Friday, August 13, 2010

Basic terminology

Rotation - how the disks spin. 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.
Sector - an area of disk enclosed within a given central angle (a pie piece)
Platter - an individual recording surface. 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.
Low level formatting - establishing the tracks and sectors.
Track - the circle of recorded data on a single recording surface for a single arm position.
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 - average time needed to move the head to a new position(specific track).
Rotational delay - average time, once the arm is on the right track, before a head is over a desired sector.
Interleave - the spacing between sectors. Sequential sectors were spaced on the media to enable the next sector to be in the correct position under the head once the host was ready to read it. At an interleave factor of 3:1, three full rotations would be required to read an entire track. Due to technological advances almost all hard disk drives since Compaq and Western Digital defined the AT Attachment standard have used an interleave factor of 1:1. Floppy disks are still usually interleaved.

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