What is the index-file allocation of disk space?

Disk allocation describes how files are stored inside a disk. One type of disk allocation is index-file disk allocation. We use these disk allocations mainly to ensure efficient disk space utilization and faster access to the disk blocks.

Index file allocations

Index-file allocation contains a special disk block known as the index block. This moves all the pointers to other blocks to one location inside the disk. Each file inside the memory has its index block. The element in the index block corresponds with the disk address in the respective file block.

Sequential and random disk access first requires access to the index block before it can read the file content.

Inside the disk

The following diagram illustrates a virtual flow of index file allocation inside the disk:

In the above diagram, the "educative" file has an index block at position 14. It contains pointers to all the disk blocks. First, we read the index block, and then read the disk blocks the pointers point to.

Advantages

Some of the advantages of index file disk allocation are as follows:

• There is no external fragmentation.

• There is no file size limitation.

• Since it has direct access to the file blocks, it provides faster access to the disk blocks.

Disadvantages

Some of the disadvantages of index file disk allocation are as follows:

• There is a trade-off for selecting the index block size. On the one hand, we want the index block to be as small as possible, since every file requires an index block. On the other hand, if the index block is too small, it is not able to hold enough pointers for larger files.

• It has a pointer overhead that burdens the memory resources.

Schemes

To tackle the aforementioned trade-off, we can use the following schemes to accommodate larger files:

• Linked scheme: When the file outgrows the index file, we link another index block to it.

• Multilevel index scheme: In this scheme, a first-level index block points to a set of second-level index blocks, which then points to the file blocks.

• Mixed scheme: In this scheme, the first twelve pointers point to direct blocks. Data for small files does not require a separate index block. The next three pointers point to indirect blocks: single indirect, double indirect, and triple indirect blocks.

The following figure illustrates how a mixed scheme works: