Re: [CentOS] IO causing major performance issues

Well the CFQ scheduler tries to do just that, but if the amount of io is overwhelming even it cannot compensate.

Writes take longer then reads and for a mirror they have to happen on both drives before they can service another request.

I suggest you put the temp database on a tempfs to avoid the problem.

Set cfq scheduler on /dev/sda if it isn't already.

Other then that you could try a different file system like xfs or jfs to see if that helps.

-Ross

-----Original Message-----
From: centos-bounces@centos.org <centos-bounces@centos.org>
To: CentOS mailing list <centos@centos.org>
Sent: Thu Nov 15 19:29:20 2007
Subject: RE: [CentOS] IO causing major performance issues

Of course IO can swamp the file system. My point is that the kernel should
at least give enough time-slices to the other processes (like sshd) so
we can still log in. It's not asking a lot from the kernel - to just log in via ssh really.

--
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antonio varni
[ technology ]

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On Thu, 15 Nov 2007, Ross S. W. Walker wrote:

> redhat@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'.
>
> -Ross
>
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