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Re: [Xen-users] RHEL6 domU migrate issues w/ higher to lower frequency C

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Subject: Re: [Xen-users] RHEL6 domU migrate issues w/ higher to lower frequency CPU's
From: Philipp Hahn <hahn@xxxxxxxxxxxxx>
Date: Wed, 19 Jan 2011 13:59:58 +0100
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Am Donnerstag 16 Dezember 2010 00:03:07 schrieb Joshua West:
> I've encountered a rather interesting/frustrating issue with RHEL6
> domU's and live migration.
> I have no problems booting a RHEL6 domU using its stock/native kernel on
> Xen 3.4.1 or Xen 3.4.3.  But in terms of live migration, there seems to
> be a problem when moving from a higher (in terms of CPU MHz) to lower
> (MHz) system -- even if the higher of the two is a much older CPU model.
> BTW, when I say "fails", what I really mean is the migration succeeds
> but the domU is no longer responsive.  I can attach to the console via
> 'xm console' but nothing is displayed, although occasionally a new line
> is printed as i bang my hands on the Enter key.  Occasionally ping works
> and occasionally I can establish a connection to the domU's port 22 and
> see the OpenSSH banner, but thats as far as I get.  Its not like the
> domU is runaway with 100% cpu.  It sits with state "-b----" (xm list).

I encountered a very similar problem with our Debian based distribution: ping 
often works, ssh-login is broken after login, xm console doesn't accept 
input, but prints kernel messages.
I think its relates to a Bug in the pvclock driver in the domU-Kernel (my 
kernel is ~2.6.32-24, but I've also not seen it fixed in 2.6.32-28), which 
also happens when migrating between two hosts with different uptime.
You might want to check is your domU-kernel contains the fix from 

At least that patch seems to have solved my problem. I found this very 
detailed bug report, which explains the problem: 

Philipp Hahn           Open Source Software Engineer      hahn@xxxxxxxxxxxxx   
Univention GmbH        Linux for Your Business        fon: +49 421 22 232- 0
Mary-Somerville-Str.1  28359 Bremen                   fax: +49 421 22 232-99

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