Lesson 4: Disk Scheduling

Question 1

—storage device that uses a magnetization process to write, rewrite and access data
—covered with a magnetic coating and stores data in the form of tracks, spots and sectors
—provide the bulk of secondary storage for modern computer systems

Answer 1

magnetic disk

Question 2

common examples of magnetic disks

Answer 2

Hard disks, zip disks, floppy disks

Question 3

—polyethylene tape coated with a magnetic particulate or film medium
—storage medium that allows for data archiving, collection, and backup

Answer 3

magnetic tapes

Question 4

—the density of bits per track is uniform on media using it
—method used in CD-ROM and DVD-ROM drives

Answer 4

Constant Linear Velocity (CLV)

Question 5

—method used in hard disks
—disk rotation speed can stay constant, and the density of bits decreases from inner tracks to outer tracks to keep the data rate constant

Answer 5

Constant Angular Velocity (CAV)

Question 6

—done by Operating Systems to schedule I/O requests arriving for the disk
—also known as I/O Scheduling

Answer 6

Disk Scheduling

Question 7

Two major components of access time (SR)

Answer 7

Seek time
Rotational latency

Question 8

[Components of access time] the time for the disk arm to move the heads to the cylinder containing the desired sector

Answer 8

Seek time

Question 9

[Components of access time] the additional time for the disk to rotate the desired sector to the disk head

Answer 9

Rotational latency

Question 10

the time to transfer the data

Answer 10

Transfer Time

Question 11

Formula for Disk Access Time

Answer 11

Seek time + Rotational latency + transfer time

Question 12

Disk Scheduling Algorithms (F/S/S/CS/L/CL)

Answer 12

FCFS = First come, first served
SSTF = Shortest Seek Time First
SCAN
C-SCAN = Circular SCAN
LOOK
C-LOOK = Circular LOOK

Question 13

—simplest of all the Disk Scheduling Algorithms
—the requests are addressed in the order they arrive in the disk queue

Answer 13

FCFS

Question 14

—requests having shortest seek time are executed first
—requests near the disk arm will get executed first

Answer 14

SSTF

Question 15

the disk arm moves into a particular direction and services the requests coming in its path and after reaching the end of disk, it reverses its direction and again services the request arriving in its path

Answer 15

SCAN

Question 16

What is SCAN also known as?

Answer 16

elevator algorithm

Question 17

modified version of SCAN disk scheduling algorithm that deals with the inefficiency of SCAN algorithm by servicing the requests more uniformly

Answer 17

C-SCAN

Question 18

What is C-SCAN also known as?

Answer 18

Circular Elevator Algorithm

Question 19

—similar to the SCAN disk scheduling algorithm except for the difference that the disk arm in spite of going to the end of the disk goes only to the last request to be serviced in front of the head and then reverses its direction from there only
—prevents the extra delay which occurred due to unnecessary traversal to the end of the disk

Answer 19

LOOK

Question 20

—the disk arm in spite of going to the end goes only to the last request to be serviced in front of the head and then from there goes to the other end’s last request.
—prevents the extra delay which occurred due to unnecessary traversal to the end of the disk.

Answer 20

C-LOOK