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Re: [Xen-users] Can Dom0 be replaced after installation?

To: Xen User-List <xen-users@xxxxxxxxxxxxxxxxxxx>
Subject: Re: [Xen-users] Can Dom0 be replaced after installation?
From: "Fajar A. Nugraha" <fajar@xxxxxxxxx>
Date: Thu, 12 Aug 2010 14:00:50 +0700
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On Thu, Aug 12, 2010 at 12:30 PM, Ken <kr.xen@xxxxxxxxxxxxx> wrote:
> Hi,
> Agonizing about a strategy for installing, when I know nothing about it.
> I suspect that in spite of the flexibility in choices for a dom0
> operating system, I think it's probably really important to get it all
> right.

At this point I usually say "it depends".
xenserver/XCP is designed for those who wants a system that "just
works" without having to worry about what the best dom0 setup is.

> 1. Assuming I install Xen, a handful of VMs and have it all set up, is
> there a way to replace dom0 with some other dom0?  If I knew I could
> change my mind later it would be much easier to set this up.

In principal, yes. A domU is basically defined by its configuration
file and backing storage. If you have both, then you should be able to
(re)create an identical domU using whatever dom0 hardware/OS.

So assuming that the old and new OS both have access to the same
storage (possibly include support for the same partition/LVM and
filesystem type) and domU configuration, changing dom0 OS is possible.
Moving from (for example) Centos to Opensuse is easy enough, but
moving from Centos to opensolaris is somewhat hard.

> 2. There seems to be a way to have a kernel which works either on the
> bare metal or as a guest, but it doesn't seem to have been tested much.
> Do I ignore this for now, and if I do can I start using it later?

Since you're new to Xen, I suggest using dom0 with bundled Xen. My
favorite is RHEL5. Although its version of Xen is slightly out of date
in terms of features, it's enough for most common use, and kept
up-to-date in terms of bugfix/security update.

>  If I
> change it on Dom0 do I need to recompile kernels on the other guests?

No. domU can be independent of dom0 kernel.

> 3. I keep expecting to need to install the hypervisor somewhere.  I
> don't see the hypervisor anywhere in Gentoo, but there's xen-sources
> which I suspect contains it.

Try asking on Gentoo list
on RHEL/Centos it's simply "yum groupinstall Virtualization"

>  Or do I need the tarball from
> www.xen.org/products/xen_source.html?

Possible, but it might not be so straightforward. See

> 4. In my understanding the dom0 needs to be extremely stable and have
> good driver support.  What distro works best for that?  I'm currently
> using Gentoo, but I would think Debian would be most stable.  If so, is
> the current difficulty with Debian an unhappy coincidence or is it
> consistent?  How about Ubuntu Server, or is that still the same problem
> since it's also a Debian?  What do people use if they just want a small,
> stable dom0 with good driver support?

I like RHEL.

> 5. Is there a way to "loan" processors to a guest if nobody else is
> using them?

By default all domUs have the same priority. See


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