How fast are your disks?

[David Schachter]

Be careful in measuring disk performance.  I did some simple analysis of
an Interphase SMD controller for the Multibus two years ago.  The claim
of the manufacturer is that it can handle two megabytes per second.  I
wrote a program which talked directly with the controller, bypassing the
(non-Unix) operating system and twiddled various parameters to see the
effect.  I made sure the data I was requesting was in the track buffer of
the controller and checked that assertion by verifying that the disk drive
(a 475 MB Fujitsu "Eagle") was quiescent (the "drive busy" light stayed
off.)

The best transfer rate I could get was one megabyte per second, on a
fast Multibus system, doing sixteen bit transfers.  I put the controller
on an extender card and used an oscilloscope to check out the hardware;
the controller simply wasn't using the available bus cycles.  (There are
some parameters one can set in the controller to control bus usage; the
one MB/second rate was achieve by telling the controller to hog the bus.)

A call to Interphase got an answer of "well, of course, it doesn't really
get two megabytes per second...."  Thanks, guys.  Real helpful.

Moral: don't trust the controller manufacturer.  Or the documentation.
You may have to measure raw hardware, with a 'scope, to get believable
answers.

				-- David Schachter

The opinions expressed above are mine, as are the facts, and most everything
else.