Safe raised niceness without interfering with system?

[Phil Pennock <Phil.Pennock@globnix.org>]

I have a box running OpenBSD 2.9-STABLE (not the one which I've just
built).  I need to sort out backups, I'm planning on burning to CD.  I
have a spare partition which I can copy data to (or run commands to
safely extract data from running systems) and prepare ISO images on, so
that's not a problem.

However, I can't afford to drop this system out of operational service,
yet don't want the write buffers to underrun.  So, I can tell cdrecord
to use speed=2 or speed=1, I can set fs=<fifo-size>, etc.  I suspect
that it's also a good plan to run at a raised[*] niceness.  Most of the
processes are standard userland and won't be affected by a delay in
getting scheduled.  However, /tmp is on MFS and I have NTP
synchronisation.

[*] as I'm using it, raised -> more negative,
    ie preferred scheduling bias

Is there a niceness which it would be a Bad Idea to use?  Eg, "not more
than -17 lest you interfere with mount_mfs"?

Or will this simply not buy me anything?  (I suspect that since most
time will be spent waiting on writes, time slices won't expire so
cdrecord should retain a high running priority, but have no practical
evidence to back this up).

If anyone has any specific points or tips, etc, please reply to me
directly and I'll summarise to the list and/or contribute to the FAQ, as
appropriate.

Thanks,
-- 
A guru went to the dentist and said, "I'd like to transcend dental medication."