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[Xen-users] Running OS instances within domU or on physical device

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Subject: [Xen-users] Running OS instances within domU or on physical device
From: "Nyers, Gabor" <Gabor.Nyers@xxxxxxxxxxxxx>
Date: Mon, 26 Nov 2007 12:26:41 -0000
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Thread-topic: Running OS instances within domU or on physical device
Hi all,

Something interesting happened the other day. Due to a boot device 
configuration mix-up, an RHEL 5 instance, which was intended to run as domU, 
accidentally got booted up on the physical hardware. What came as quite a 
surprise is, that it actually ran pretty good. I haven't performed extensive 
tests, but for all I could see it run without a hitch.
This made me wonder if there are any (conceptual or proven and tested) 
arguments against this. What problems can arise from using an OS instance both 
on physical hardware, as well as within a domU? Is there any interesting 
application of this? (For one, I can imagine a case, when a non 24x7 VM gets 
started on a physical machine in order to cope with periodic heavy loads)

For those who wonder how this could happen, a quick overview: I had a couple of 
RHEL 5 domUs, each of them installed on dedicated SAN LUNs. I installed RHEL on 
the LUNs using Kickstart scripts, which also installed and configured GRUB, 
thereby making them bootable. As it turned out, one of the physical nodes' BIOS 
was configured to boot from SAN. So at its first reboot after the installation 
of the domUs, the BIOS booted from the SAN LUN, which contained the RHEL 
instance, originally intended as a domU. The rest, as they say, is history ;-)


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