Contiguous Allocation

Each file occupy a set of contiguous blocks on the disk

 

Advantages:

The number of disk seeks required for accessing contiguously allocated files is minimal,as is seek time when a seek isfinally needed.

Accessing a file that has been allocated contiguously is easy

 

Disadvantages:

One difficulty is finding space for a new file ,which is also called dynamic storage-allocation problem

Determing how space is needed for a file.We don't know the size of the file to be allocated,It will cause some problems

 

Linked Allocation

Each file is a likned list of disk blocks.Each block contains a pointer to the next block

 

Advantages:

solves all problems of contiguous allocation

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

 

Disadvantages:

It can be used effectively only for sequential-access files.It is in efficient to support a direct-access capability for linked-allocation files

It need the space required for the pointer

Reliability,the pointer may lost.

 

Indexed Allocation

solves this peoblem by bringing all the pointers together into one location:the index block

 

Advantages:

indexed allocation supports direct access, without suffering from external fragmentation.

Disadvantages:

we don't know how large the index block  should be.Some method will have to be available to deal with the issue