Contiguous Allocation
Advantage

1.It's easy to access a file.Both aequential and direct access can be supported by contiguous allocation.
Disadvantage
1.It's hard to find space for a new file.

2.Another problem is to determine the space for a new file, we don't know the size of a file. It may costs a lot of waste of the memory and time.

 

Link Allocation
Advantage

1.There is no external fragmentation with linked allocation,and any free block on the free-space list can be used to satisfy a request.

2.The size of a file need not be declared when that file is created.A file can continue to grow as long as free blocks are available.It's never necessary to compact disk space.

Disadvantage

1.It can only be used only for sequential-access files.It's inefficient to support a direct-access capbility for linked-allocation files.

2.Another disadvantage is the space required for the pointers.Each file required slightly more space than it would otherwise.

3.The third problem is that the reability of this method is low, since the pointer may lost.

 

Index Allocation
Advantage

1. It's an improvment to the contiguous allocation, it collects each pointers of a file and list them together by sequence.It get rid of the fragement problem.

Disadvantage

1.The shortcoming is that it can't support random access. And the size of a file is determined by the size of index blocks, several methods like multilevel index is needed to solve the problem.