SSTF:
Initial position 50. So, shortest sequence will be
50 45 60 70 80 90 25 20 10
and the corresponding distances will be
0 5 15 10 10 10 65 5 10
giving total distance of 130.
SCAN:
Here requests from 50 are serviced in ascending order of their track number (as the movement is from 50 to 100) and at the end of the disk ( not sequence as in LOOK scheduling), the remaining requests are serviced in the descending order. So, the service order will be
50 60 70 80 90 100 45 25 20 10
and the corresponding distances
0 10 10 10 10 10 55 20 5 10
giving a total distance of 140
So, extra distance of SSTF = 130 - 140 = -10
SOURCES ::
http://www.cs.iit.edu/~cs561/cs450/disksched/disksched.html
http://www4.comp.polyu.edu.hk/~csajaykr/myhome/teaching/eel358/ds.pdf