Disk Storage - Basic Terminology

Basic Terminology

• Platter – An individual recording disk. In a hard disk drive we tend to find a set of platters and developments in optical technology have led to multiple recording layers on a single DVD's.
• Rotation – Platters rotate; two techniques are common:
  • Constant angular velocity (CAV) keeps the disk spinning at a fixed rate, measured in revolutions per minute (RPM). This means the heads cover more distance per unit of time on the outer tracks than on the inner tracks. This method is typical with computer hard drives.
  • Constant linear velocity (CLV) keeps the distance covered by the heads per unit time fixed. Thus the disk has to slow down as the arm moves to the outer tracks. This method is typical for CD drives.
• Track – The circle of recorded data on a single recording surface of a platter.
• Sector – A segment of a track
• Low level formatting – Establishing the tracks and sectors.
• Head – The device that reads and writes the information—magnetic or optical—on the disk surface.
• Arm – The mechanical assembly that supports the head as it moves in and out.
• Seek time – Time needed to move the head to a new position (specific track).
• Rotational latency – Average time, once the arm is on the right track, before a head is over a desired sector.
• Data transfer rate - The rate at which user data bits are transferred from or to the medium, technically this would more accurately be entitled the "gross" data transfer rate.