The organization

The organization

platter

Facts

• When the hard drive is operating, the disk is rotating at constant speed

1. To read or write, the disk head must be positioned on the desired track and at the beginning of the desired sector

2. Once the head is in position, the read or write operation is then performed as the sector moves under the head

Terms

• Seek time is the time it takes to position the head on the desired track
• Rotational delay or rotational latency is the additional time it takes for the beginning of the sector to reach the head once the head is in position
• Transfer time is the time for the sector to pass under the head (read or write data)

The equation

Transfer Time

• b = number of bytes to be transferred

• N = number of bytes on a track/cylinder

• r = rotation speed, in revolutions per second

Disk Scheduling Policies 

First-in, first-out

• Process requests sequentially

• —Approaches random scheduling in performance if there are many processes

• —Example: 55, 58, 39, 18, 90, 160, 150, 38, 184

Shortest Service/Seek Time First

• —Select the disk I/O request that requires the least movement of the disk arm from its current position

• —Requests for tracks far away from the current position may never be served, if requests for closer tracks are issued continuously

• Example: 55, 58, 39, 18, 90, 160, 150, 38, 184

SCAN (aka Elevator Algorithm)

• —Arm moves in one direction only, satisfying all outstanding requests until it reaches the last track in that direction

• Direction is reversed

• —Example:  55, 58, 39, 18, 90, 160, 150, 38, 184

C-SCAN

• —Restricts scanning to one direction only

• When the last track has been visited in one direction, the arm is returned to the opposite end of the disk and the scan begins again