IOWAIT block layer problem

Al Boldi
2005-07-05T03:44:50Z

André Tomt wrote: {
>>>>On 7/4/05, Al Boldi <a1426z@gawab.com> wrote:
>>>>Hdparm -tT gives 38mb/s in 2.4.31
>>>>Cat /dev/hda > /dev/null gives 2% user 33% sys 65% idle
>>>>
>>>>Hdparm -tT gives 28mb/s in 2.6.12
>>>>Cat /dev/hda > /dev/null gives 2% user 25% sys 0% idle 73% IOWAIT

The "hdparm doesn't get as high scores as in 2.4" is a old discussed to
death "problem" on LKML. So far nobody has been able to show it affects
anything but that pretty useless quasi-benchmark.

>>>No errors though, only sluggish system.

Really sluggish or just "benchmark-sluggish"?
If the former, try selecting a different IO elevator/sheduler.

> When you compare 2.4.31 with 2.6.12 don't you see this problem on your
> machine?
> If you have a fast system the slowdown won't show, but your IOWAIT
> will be higher anyway!

Nothing wrong with 73% iowait
Beware 2.4 didn't export that statistic at all to userspace, so 0% iowait
gets reported from most 2.6-ready reporting tools on 2.4.

}

1. hdparm only gives you an indicator.
2. Really sluggish, as in unresponsive to commands.
3. Tried all IO elevators/shedulers to no avail.
4. In 2.4 IOWAIT is added to sys, as in 8% IOWAIT.