Finding out where biowait is stuck

[Stuart Brooks <stuartb%cat.co.za@localhost>]

I have a problem where a process is getting stuck in a biowait and I was 
wondering if there is any way to find out where in the code (or in the 
kernel) this is happening. Once it is in this state it is unkillable so 
it doesn't process signal 11 to get a core dump. I am 90% sure it is the 
access to an external disk array which is causing it but I can't find a 
way to verify this. I have control of the source code so can compile it 
with symbols etc.

Are there any tools I can use to debug this. At present I have scripted 
a ps and fstat to log periodically so I can check these when it occurs 
but I don't know of anything else which would give more focussed 
information. The problem is it only happens every few days or so.

Any help would be greatly appreciated,

Stuart

PS. Would this be better sent to Users?