Mark,
I would have responded to this thread earlier but I was out late last 
week.  We had tons of problems with our ldm server running Linux last 
fall which sounds very similar to the behavior you are describing.  I 
had migrated our LDM from Freebsd to Linux (Slackware, not that it makes 
much difference) and had huge problems with I/O waits bogging the entire 
system down. ...
What version of the kernel were you running?  For example
   $ uname -r
   2.6.11-1.14_FC3smp
Versions of the 2.6 kernel earlier than 2.6.10 (I think) appear to have
a very bad I/O performance bug.
Regards,
Steve Emmerson