Serial Console and Slow SCSI Disk Access?

Jeff Gold
2006-06-12T21:28:04Z

Does it make sense that enabling a serial console would reduce the
bandwidth of a SCSI disk by more than a factor of ten? This seems crazy
to me but there's no arguing with empirical facts. I have an IBM x345
system on which I installed an unmodified 2.6.16.20 kernel built with
gcc-4.0.2-8.fc4 (the kernel configuration file I used can be found at
http://augart.com/jgold/kconfig). With the control kernel command line
I get about 70 MB/sec with hdparm -t but when I add "console=ttyS0,9600
console=tty0" I get about 1.6 MB/sec instead.

Here are the complete kernel command lines from grub.conf (the second
one doesn't actually include a backslash in grub.conf -- that's just to
avoid email wrapping):

ro root=/dev/sda3 pci=biosirq loglevel=7

ro root=/dev/sda3 pci=biosirq console=ttyS0,9600n8 console=tty0 \
loglevel=7

I have not been able to connect this behavior to any known kernel bugs
using Google or mailing list archives. What might I be doing wrong?

Jeff