[CentOS] IO causing major performance issues

Ross S. W. Walker rwalker at medallion.com
Thu Nov 15 22:45:10 UTC 2007

redhat at mckerrs.net wrote:
> Antonio Varni wrote:
> > 
> > Hello everyone.
> > 
> > I'm wondering what other people's experiences are WRT systems becoming
> > unresponsive (unable to ssh in, etc) for brief periods of time when
> > a large amount of IO is being performed.  It's really starting to
> > cause a problem for us.  We're on Dell PowerEdge 1955 blades 
> > - but this same
> > issue has caused us problems on PE1950, PE1850, PE1750 servers.
> > 
> > We're running Centos 4.5 right now. I know Centos 5 includes 
> > ionice, more
> > io scheduler/elevator selections like deadlock/etc. Perhaps that would
> > fix this issue.  We're running the latest PERC firmware.
> > 
> > The specific issue I'm referring to at this point is on a 
> > system running
> > mysql. All mysql data files are on a netapp filer but mysql's 
> > tmp directory
> > is on local disk.  Whenever a lot of temp tables are created (and thus
> > written and deleted from local disk quickly) we can't even 
> > log in to the
> > machine - and our monitoring system gets all freaked out and we get
> > lots of pages, etc... FYI this is two disks with hardware raid 1.
> > 
> > Is it just me? Or is this specific to Dell systems, or is this just
> > the state of the Linux kernel these days? Is there some magical patch
> > I can apply to make this issue go away :)
> > 
> > 
> > Thanks in advance for any insight into this issue.
> > 
> > Antonio
> I have noticed similar behaviour on all sort of linuxes (in 
> particular, ssh into the box is really slow when it's doing 
> IO) and wondered why, but never really thought about 
> investigating any further.
> Unfortunately, I do a lot of work with solaris and the funny 
> thing is that I have *never* seen a solaris kernel exhibit 
> this sort of behaviour. Even if it is installed on normal 
> IDE/SATA disks. And, in fact, even if installed on the exact 
> same hardware.
> Now I'm curious.....especially given that I'm right in the 
> middle of pushing to get rid of solaris in favour of RHEL.

It really depends what the system is doing, what services you are
running and how you have it configured.

You had Solaris installed, what services was it running?

You had Linux installed, what services was it running?

Database temp tables and logs can generate an enormous amount of
io which can swamp the file systems of any system.

I have seen it on Windows and Linux, so I don't see why Solaris
would be any different.

You could always try a different scheduler to see if that helps,
for instance if you are using 'cfq' try 'deadline'.


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