Index Allocation

It is the Disk allocation technique in which each file has its own index block which is an array of disk block addresses. The n^th entry in the index block points to n^th block of file.

Whenever a file is created its index block is initialized, free block is taken from the disk and its entry is made into index table. whenever  file uses a new block its entry is made into index table. Index table consist of the addresses of all disk blocks on which a particular file is written.

The main advantage of using this technique is that it support direct file access. Another advantage is there is no external fragmentation. There is no need to re define the file size.

Problem associated with this technique is that what should be the size of index blocks? If we use one block for per file index and file is too large than it will not be possible to maintain index on single disk block. To overcome this problem linked index scheme is used where index block are linked together to form a large file index.