How to make 4.2bsd ``/etc/dumps faster

[Don Speck]

    After reading Chris Torek's trial of my dump mods I waited until
I could try it on a TU80 myself before saying anything.  Sorry if I
seemed too silent...

    I timed the TU77 and found that a tight write() loop only makes
it go at 110 ips, not 125.  I was mistaking the smooth motion of the
reels for top speed - the vacuum column chatter should have told me
that it's coming to a stop between every tape write.  Trying this on
a TU80 on a 750, the TU80 does stream, but keeps going in and out of
100 ips mode every few seconds.  (This is in single-user mode).  The
750 can just *barely* finish the current write() and start another
one before the interrecord gap flys by, even though the TU80 is
stretching that interrecord gap as far as it can (1.2 inches).

    Dump has to do more processing between writes than the tight-loop
write() program.  When running dump, the TU80 tries 100 ips a few
times but discovers that the time between tape blocks is over the
limit.	Thereafter the TU80 just sticks to 25 ips streaming.

    So the problem is that overhead on write() is much higher than
I'd dreamed.  Unless someone knows offhand where the time goes, I'll
have to profile the kernel and find out where.	(Probably physio()).
Btw, how do I get an alternate profiling clock?

    To those who've asked for the dump mods, please be patient.  I
don't want to give out any more copies of it until it works *well*.

			Don Speck   Caltech CS	(818) 356-6886