Consider a disk that is scheduled using the Elevator policy. That is, the disk head scans from the first cylinder to the last and back repeatedly, processing I/O requests along the way. However, the scan direction changes whenever there are no I/O requests in the disk queue in the direction of the scan, but there are I/O requests in the disk queue in the opposite direction. Let the disk have an average rotational latency of 0.5 time units and transfer time of 0.1 time units per sector. The seek time for processing a request is 0, if the request is in the same cylinder that the disk head is currently in. Otherwise, let the seek time be 1+(k-1)*0.1 time units, where k is the number of cylinders travelled.
Let the following table represent a stream of I/O requests for the disk:
Request Arrival Time Cylinder Number of Sectors
R1 t 36 2
R2 t+4 15 5
R3 t+5 25 2
R4 t+6 4 1
R5 t+9 98 5
R6 t+12 36 2
R7 t+15 27 1
R8 t+16 57 5
Assume that the disk head is at cylinder 20 at time t, and no other requests enter the disk queue until all of the above requests are processed. Determine the schedule of processing for the above stream of requests.
Based on the processing schedule, compute the wait times and turnaround times for the various requests. Note that the wait time for a request is the difference between the time at which the disk starts processing the request and the time at which the request arrives; the turnaround time is the difference between the time at which the request processing is completed and the arrival time of the request.
Identify the true statement among the following, with respect to the wait and turnaround times of the various requests.
A. R7 has the largest turnaround time.
B. R3 has the largest wait time.
C. R3 has the largest turnaround time.
D. R5 has the largest turnaround time.