disk head scheduling

Question 1

what is the aim of disk scheduling and what three things are we trying to improve

Answer 1

ordering requests to the disk so its moving its head in an optimal way
fairness, throughput and response time

Question 2

what are some of the disk scheduling techniques

Answer 2

fcfs: first come first served
sstf/ssf: shortest seek time first
scan/elevator scheduling
cscan: circular scan
edf: earliest deadline first
scan + edf

Question 3

scan + edf

Answer 3

priority based with the head moving in one direction

Question 4

how does fcfs work

Answer 4

always completing the first request in the queue

Question 5

what are 2 positives of fcfs

Answer 5

shortest response time and fair as no request is ever being held back in the queue

Question 6

what are negatives of fcfs

Answer 6

leads to longer seek times due to the random head movements
also poor throughput as the head may have to travel a while to get the next request

Question 7

sstf/ssf

Answer 7

find the request closest to the one just serviced so the head moves the shortest distance possible

Question 8

what are positives of ssft/ssf

Answer 8

smoother head movements
short seek times and high throughput as the head is moving as little as possible

Question 9

what are negatives of sstf/ssf

Answer 9

each process has longer response times as other processes can jump ahead
the head usually stays around one position (middle)
can lead to starvation as processes not close to the others wont get serviced

Question 10

scan/elevator

Answer 10

go for the shortest head movement in one direction until no more requests in that direction

Question 11

what is a positive of scan

Answer 11

no starvation

Question 12

what is a negative of scan

Answer 12

could be unfair as requests clustered at the edge of the disk may get two chances of data access

Question 13

cscan

Answer 13

the head should always move in one direction and jumps back to the opposite side when all requests on that side are complete

Question 14

+ cscan

Answer 14

improves fairness

Question 15

• cscan

Answer 15

takes longer due to going back
so lower throughput

Question 16

edf

Answer 16

chooses the request with the highest priority and if there are multiple then it reads the closest one

Question 17

what is edf used for

Answer 17

real time systems

Question 18

soft real time system + example

Answer 18

missing the deadline is file
e.g. playing a video

Question 19

hard real time system

Answer 19

achieving deadlines is critical

Question 20

• edf

Answer 20

long seek times
long deadlines may have to wait a long time