the file.

  • Inverted Index − This method is used in databases to store indexes of records. In this method, a separate index block is created for each record type, and each block contains pointers to the data blocks that contain records of that type. This method is useful for fast access to specific types of records.

File Allocation Table

File Allocation Table (FAT) is a file system that uses a table to store information about the allocation of files on a disk or other storage media. In a FAT file system, the file allocation table is a data structure that contains a list of entries, each of which represents a block of storage space on the disk. The entries in the file allocation table indicate whether a block of storage space is free or allocated, and if it is allocated, they indicate which file or directory the block is associated with. When a file is created, the operating system searches for a series of free blocks of storage space on the disk and records the allocation of these blocks in the file allocation table. As the file is modified or expanded, the operating system updates the entries in the file allocation table to reflect the new allocation of blocks.

The FAT file system has several advantages. It is a simple and efficient file system that is well-suited to small disks and low-powered devices. It is also widely supported by many operating systems and can be used on a variety of storage media, including hard disks, floppy disks, and flash drives.

However, the FAT file system also has some limitations. It can be susceptible to file fragmentation, where files become fragmented across multiple non-contiguous blocks of storage space on the disk. This can slow down file access times and reduce overall system performance. Additionally, the file allocation table can become corrupted, leading to data loss or disk errors.

Conclusion

File allocation methods are an important aspect of computer operating systems, as they determine how files are stored and accessed on disk or other storage media. These methods are designed to ensure the efficient storage of files and data, which is essential for the seamless functioning of computer systems. The selection of a file allocation method depends on various factors such as the number and size of the files that need to be stored, the specifications of the operating system and applications, and the speed and capacity of the storage media. The chosen file allocation method should be able to cater to the storage requirements while simultaneously preventing data fragmentation.

Updated on: 07-Apr-2023

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