MadeEasy Test Series: Operating System - Disks

Question

Consider linked allocation system, a program has just read the 15th disk block. After some point of time, If it next want to use the 20th   block, then how many disk blocks must the program access?

a: 5

b: 10

c:15

d:20

Comments

@VS:- The key point here is after some point of time, 

So we don't know after some point of time whether we are at 15 or some other.And in that case we will take the worst  case.We will start from 1 block and access 20 blocks.

I questions says,we are and now from 15 we need to access 20th block,then 5 will be correct

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@rahul sharma 5

But, once we reached 15 th block why can't we store the addr of 15th block in a separate variable and then use it latter on !

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@VS:- May be some insertion deletion done and that 15 block address is not a valid address now.Assume you are at 10 th node at linked list.Now after some time you need to goto 15th node.You dont know where you are after some one.Begining or and or middle(anywhere).Even if you save 10th node address earlier,then whether it is valid now or not,we dont know.

Answer 1

 Linked List Allocation

In this scheme, each file is a linked list of disk blocks which need not be contiguous. The disk blocks can be scattered anywhere on the disk.

Therefore we need 20 Block Access In Worst Case

Answer 2

Linked allocation follows Sequential Access.  So clearly if it next want to access the n'th   block, then it must have to start its access from first block. So to read 20th block in linked allocation , the reading will start from 1st block and total 20 disk block should be access here.