System is Ubuntu 10 LTS 64bit (2.6.35.31), I'm running on xen 4.0, no services active, cron stopped, scheduler is cfq for the disk /usr is mounted from:

time find /usr -exec stat {} \; > /dev/null 2>&1 &

giving

real    0m35.760s
user    0m0.270s
sys 0m3.910s

and

time ionice -c3 find /usr -exec stat {} \; > /dev/null 2>&1 &

giving

real    0m36.110s
user    0m0.310s
sys 0m4.100s

which is exactly as expected, now I run both at the same time:

time find /usr -exec stat {} \; > /dev/null 2>&1 & time ionice -c3 find /usr -exec stat {} \; > /dev/null 2>&1 &

where to my believe the ioniced version should be much slower while the straight version should be as fast as if running alone. but:

straight:

real    1m10.430s
user    0m0.320s
sys 0m3.940s

ioniced:

real    1m10.230s
user    0m0.250s
sys 0m4.020s

which implies that ionice did not work at all. Any hints?