1.Accessing a file that has been allocated contiguously is easy.Sequential and direct access can be supported by contiguous allocation.

One difficulty is finding space for a new file.All these algorithms suffer from the problem of the external fragmentation.Another problem with contiguous allocation is determining how much space is needed for a file.

2.Linked allocation solves the external fragmentation and size declaration problems of the contiguous allocation.

It is inefficient to support a direct-access capability for linked-allocation files.Another disadvantage is the space require for the pointers.Yet another problem of linked allocation is reliability.

3.Indexed allocation supports direct access,without suffering from external fragmentation.

Indexed allocation does suffer from wasted space.The pointer of overhead of the index block is generally greater than the pointer of overhead of the linked allocation.