wizardly help needed (785 with slow UDA)

[System Mangler]

    The UDA-50 has a set of pluggable jumpers that set the "Unibus delay" -
the amount of time the UDA waits between DMA requests to give other devices
a chance at the bus.  This can be set for 0us, 6.7us, or 10us.  The burst
transfer rate at each setting is:
	0us	800 kilobytes/second
	6.7us	350 kilobytes/second
	10us	250 kilobytes/second
Some devices with very little buffering (RK07's and RL02's) will get data-
lates if competing with a UDA set to any but the slowest setting, so I guess
DEC has been tending to ship them set for 10us.  I found that an RL02
sharing a 750 Unibus with a UDA and an Interlan would get data lates
on ANY of the three settings.  (So we sold both the RL02 *and* the
UDA, and bought Eagles).
    You might also try fiddling with "tunefs -d", which sets the rotational
distance between consecutive blocks of files.  When set optimally, the cpu
asks for the next block just as it comes under the heads.  Unfortunately,
if you use the default value, the cpu asks for the block just AFTER it
passes under the heads.  When we still had UDA-50's on our 750's I found
that the optimal value was a whole revolution - which you specify as 0,
to avoid forcing a track switch and the consequent quarter revolution
of head-switching delay that is built into the sector numbering.
(E.g, track 1 sector 0 is 1/4 revolution away from track 0 sector 0).
    Of course if you really need speed you don't use RA81's...

				Don Speck	speck at cit-vax.arpa