aims:

1.Effective use of disk space

2.Quick access to files

 

There are three common types of distribution methods:

1.Continuous allocation

2.Link assignment

3.Index allocation

 

Contiguous allocation

The continuous allocation method requires each file to occupy a contiguous block of disks.

 

advantage:

1. Simple - just record the starting position (block number) and length of the file.


2. Accessing files is easy and requires minimal seek time

Sequential access

direct interview.

 

Disadvantages:

1. It is difficult to find space for new documents


2. For the first time, the best, the worst


3. The file is difficult to grow

 

Linked allocation

Link assignments solve all the problems of continuous allocation.

 

With link assignment, each file is a linked list of disk blocks:

• Disk blocks are distributed anywhere on the disk.


• Each directory entry has a pointer to the first block of the file.


• The pointer is initialized to nil (the value of the footer pointer) to identify an empty file.

 

advantage:

1. Simple - just the starting position


2. File creation and growth is easy

 

Disadvantages:

1. Can not be randomly accessed


2. The link pointer between the block and the block takes up space


3. There are reliability issues

 

Indexed allocation

Features:

1. Put all the data block pointers into the index block

The ith entry in the index block points to the ith block of the file.

• The directory entry includes the address of the index block

2. Each file has an index block;

• Array of disk block addresses

3. Index allocation supports direct access, and there is no external fragmentation problem


4. The index block itself may waste space

Link scheme: An index block is usually a disk block.For large files, multiple index blocks can be linked together.

Multi-layer index: Indirect addressing similar to memory (primary, secondary, indirect...)