minow@mountn.dec.com (Martin Minow) (05/11/89)
Summary: people notice that their 3.5 inch Quantum (40/80 Mbyte) disks occassionally do some I/O without any prompting from the host. They asked why. I talked with my local Quantum technical support person. Here's his answer: The driver uses an optical encoder and servo cylindars to locate data on the disk. The optical encoder uses an etched glass plate bonded to the head mechanism. Servo information is written on cylindars -1, -2, -3 and 835, 836, 837 (these are outside the data storage area). To seek to a particular data cylindar, the drive combines the optical track counter information with the micro-positioning information from the servo cylindars. (the exact values are a function of the desired cylindar number and the two servo values.) As the drive warms up, the mechanism expands and must be recalibrated. Quantum drives contain a thermistor that triggers recalibration when the temperature changes by 2 degrees-Celsius. Recalibration is also done on a timed basis. (Seems to be about every 2 minutes on my drive.) The recalibration requires reading the servo information on both the inner and outer cylindars. This is a low-priority task, and is interruptable by user I/O requests. So, the message is that it's intentional and you shouldn't be concerned. Martin Minow minow%thundr.dec@decwrl.dec.com <- best mail address The above does not represent the position of Digital Equipment Corporation.