From: Neil Brown <neilb@suse.de>
To: CaT <cat@zip.com.au>
Date: Mon, 19 Dec 2005 10:36:53 +1100
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Message-ID: <17317.62101.886020.592277@cse.unsw.edu.au>
Cc: linux-kernel@vger.kernel.org, Nick Piggin <nickpiggin@yahoo.com.au>
Subject: Re: anticipatory scheduler and raid rebuild
In-Reply-To: message from CaT on Tuesday December 13
References: <20051213052329.GM4212@zip.com.au>
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On Tuesday December 13, cat@zip.com.au wrote:
> I'll be able to play a bit more with this later but for now I thought
> I'd toss it into the wilderness. 

Thanks.

> 
> I had jsut setup a nice little server with WD 10k drives and s/w raid 1.
> The kernel is 2.6.14.3. The CPU is a p4 3Ghz and it's an Intel 82875P
> chipset. In order to test that it'll build ok with missing disks I
> pulled one out, booted, shutdown, put it back in and rebooted. I then
> went on to try and get one of the raids to rebuild with:
> 
> mdadm --manage -a /dev/md6 /dev/sdb8
> 
> And then the server slowed to a crawl. Well not even that. It slowed to
> the point of freezing and occasionally stuttering with activity other
> then the rebuild. I got a similar reaction when it was rebuilding
> it.

I've heard reports of this sort of thing before I think, but I'm
wondering why I never experience it.
What sort of drives do you have? What controller?
What filesystem are you running over the raid1?

> 
> So, does my hardware suck and AS is pushing it beyond its limits or is
> AS unsuitable for the task I am putting it through or is AS buggy and
> all should be well with it?

I suspect it is an odd interaction between md/raid1/rebuild and AS.
AS tries to guess how a process is behaving and the raid1/rebuild
process probably is confusing it.  But it is hard to say how until I
can reproduce it.

NeilBrown

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