iostat question

bichonfrise74 · 08-18-2008, 12:00 AM

Iostat shows that the Blk_read/s and Blk_wrtn/s is around 500 blocks / seconds and yet the %iowait is at 10% only.

Shouldn't the %iowait being huge if there is constant reading and writing to the disk???

Thanks.

syg00 · 08-18-2008, 12:21 AM

The I/O schedulers don't issue the physical I/Os immediately.
The are delayed, sorted (by block), and queued. That way multiple blocks (from multiple issued I/Os) that are "close" will be transferred together. There are various schemes to (attempt) to ensure no process gets blocked or starved - hence the existence of multiple I/O schedulers that you can choose to use.

bichonfrise74 · 08-18-2008, 05:21 PM

If you say that it IO scheduler and the Physical IO are not happening at the same time, then how long show I wait? Because I've been monitoring the box for half an hour, and %iowait still shows the box being at 10% only.

syg00 · 08-18-2008, 09:30 PM

I don't understand what you are asking.
The scheduler(s) manage the I/O for you, delaying the physical I/O variously from some few milliseconds to a few seconds. You shouldn't be able to discern this in a normal environment with mixed I/O and CPU consumption.

bichonfrise74 · 08-20-2008, 08:11 PM

I'm just confused because I know there is a lot of activity based on result of iostat (unless I am not interpreting the result of iostat correctly) but somehow top does not show any activity in %iowait.

syg00 · 08-20-2008, 08:31 PM

What you are seeing looking at is "activity" from the application(s) point of view. Those I/Os get buffered up and issued at the discretion of various components - chiefly the scheduler. This optimises the usage of the disk - things like head movement. As you've noticed, this (usually) works a treat.
Developers (and users) being protected from themselves ....

cyclepathology · 08-20-2008, 11:36 PM

Quote:

Originally Posted by bichonfrise74

Iostat shows that the Blk_read/s and Blk_wrtn/s is around 500 blocks / seconds and yet the %iowait is at 10% only.

Shouldn't the %iowait being huge if there is constant reading and writing to the disk???

Thanks.

These questions are easier to answer if you've got some sort of baseline to compare it to. I've got some Oracle servers that normally run about 10 times that much I/O. But I know that's normal, so it's OK. Big iron hardware, EMC Symmetrix disk array. On my desktop system, 500 blocks/sec would be unusual, and I'd expect the SATA drive to be a bottleneck.

It depends.

bichonfrise74 · 08-21-2008, 11:52 AM

That makes a lot of sense! Without a baseline to compare, I am not sure if the %iowait is high or not. Thanks!