# Recent questions tagged disk-scheduling

1
Consider a disk system having $60$ cylinders. Disk requests are received by a disk drive for cylinders $10,22,20,2,40,6$ and $38$, in that order. Assuming the disk head is currently at cylinder $20$, what is the time taken to satisfy all the ... cylinder to adjacent one and Shortest Seek Time First (SSTF) algorithm is used? $240$ milliseconds $96$ milliseconds $120$ milliseconds $112$ milliseconds
2
Disk request 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 $6\text{ ms}$ per cylinder. The total seek time, if the disk arm scheduling algorithm is first-come-first-served is $900$ ms $850$ ms $360$ ms $876$ ms
3
A disk has $200$ tracks (numbered $0$ through $199$). At a given time, it was servicing the request of reading data from track $120$, and at the previous request, service was for track $90$ ... (Shortest Seek Time First) and FCFS (First Come First Serve)? $2$ and $3$ $3$ and $3$ $3$ and $4$ $4$ and $4$
1 vote
4
Consider a disk queue with I/O requests on the following cylinders in their arriving order: $6,10,12,54,97,73,128,15,44,110,34,45$. The disk head is assumed to be at cylinder $23$ and moving in the direction of decreasing number of cylinders. Total number of cylinders in the disk is $150$. The disk head movement using SCAN –scheduling algorithm is: $172$ $173$ $227$ $228$
5
Consider the following five disk 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 ... $S$,but before $T$. The head reverses its direction of movement between servicing of $Q$ and $P$. $R$ is serviced before $P$.
6
Write a program to implement the three disk-arm scheduling algorithms. Write a driver program that generates a sequence of cylinder numbers $(0–999)$ at random, runs the three algorithms for this sequence and prints out the total distance (number of cylinders) the arm needs to traverse in the three algorithms.
7
A slight modification of the elevator algorithm for scheduling disk requests is to always scan in the same direction. In what respect is this modified algorithm better than the elevator algorithm?
8
Disk requests come in to the disk driver for cylinders $10, 22, 20, 2, 40, 6,$ and $38,$ in that order. A seek takes $6\: msec$ per cylinder. How much seek time is needed for First-come, first served. Closest cylinder next. Elevator algorithm (initially moving upward). In all cases, the arm is initially at cylinder 20.
9
Consider a disk system with $100$ cylinders. The requests to access the cylinders occur in the following sequences: $4, 34, 10, 7, 19, 73, 2, 15, 6, 20$ Assuming that the head is currently at cylinder $50$, what is the time taken to satisfy all requests if it takes $1$ ms to move from the cylinder to adjacent one and the shortest seek time first policy is used? $357$ ms $238$ ms $276$ ms $119$ ms
10
Compare the performance of $C-SCAN$ and $SCAN$ scheduling, assuming a uniform distribution of requests. Consider the average response time (the time between the arrival of a request and the completion of that request’s service), the variation in response time, and the effective bandwidth. How does performance depend on the relative sizes of seek time and rotational latency ?
11
Suppose that a disk drive has $5,000$ cylinders, numbered $0$ to $4,999$. The drive is currently serving a request at cylinder $2,150$, and the previous request was at cylinder $1,805$. The queue of pending requests, in $FIFO$ ... for each of the following disk-scheduling algorithms ? $a. FCFS$ $b. SSTF$ $c. SCAN$ $d. LOOK$ $e. C-SCAN$ $f. C-LOOK$
12
Explain why $SSDs$(Solid State Drives) often use an $FCFS$ disk-scheduling algorithm.
13
None of the disk-scheduling disciplines, except $FCFS$, is truly fair (starvation may occur). a. Explain why this assertion is true. b. Describe a way to modify algorithms such as $SCAN$ to ensure fairness. c. Explain why fairness is an important goal in a ... d. Give three or more examples of circumstances in which it is important that the operating system be unfair in serving $I/O$ requests.
14
Why is rotational latency usually not considered in disk scheduling ? How would you modify $SSTF$, $SCAN$, and $C-SCAN$ to include latency optimization ?
15
Explain why $SSTF$ scheduling tends to favor middle cylinders over the innermost and outermost cylinders.
16
Is disk scheduling, other than $FCFS$ scheduling, useful in a single-user environment ? Explain your answer.
17
Disk request come into disk driver for cylinders 5, 17, 60, 125, 28, 170, 8, 32. Total moves using SCAN algorithm when disk head is currently positioned at 35 and moving toward higher cylinder number________.(Disk system has 200 cylinders (0 - 199)
18
1 vote
19
1 vote
20
I got LOOK ==> 260 CSCAN ==> 383
1 vote
21
Suppose a disk has 100 cylinders, numbered from 0 to 99. At some time the disk arm is at cylinder 20, and there is a queue of disk access requests for cylinders 10, 22, 6, 38, 2, 40, 20. No of seeks in FCFS SSTF
22
How does LOOK algorithm decides that this is the last request? What technique is used to determine that after this request, instead of going to the end of disk, the direction should be changed?