The Rotating Nature Of HDDs

• Platers revolve at a fixed speed.
• We have disks revolving at 5400 Rounds per minute (RPM), 7200 RPM, 10000 RPM and 15000 RPM.
• The arm moves only in or out (i.e. between cylinders). To access different sectors on a disk - the platers need to rotate.
• To get to a given location on the disk, it needs to rotate to the right angle, and the arm needs to move in or out to the right cylinder.
• This operation is called "seek".
• The worst seek operation will require a full rotation of the platters.
• For an 10000 RPM disk - the rotation time is 6 milli-seconds.
• Add the time to move the arm - and you get several milli-seconds of seek time.

Originally written by guy keren