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

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

> >> 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.

Sure.  I was referring to the specific case of Fedora 7 and why there weren't 
separate kernel packages for dom0 and domU - you should use the same kernel 
for both (in the case that you're running a Fedora guest).

If you're running a range of different distros, the best plan is to run each 
install on that distro's Xenified kernel rather than trying to use the same 
kernel for each of them.

> 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.

Been there, done that :-(

I just use commandline tools on CentOS 4.x, and have built my own kernels for 
all the domains.  But that's my development box, so this is what I'd do 
anyhow.

> 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.

*nod*

But if a distro supports a single -xen kernel rather than -xen0 and -xenU then 
it's nothing to worry about; the extra cruft required by dom0 is mostly 
device drivers as modules, so you don't actually need them in memory.

The nicest way to handle this is probably to boot all the guests using pygrub 
and let them decide which kernel they want to boot off their own filesystem.  
The guest OS can be responsible for updating this kernel as necessary, etc.

Cheers,
Mark

-- 
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!

_______________________________________________
Xen-users mailing list
Xen-users@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-users