Andrew S. Tanenbaum (OS) Edition 4 Exercise 4 Question 18 (Page No. 334)

Question

Consider a file whose size varies between $4\: KB$ and $4\: MB$ during its lifetime. Which of the three allocation schemes (contiguous, linked and table/indexed) will be most appropriate?

Comments

table/indexed?

Answer 1

Contagious Allocation – 

• This is not a preferred method for allocation if the file size varies.
• It requires a contagious sequence of blocks large enough to handle the entire file.
• Since the size of the file specified in this question varies, either you have to pre-allocate 4 MB of space (which could result in wastage if the file size is minimum), or the file will need to be copied whenever it grows, to a contagious block of free space.
• Supports Random access.

Linked Allocation – 

• This is a preferred way for the above file if random access is not needed, rather sequential access is needed.
• This method does not require contagious blocks of free space, rather it will link new blocks whenever a new block is needed from the free blocks available.
• Does not support Random Access.

Indexed Allocation – 

• This is also a preferred way for the above file if random access is needed.
• A table called Index Table is used to store all the physical addresses of the data blocks of a file.
• Blocks can be added or removed as per needed.
• More complex than linked allocation.
• Overhead for storing the Index Table is huge (greater overhead than Linked Allocation).

As the question does not specify if it requires a random access or not, Linked Allocation is the most appropriate answer considering the file is accessed sequentially.