linux-kernel - Re: [PATCH] xen: wait up to 5 minutes for device connection

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Message-ID: <4A51AB17.5050404@redhat.com>
Date:	Mon, 06 Jul 2009 09:43:19 +0200
From:	Gerd Hoffmann <kraxel@...hat.com>
To:	Valdis.Kletnieks@...edu
CC:	Christoph Hellwig <hch@...radead.org>,
	Paolo Bonzini <pbonzini@...hat.com>,
	linux-kernel@...r.kernel.org,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>
Subject: Re: [PATCH] xen: wait up to 5 minutes for device connection

On 07/04/09 17:15, Valdis.Kletnieks@...edu wrote:
> On Fri, 03 Jul 2009 23:21:35 +0200, Gerd Hoffmann said:
>
>> Usually it is *much* faster, but when the host is quite loaded it can
>> take a unusual long time.  With 10 seconds it happends in practice now
>> and then that a virtual machine fails to boot just because the virtual
>> root disk didn't show up fast enough.
>
> Are people actually trying to boot a guest on a host machine so loaded that
> disks take that long to show up - and expect things to work in any sane
> matter?

Even on machines which can handle their load just fine under normal 
circumstances you may see that behavior in case of load peaks.  Booting 
a bunch of virtual machines at the same time can trigger it for example.

> I'm tempted to suggest that booting under conditions like that is almost
> deserving of its own kernel Tainted flag.  If devices aren't showing up in
> a timely manner, we probably need to be leery of any other kernel timeout
> values as well...

It is pointless to taint just because of a unusual delay at some random 
point in time.  A virtual machine can see higher delays due to load 
peaks on the host anytime.  The flag wouldn't carry any useful information.

You could add TAINT_VIRT to flag *all* vm guests, but as the boot log 
gives you that information already it is pointless too IMHO.

cheers,
   Gerd

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