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

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

I'm cc-ing Emre because he's trying to run domUs on baremetal too and some of 
this will be relevant to him.

I think this ought to work, so I'm glad that it worked for you ;-)

The only problem with this sort of thing is where device names etc change 
(e.g. some newer XenLinux kernels use hvc0 for the console, others use xvc0 
for the console, older ones may use ttyS0 - makes it difficult to configure 
for).  However, given modern distros identify filesystems by their label 
rather than their device, the device naming should be less of a problem.  
You'll probably need to specify different commandline options to the kernel 
depending on whether it's a native one or not (e.g. different console= value, 
any Xen-specific or native-specific options).

You also need to add the Xen console device to /etc/securetty(s) and the 
native serial console device also, if you use one.

The main gripe with dual booting between native and Xenified is possibly going 
to be that the hardware detection routines of your distros will get confused 
by (virtual or real) hardware coming and going depending on how you booted.  
I'm not quite sure how they handle this, or how much of a problem it'd be in 
practice.  This is also likely to be an issue for HVM/PV dual booting.

Unfortunately, different niggles are also likely to come up depending on which 
XenLinux you use. :-(

Cheers,
Mrak

> > [...]
> > 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)
> > [...]
>
> Hmm, I see not much useful application in this.
>
> For this type of load balancing, you would rather let the domU run alone
> on a single host, and migrate other away to less used servers.
>
> Having to reboot every time the load gets higher seems a bit odd for
> real production.
>
> So, this experiment might be an interesting result, but I see not
> practical use.
> On the other hand, it's not even such a wonder that this works: if you
> installed a normal kernel to boot from, there is not much difference
> between a "normal" system and a (PV - HVM is even less different, if at
> all)domU - the only thing different is that you have a xen kernel, and
> you often disbale all but one serial console - nothing special that
> keeps you from using the system directly on the hardware instead with a
> hypervisor in between.
>
> Maybe it's interesting and usable for desktop/laptop users - I don't know?
>
> Henning
>
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-- 
Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!

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