describe advantages and disadvantages of each allocation method for disk space

 

contiguous allocation

advantage:

Accessing a file that has been allocated contiguously is easy.For sequential access,the file system remembers the disk address of the last block;

It can prevent loss of significant amounts of disk space to external fragmentation.

disadvantage:

It's difficult to find space for a new file;

All files are allocated and deleted,the free disk space is broken into little pieces so it can't store big data;

It can be a problem to determining how much space is needed for a file ;

It allocate too little space to a file

preallocation may be inefficient

 

Linked Allocation

advantage:

It solves all problems of contiguous allocation.

A file can continue to grow as long as free blocks are available.

The use of file-allocation table

disadvantage:

The major problem is that it can be used effectively only for sequential-access files.

The space required for the pointers.

Linked allocation cannot support efficient direct access.

 

 

Indexed Allocation

advantage:

Indexed Allocation solved the problem that linked allocation cannot solve by bringing all the pointers together into one location:

the index block

Indexed allocation supports direct access,without suffering form external fragmentation.

disadvantage:

It's difficult to determining how large the index block should be