Disk Scheduling Quiz 2

Consider the following five disk access requests of the form (request id, cylinder number) that are present in the disk scheduler queue at a given time. (P,155),(Q,85),(R,110),(S,30),(T,115) Assume the head is positioned at cylinder 100. The scheduler follows Shortest Seek Time First scheduling to service the requests. Which one of the following statements is FALSE?

• T is serviced before P.

• Q is serviced after S, but before T.

• The head reverses its direction of movement between servicing of Q and P.

• R is serviced before P.

Disk requests come to a disk driver for cylinders in the order 10, 22, 20, 2, 40, 6 and 38 at a time when the disk drive is reading from cylinder 20. The seek time is 6 ms/cylinder. The total seek time, if the disk arm scheduling algorithms is first-come-first-served is

• 360

• 850

• 900

• None of the above

Consider a storage disk with 4 platters (numbered as 0, 1, 2 and 3), 200 cylinders (numbered as 0, 1, ... , 199), and 256 sectors per track (numbered as 0, 1, ... , 255). The following 6 disk requests of the form [sector number, cylinder number, platter number] are received by the disk controller at the same time: [120, 72, 2] , [180, 134, 1] , [60, 20, 0] , [212, 86, 3] , [56, 116, 2] , [118, 16, 1] Currently the head is positioned at sector number 100 of cylinder 80, and is moving towards higher cylinder numbers. The average power dissipation in moving the head over 100 cylinders is 20 milliwatts and for reversing the direction of the head movement once is 15 milliwatts. Power dissipation associated with rotational latency and switching of head between different platters is negligible. The total power consumption in milliwatts to satisfy all of the above disk requests using the Shortest Seek Time First disk scheduling algorithm is _______.

• 85

• 115

• 155

• 55

Consider the disk system with 100 cylinders. The request to access the cylinders occur in the following sequence. 4, 37, 10,7,19,73,2,15,6,20 Assuming the head is currently at cylinder 50 what is the time taken to satisfy all requests if it takes 1 ms to move from one cylinder to adjacent one and shortest seek ime first algorithm is used.

• 95 ms

• 119 ms

• 233 ms

• 276 ms

Consider a disk queue with requests for I/O to blocks on cylinders 47,38,121,191,87,11, 92, 10. The C-LOOK scheduling algorithm is used. The head is initially at cylinder number 63, moving to wards larger cylinder numbers on its servicing pass. The cylinders are numbered from 0to 199. The total head movement (in number of cylinders) incurred while servicing these requests is____ .

• 346

• 146

• 546

• 446