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Find the number of head movements in cylinders using SCAN scheduling.

Consider the following disk request sequence for a disk with 100 tracks.

98, 183, 37, 122, 14, 124, 65, 65, 67

Head pointer starting at 53 (current position of R/W heads) and moving in left direction.Find the number of head movements in cylinders using SCAN scheduling.

3Comments
kn nk @knkn
16 Mar 2016 06:28 pm

if head is moving in left direction then
head movements in left direction 53 -> 14 = 39
head movement in right direction 14 -> 183=169
total head movements = 39 +169 = 208

While in SCAN scheduling,
head movements in left direction 53 ->183 = 130
head movement in right direction 183 -> 14=169
total head movements = 130 +169 = 299

Jitendra Verma @jitendraverm
16 Mar 2016 11:30 pm

Wrong analysis.It's the SCAN algorithm you're implementing LOOK Scheduling.

Jitendra Verma @jitendraverm
16 Mar 2016 11:29 pm

In SCAN Scheduling,the disk arm starts at one end of the disk,and moves towards the other end,servicing requests until it gets to the other end of the disk,where the head movement is reversed and servicing continues.

Here in this question,R/W head is at  53 position and moving in left direction so it will serve all the requests till 14.

Take a look.

53-->37

37-->14

Now it will reach the end of the disk so this will also be a track movement.

14-->0

Again again it will start servicing the request in opposite direction.

0-->65

65-->67

67-->98

98-->122

122-->124

124-->183.

So,total head movements will be 236.