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Consider a disk I/Osystem in which an I/0 request arrives at a rate 100 IOPS. The service time Rs=8ms.

Calculate the following measures of disk performances:
1) Utilisation of I/O controller (U).
2) Total Response Time (R).
3) Average Queue Size.
4) Total time spent by request in the Queue.

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Given :

Arrival Rate (a) = 100 IOPS, So

$R_a = \frac{1}{a} = \frac{1}{100}$

$= 0.01 \times 10^2 = 10 ms$

Rs = 8 ms Given

(a) Utilization of i/o controller is

$U = \frac{Rs}{Ra} = \frac{8}{10} = 0.8$ i.e. 80%

(b) Total Response time $(R) = \frac{Rs}{1-U}$

$ \frac{8}{1-0.8} = 40 ms$

(C) Average Queue Size $= \frac{U^2}{1-U}$

$\frac {(0.8)^2}{1-0.8} = 3.2 ms$

(d) Total time spent by request in Queue = U x R

= 0.8 x 40 = 32 ms

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