UGC NET CSE | August 2016 | Part 3 | Question: 54

Question

An experimental file server is up $75$% of the time and down for $25$% of the time due to bugs. How many times does this file server have to be replicated to give an availability of at least $99$% ?

• A. $2$ 
• B. $4$
• C. $8$
• D. $16$

Answer 1

Lets us check each option

• a) if there are 2 replications, probability of failing both at same time = .25*.25 = .0625 =6.25%
  Availiability = 100-6.25=93%  //Not the answer
• b) In case of 4 replications probability of failing all at same time =(0.25) ⁴ =0.0039=0.39%
  Availiability = 100-0.39 ~ 99% //Hence the answer

No need to check furthur as all options are more than 4

Answer is B

Answer 2

25%=0.25....down tine

100%-99%=1%... this translates to 0.01

therefore, no. of replications = n log (0.25)= log 0.01

                                              n = (log(0.25)) / ( log (0.01))

                                                  = 4 replications