> Now, what CPU and what disk controller do you use?
 
 A3k SCSI plus GVP G-Force 68040.

> P5 DMA hardware (other than the CS Mk.3) is known to block the CPU out of
> memory for extended times. Others might do the same.  Michael Hitch
> introduced a hack to limit this problem some years ago.
> 
> 68040 and 68060 machines do the same when flushing caches btw. Shouldn't
> matter at 38400 bps.
> 
> > tty00 33600     # modem port and speed
> 
> So, if you have LEV6_DEFER, and see "silo voerflows" at  speed that
> low, I suggest you make sure you first remove locore.o and recompile
> the kernel.  If you still have the problem, some DMA device is
> misbehaving.

 I have recompiled the kernel with LEV6_DEFER and defaults for
 SERIBUF/SEROBUF. Ftp transfers are up now by x10 - x100, at about
 2k/s. Not too bad. I still see the occasional silo overflow, but
 a lot less often.