I/O or Threading Suffer

hello,

i got problems with mysql under freebsd 5.2.1 and CURRENT.

i got these problems on two machines with different hardware, but both are
single cpu machines.
used scheduler is ULE.

problem description:
under high i/o load the mysqld isnt responding any more.
i tested this with dd if=/dev/urandom of=testfile bs=128k on the hdd an was
not
able to become any ouput from simple select-statements within a long period
of time
(above minutes).
i dont know if this problem occurs from mysql and its threads or if the i/o
subsystem has
any problems to deal fast enough with two intensives read and write
processes and its
priorities.

best regards,
christian