MGMT 1050 Lecture Notes - Lecture 21: Hard Disk Drive Performance Characteristics
MGMT 1050 Lecture 21 Notes – Topmost Platter
Introduction
• Only the topmost platter and head are fully visible.
• The entire assembly is sealed to prevent dirt particles from wedging between the heads
and the disk platter.
• Since this situation could easily destroy the drive.
• Even a particle of cigarette smoke is much larger than the space between the head and
the disk.
• When the disk is stationary, the head rests in a parked position on the edge of the drive.
• The head has an aerodynamic design, which causes it to rise on a cushion of air when
the disk platter spins.
• The operation required to locate an individual block of data.
• First, the arm moves the head from its present track until it is over the desired track.
• The time that is required to move from one track to another is known as the seek time.
• Since the distance between the two tracks is obviously a factor, the average seek time is
used as a specification for the disk.
• Once the head is located over the desired track, the read/write operation must wait for
the disk to rotate to the beginning of the correct sector.
• The time for this to occur is known as the rotational latency time, or sometimes as
rotational delay or simply latency time.
• The latency time is obviously variable, depending on the position of the disk.
• As a best case, the head is just about to enter the sector, and the rotational latency time
is 0.
• At the opposite extreme, the head has just passed the beginning of the sector.
• A full rotation is required to reach the beginning of the sector.
• This time can be calculated from the rotational speed of the disk. Both situations are
equally probable.
• On average, the disk will have to rotate half way to reach the desired block.
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