Enterprise-class versus Desktop-class Hard Drives

Revision 1.0

Enterprise class drives provide a more sophisticated compensation for vibration by sensing the

vibration motion of the drive, and by sensing head position and track alignment. The drive can

then react with additional actuator strength or wait for the spindle motor to spin the target media

location under the head again so that it can re-attempt access. Some forms of misalignment

compensation can have a dramatic impact on performance due to the time required to spin the

data target location back under the read/write head. Performance loss of up to 90% has been

observed in some tests. The amount of performance loss is dependent on the frequency and

strength of the vibration.

1.1.4.3 Misalignment detection

Most drives incorporate multiple “servo wedges” in the track. Drive firmware constantly

monitors these wedges to determine the location of the head in relation to the track. If the drive

firmware detects a misalignment, it will hold the read or write and wait for the target location to

spin under the head again. Desktop class drives have fewer “servo wedges”, or a single

combined servo/data path processor. With a single processor, the drive is unable to servo the

head while writing to media. Most desktop drives do not have dedicated servo and data path

processors or the firmware compensation algorithms that make them more susceptible to RV

errors, which results in poor performance due to vibration. Most enterprise class drives have

dedicated servo and data path processors with servo algorithms in the drive firmware to enforce

this compensation.

1.1.4.4 Vibration Sensors

Most enterprise drives implement a vibration sensor circuit on the drive’s electronic board.

These sensors can detect the movement of the drive when the servo is moving the heads,

which provides a reliable method of positioning the heads and determining when it is safe to

read or write data. However, this can be less beneficial if there is a single, combined servo/data

path processor. Enterprise class drive designs include a closed loop feedback system between

the magnetic head and the spindle(s) to sense vibration anomalies and react accordingly.