File Allocation Methods
File allocation is an essential part of the computer science working in the operating system and file system. It decides where files are written, read and how they are controlled or stored on storage media including the hard disk, SSD, and other types of memory. Understanding file allocation methods is useful in the design of storage solutions and if there is a problem with storage performance .
This blog aims to provide an insight to the layman on the basic file allocation techniques, its usages and pros and cons of each methods used by operating system. , and also solve five common questions people have about file allocation to reduce confusion.
Introduction to File Allocation
File allocation encompasses a method by which operating systems assign files on a disk. Every file is associated with some sort of space on the storage device and the way that space is managed and allocated can cause significant performance differences.
The primary goals of file allocation methods are:
- Efficient Space Utilization: Minimizing wasted space.
- Quick Access: Bringing file access time closer to the time required by online applications.
- Scalability: I support small as well as large files with same efficiency.
- Robustness: Creating an abridged back-up method of hard disk in the event the entire system malfunctions.
Thus in order to accomplish these goals, various methods of insourcing and outsourcing have been established which have each own distinctive strategy and compromise.
Types of File Allocation Methods
1. Contiguous Allocation
In consecutive allocation, each file is stored in a single segment of the memory, one after another. The staring address and the number of bytes allocated to the file is stored in the directory entry of the file.
How it Works:
- There is the file system that looks for an empty block big enough to fit the said file.
- Once located it assigns the entire block to the file.
Advantages:
- Fast Access: Since the data of the file is stored in a cluster together with all parts of it, consecutive data access is very efficient.
- Simple Implementation: The file system can very much compute where in the file any part of the file is using mere calculations.
Disadvantages:
- External Fragmentation: Periodically, free space is scattered all over the place in the disk and getting large free space becomes a problem.
- Difficult File Expansion: In other words, if a file has to take more space, then the Alberta scheme requires that it be relocated to a block of a larger size, which is a lengthy process.
Use Cases:
Contiguous allocation is well suited for those systems where files are moderate in size and the sizes are predetermined such as the embedded operating system or some early operation systems.
2. Linked Allocation
In linked allocation each file is spread on the disk such that each piece of the file is linked with the other part of the same file. Moreover, every block has a link to the next block of text in the sequence.
How it Works:
- The file system begins the file by providing the first block in the file system block storage table.
- Every block is linked with the next one, hence they make a chain like data structure.
- In general, the last block of the chain includes a special sign to mark the file is completed.
Advantages:
- No External Fragmentation: It means that any free block can be used for storing files and it is impossible to spend free space for some other purpose.
- Easy File Expansion: Files can just increase by extension, where other blocks are just added to the chain.
Disadvantages:
- Slower Access: All the above means that to get to one part of the file, one may have to traverse the chain which can be cumbersome.
- Increased Overhead: In order to implement the linked blocks, each block has to store a pointer which takes up space, thus decreasing it’s raw storage capacity.
- Potential for Broken Links: This includes the fact that if a pointer is corrupted then parts of the file cannot be accessed.
Use Cases:
Linked allocation applies where systems require compactness more than accessibility such as storing logs.
3. Indexed Allocation
Indexed allocation involves the use of an index block to contain pointers to each of the blocks allocated to each file. This means that the means that availability of storage does not have to be in continuous blocks or make use of linked lists.
How it Works:
- Each file has an index block that contains an array of pointers.
- Each pointer corresponds to a block of data on the disk.
Advantages:
- Direct Access: A data structure must be kept at one place, giving direct access to the contents of files.
- No External Fragmentation: As with linked allocation, any free block may be taken.
Disadvantages:
- Index Block Overhead: In a particular case, the volume of the index block can be comparable to the file’s size especially when a file is small.
- Complex Management: Changes for the maintenance of the index block and the access to the block are complex.
Use Cases:
It is also effective when many people read large files with the need to jump around the record in a file, for instance, database or multimedia files.
Comparison of File Allocation Methods
Aspect | Contiguous Allocation | Linked Allocation | Indexed Allocation |
---|---|---|---|
Access Speed | Fast | Slow (Sequential) | Fast (Random) |
Space Efficiency | Low | High | Medium |
File Expansion | Difficult | Easy | Moderate |
Fragmentation | External | None | None |
Complexity | Low | Medium | High |
Applications of File Allocation
- Operating Systems: Allocation techniques are employed in such file systems as FAT (File Allocation Table) where linked, and NTFS (New Technology File System) where indexed, allocation techniques are applied.
- Multimedia Storage: Indexed allocation is special used for processing large multimedia files.
- Log Management: This kind of allocation is effective for a system that produces sequential log data.
- Embedded Systems: Continuous allocation is applicable in those devices for which simplicity is essential.