[CentOS] Finding i/o bottleneck

[Chad Gross]

Hi Nicolas,

While this doesn't exactly answer your question, I was wondering what
scheduler you were using on your GFS2 (Note: I have not used this file
system before) block. You can find this by issuing 'cat /sys/block/<insert
block device>/queue/scheduler' ?

By default the system uses cfq, which will show up as [cfq] when catting the
scheduler as I showed above. This is not the most optimal scheduler for a
webserver. In most cases you'd be better off with deadline or noop. Not
being familiar with GFS2 myself, I did skim this article, which makes me
think noop would be the better choice:

http://www.redhat.com/archives/linux-cluster/2010-June/msg00027.html

 This could be why you are seeing the processes waiting on I/O.


Chad M. Gross


On Tue, Sep 20, 2011 at 2:55 PM, Nicolas Ross <rossnick-lists at cybercat.ca>wrote:

> Hi list !
>
> We have a very busy webserver hosted in a clustered environment where the
> document root and data is on a GFS2 partition off a fiber-attached disk
> array.
>
> Now on busy moments, I can see in htop, nmon that there is a fair
> percentage
> of cpu that is waiting for I/O. In nmon, I can spot that the most busy
> block
> device correspond to our gfs2 partition where many times, it shows that
> it's
> 100% busy and is read all along.
>
> Now, I want to know what files are being waited for. With lsof I can get a
> listing of open files, but it doesn't gives me if a file is just opended in
> ram or if it's being waited for...
>
> What tools besides lsof, nmon, htop, atop can help me find that info ?
>
> I am under RHEL/CentOS 6.1.
>
> Thanks
>
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