linux-kernel - Re: Re: Performance issues with Areca 1680 SAS Controllers

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Date:	Thu, 20 Aug 2009 11:00:02 +0300
From:	Pasi Kärkkäinen <pasik@....fi>
To:	Drew <drew.kay@...il.com>
Cc:	Wang Jinpu <jack_wang@...sh.com>,
	Nick Cheng <nick.cheng@...ca.com.tw>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Michael Fuckner <michael@...kner.net>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-scsi <linux-scsi@...r.kernel.org>,
	Erich Chen <erich@...ca.com.tw>
Subject: Re: Re: Performance issues with Areca 1680 SAS Controllers

On Wed, Aug 19, 2009 at 07:29:07PM -0700, Drew wrote:
> > The problem is while Areca is doing the flushing _all_ IOs are really slow,
> > including the other raid-10 array for the OS, which uses totally separate physical disks.
> >
> > Opening another shell in screen takes at least 30 seconds, starting "top"
> > takes forever etc..
> >
> > While Areca is flushing the caches (and all the IOs are slow), "iostat 1"
> > doesn't show any "leftover" IOs from the benchmark. So the benchmark was
> > really using direct IO, bypassing kernel caches.
> >
> > I tried with different io-schedulers (cfq,deadline,noop) but they didn't
> > have big effect.. which makes sense, since the OS/kernel is not doing any
> > big IO when the 'stalling' happens.
> >
> > Is there some way to make Areca NOT use all cpu-power for cache flushing?
> 
> Is top showing 100% cpu usage?
> 

No, cpu usage is almost 0%. I was talking about cpu-usage of _Areca_ controller.

When the disktest-benchmark ends, Linux cpu usage goes to 0%, and there's no IO
anymore (checked with "iostat 1"). 

But _Areca_ is still flushing it's caches, and all the IO will be slow, even
when there's no load in _Linux_. 

-- Pasi

> It wouldn't surprise me if the Areca's internal bus / processor is
> getting swamped. With the RAID60 chewing through two PQ calculations
> over 14 disks I'd expect the performance of other arrays on the
> controller would take a hit
> 
> I've seen similar symptoms when moving massive amounts of data between
> disks on my servers. IO maxes out the bus, system responsiveness goes
> down as evidenced by slowly loading apps, and top still shows minimal
> CPU usage.
> 
> 
> -- 
> Drew
> 
> "Nothing in life is to be feared. It is only to be understood."
> --Marie Curie
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