Since I'm (currently) running an ext3 partition
this is currently under suspicion for the total system lockups under jack.
If you have two different disks in your box, I've found the kernel 'feature'
ideautotune can also cause lock-ups when disks are put under stress. You can
disable this in your boot loader, fortunately. Booting into failsafe mode (on
some distros) also disables ideautotune.
I'm still suprised that something which causes total lock-ups on otherwise
stable machines found it's way into the stable kernel series. I don't see
what's wrong with using hdparm if you want to optimise your drives.
Cheers
Daniel