[mdlug] Serializing ide controllers

[Robert Meier]

Stan,

I hope the following, gleaned from google (not personal experience) will help.

> This is happening on more than one drive.
> (There are 11 drives in the system.)

An ordinary x386 machine has only 14 interrupts, (a limitation low enough
that has mandated programmer thriftiness from the 1980s).

Increases in cpu speed has reduced the time required to handle interrupts,
but increasing peripheral speed has increased the frequency of interrupts.
To date, cpu speed is clearly ahead, but losing its edge as processor
advances are slowed by physical limits, and peripheral speed catches up.

For typical systems with only 1 or two ide controllers, reserving typically
2 and sometimes an expensive 4 interrupts is sufficient to prevent any
drive requests from colliding.  With 11 drives, it is infeasible to reserve
an interrupt for each and frequent collisions are almost inevitable.