On 9 Mar 2007, at 15:22, Brent Baisley wrote:
When you say "70% iowait" are you referring to vmstat results? There are a lot of things that can be causing iowait, the most obvious being the disks are busy. In which case giving MySQL more memory won't really help, unless it's something that can be solved with caching.
Both top and vmstat. It seems even the most trivial traffic makes the disks go nuts. The root of the problem is whatever is causing so much disk activity. I guess if MySQL buffers are set too big, it could be squeezing other apps, OTOH, if they are too small, MySQL itself will be generating more disk activity. However, there's no particular evidence that I'm hitting swap. A key issue is that I can't tell if it's MySQL or PHP that's causing it - if I stop one, of course the other stops too. In top, mysql and php processes are often only showing a few % CPU, yet iowait can be over 80%, so it's really not clear what's eating it.
From what I've read, next time I'll host mysql on XFS partitions...
What does your context switching rate look like? It's possible your system is busy juggling without actually accomplishing anything. What constitutes a high context switch rate is dependent on the hardware and os.
That sounds plausible, and I'd not looked at that. Most of the time that seems to be around 200, with busy periods of 500 and peaks of peaks of 1100. I don't know if that's high, low or about right.
In MySQL, what are your threads_created and opened _tables numbers look like from show status? (not show innodb status).
I have 14,000 opens after 1.5 million queries. There are some databases that are low traffic but that have around 175 tables each.
To all of you that have offered help, thank you very much. I've now got Peter Zaitsev on the case, so hopefully he'll be able to pinpoint my problem areas.
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