L-6: Hard Disk

L-6.1: Hard Disk Architecture in Operating System in Hindi

Hard Disk Architecture

Rotation -> Uni-direction -> Clockwise or Anticlockwise
Platter -> Surface -> Track -> Sectors -> Data
Total Disk Size = No. of Platters x No. of Surface in Each Platter x No. of Tracks in Each Surface x No. of Sector in Each Track x Data Per Sector

For Calculations Remember Powers of 2

2^1=2, 2^2=4, 2^3=8, 2^4=16, 2^5=32, 2^6=64, 2^7=128, 2^8=256, 2^9=512, 2^10=1024, 2^11=2048, 2^12=4096, 2^13=8192, 2^14=16384, 2^15=32768

1 K = 2^10 ~ 10^3
1 M = 2^20 ~ 10^6
1 G = 3^30 ~ 10^9
1 T = 2^40 ~ 10^12

Ques: Find No. of bits Required and Memory Size of the Disk With. 8 Platters, 256 Tracks on each surface, 512 Sector on Each Track and 512KB on each Sector

Ans: Memory Size = P * 2 Surface By default * T * S * D = $8 * 2 * 256 * 512 * 512KB = 2^3 * 2^1 * 2^8 * 2^9 + 2^9(2^{10}) = 2^{40} = 1 TB$

No. of Bits = $log(2^{40}) = 40$

L-6.2: Disk Access Time with Example | Seek Time, Rotational Time and Transfer Time

Disk Access Time

Seek Time -> Time taken by Read/write head to reach desired track.
Rotation time -> Time taken for one full Rotation (360.)
Rotational Latency -> Time taken to reach desired sector.
- $\text{Rotation Time} / 2$ ~ Average of All rotations
Transfer Time ->
- $\text {Data to be Transfer / Transfer Rate}$
Transfer Rate (Data Rate) ->
- $\text{ No. of Heads(or Surface) x Capacity of one Track x No. of Rotations per second}$

L-6.3: Disk Scheduling Algorithm | Operating System

Goal -> To minimize the seek time (no. of tracks Movement)

The whole track is read in one go, so the scheduling depends on the seek time (time to reach a track).
It is done by Actuator Arm back(outside) and Forth (inside) movement.

Disk Scheduling Algorithms:

FCFS (First come first Serve)
SSTF (Shortest Seek time first) -> Nearest Cylinder Next
SCAN -> Elevator Algorithm
LOOK
CSCAN (Circular Scan)
CLOOK (Circular Look)

Common Ques for All Algorithms

Ques A disk Contains 200 tracks. Request queue contains track no. (82, 170, 43, 140, 24, 16, 190) respectively. Current Position of R/W head = 50 -> Calculate total no. of tracks movement by head using All Algorithms -> If R/W head takes 1ns to move from one track to another then total time taken ___ ?

L-6.4: FCFS in Disk scheduling with Example | Operating System

FCFS Advantage -> No Starvation Disadvantage -> Performance (No optimisation)

Ans: Seek Time = abs(82-50) + abs(170-82) + abs(43-170) + abs(140-43) + abs(24-140) + abs(16-24) + (190-16) = 642 => (82-50) + (170-82) + (170-43) + (140-43) + (140-24) + (24-16) + (190-16) = 642

         50->82
            └82-------->170┐
      ┌ 43<-------------170┘
      └ 43---------->140┐
     24<-------------140┘
  ┌16<-24┘
    └16------------------>190

Back-n-Forth
170-50 + 170-43 + 140-43 + 140-16 + 190-16 = 642
Total Time taken = 642ns

L-6.5: SSTF in Disk scheduling with Example | Operating System

SSTF

Advantage -> Least Seek time (Average cases), Less Response time
Disadvantage -> Starvation, More Overhead to find nearest

         43<-50
     24<-43
   16<-24
   16--------------->82
                     82--->140
                           140-->170
                                 170->190

(50-16)+(190-16) = 208
Total Time Taken = 208 ns

L-6.6: SCAN Algorithm in Disk scheduling with Example | Operating System

SCAN:

Advantage -> It work as an ‘Elevator’ Algorithm it means, in Initial Direction Track Move Upto the Last Track (0 or n-1) to full fill the need of some new request at runtime
Disadvantage -> If Dynamically a New Request come after, Direction change from edge toward different edge, than it need to wait for long time to come back

Direction Given: Starting Direction toward larger value

             50--->82
                   82--->140
                         140-->170
                               170->190
                                    190->199
           43<-----------------------------199
     24<-43
   16<-24

(199-50)+(199-16) = 332
Total time taken = 332ns

L-6.7: LOOK Algorithm in Disk scheduling with Example | Operating System

LOOK Same as SCAN with Track not goes upto the edge tracks without need.

Direction Given: Starting Direction toward larger value

             50--->82
                   82--->140
                         140-->170
                               170->190
           43<------------------------190
     24<-43
   16<-24

(190-50)+(190-16) = 314
Total time taken = 314 ns

L-6.8: C-SCAN Algorithm in Disk scheduling with Example | Operating System

C-SCAN -> Circular Scan

50->82->140->170->190->(199) Rightmost ->Left Most (0) -> 16 -> 24 -> 43

(199-50) + (199-0) + (43-0) = 391

L-6.9: C-LOOK Algorithm in Disk scheduling with Example | Operating System

50->82->140->170->190-> Left ->16 -> 24 -> 43

(199-50) + (190-16) + (43-16) = 341