Re: [Xen-users] Xen a couple of questions

[Nico Kadel-Garcia <nkadel@xxxxxxxxx>]

Mark Williamson wrote:
> > 1. Regarding Centos and Fedora core 7 compared with fedora core 5. I've
> > seen that on fedora core 5 when you want to install xen you have to
> > install the following packages: xen, kernel-xen0 and kernel-xenU (of
> > course with the dependencies needed). But on Centos, FC7 and I think
> > redhat versions, you only have to install xen and kernel-xen, you don't
> > have any kernel for the guest system. In my case I could only start a xen
> > guest (on FC7) with an older kernel-xenU installed from FC version 5.
> >
> > My question is: Why does the newer releases of linux has xen kernel
> > prebuilt but just for dom0, not for the guest systems, and you can't even
> > find a domU kernel special for those systems?
> >     
> The same kernel will work for both; there's no need to have a different kernel 
> for the domUs.
>   
Hmmph. the same kernel *can* work for both. There may be subtlely 
different behaviors that are beneficial to DomU, although I'm not sure 
in detail what they are: I've just run into that sort of thing behavior 
in heterogeneous deployments.
No, I think the issue is that CentOS is pegged to RedHat's kernel 
release model, where an RHEL deployment is supposed to be stable and 
consistent throughout the lifespan of the operating system.  For 
reliable behavior in such an environment, your DomU *must* have a kernel 
as similar as posible to that deployed by RedHat. Dom0 can be forced to 
be more recent to get critical features (shoving Xen Dom0 into a 2.6.9 
kernel is just asking for pain, though.) So Dom0 pretty much needed a 
much newer kernel.
Notice that for RHEL and CentOS 4.5, which now can gracefully be 
installed as DomU's on top of a 5.0 Dom0, they only have kernel-xenU 
packages, not kernel-xen packages. If you want a 4.5 machine as a Dom0, 
you need to use the xensource kernel or roll your own. And do *not* try 
to backport virt-manager to CentOS 4.5 without being prepared for a lot 
of pain.
There's also the issue of kernel size: when you're doing 
micro-deployments (stripped down DomU's for firewall or similar mini 
setups) there are some advantages to teeny-tiny kernels, and since you 
have a consistent environment of necessary hardware drivers, you can 
actually do it. But it's a pain to support, and it also lets anyone 
doing a "uname -a" find out that you're in a Xen guest environment.
So there are tradeoffs.

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