Indexed Allocation in File Systems: Efficient File Access and Storage Management

Learn about indexed allocation, a method used in file systems to improve file access efficiency. This guide explains how indexed allocation uses index blocks to store disk block pointers, its advantages over FAT (File Allocation Table), and its impact on file system performance and storage utilization.

Indexed Allocation in File Systems

Limitations of File Allocation Tables (FAT)

File Allocation Tables (FATs), while improving upon linked list allocation, still have limitations. As the number of blocks on a disk increases, so does the size of the FAT. This consumes significant storage space and can lead to performance issues, especially if the FAT needs to be cached in RAM, where space is also limited. This makes FAT inefficient for disks with a huge number of files or large files.

Indexed Allocation

Indexed allocation addresses these limitations by using an index block to store all the disk block pointers for a single file. This means that the directory entry for a file only needs to store the address of the index block, rather than pointers to all the blocks in the file. This makes accessing any part of the file extremely efficient.

Advantages of Indexed Allocation

• Direct Access: Allows random access to any block in the file.
• Improved Reliability: A bad data block only affects that one block, unlike FAT, where a corrupted FAT entry could potentially make an entire file inaccessible.

Disadvantages of Indexed Allocation

• Index Block Failure: Corruption or failure of the index block results in loss of the entire file.
• File Size Limitation: The maximum file size is limited by the number of pointers the index block can hold.
• Wasted Space for Small Files: Using an index block for a very small file is inefficient.
• Increased Pointer Overhead: Requires storing pointers to all data blocks.