Fujitsu 2361's, SC7003's, 4.3BSD

[System Mangler]

In article <1305 at cit-vax.Caltech.Edu> I write:
> The sector search "optimization" code, which is even more wrong in
> 4.3bsd, will degrade throughput when several drives are active on
> one controller. ...  Making it work correctly would require more
> cooperation between the hp driver and the mba code, and isn't worth it.

In article <4892 at mimsy.UUCP>, chris at mimsy.UUCP (Chris Torek) asks:
> Why not?

The object of sector searching is to avoid tying up the controller
with disk rotational latency.  It only gains something if another
drive is ready to do a transfer immediately.  This is an unlikely
event on a low-end VAX, which doesn't have enough CPU power to
keep a Massbus controller that busy, and which can ill afford the
additional interrupts.

For sector searching to work properly, the disk has to repeat the
search until it finds the controller ready to transfer immediately.
If it were to quit after one search (like in 4.3 BSD), it gets put
in a FIFO queue, the same as if it hadn't searched at all.

My first attempt at a rewrite did not quit after one search; it
searched forever, because it depended on the mindist/sdist/maxdist
numbers being sane, and they weren't.  4.3 BSD has all the sdist and
maxdist numbers interchanged.  That is the great weakness of sector
searching:  it requires tuning numbers, and obviously, nobody cares
about it enough to even make sure the numbers are in the right order!
If it's not going to be done right, why do it at all?

Don Speck   speck at vlsi.caltech.edu  {seismo,rutgers,ames}!cit-vax!speck