1.Contiguous Allocation:

Advantages:

1.Easy to access. This enables system to access sequential files easily.

2.Hight speed of sequential access.

Disadvantages:

1.Required to distribute sequential space for a file.

2.Have to know file’s length in advance.

3.Can’t delete and insert the record flexibly.

4.For those files which were growing dynamically, it’s hard to figure out the file size at the beginning.

 
2.Linked Allocation

Advantages:

1.Eliminates external fragmentation of the disk, boosts the use ratio of external memory.

2.Easy to insert, delete and modify the records.

3.It’s able to fix the dynamic growth and needn’t to calculate the file size in advance.
Disadvantages:

1.Only valid for sequential access to files, does not support direct access effectively.

2.The pointer will occupy space and waste disk space.

3.Insufficient reliability due to the possible loss or damage of the pointer.

4.Low performance while searching a file with big size.


3.Index Allocation

Advantages:

1.This supports direct access to the blocks occupied by the file and therefore provides fast access to the file blocks.

2. It overcomes the problem of external fragmentation.

Disadvantages:

1.The pointer overhead for indexed allocation is greater than linked allocation.

2.For very small files, say files that expand only 2-3 blocks, the indexed allocation would keep one entire block (index block) for the pointers which is inefficient in terms of memory utilization.