The Rotating Nature Of HDDs
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- 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.