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

[Nico Kadel-Garcia <nkadel@xxxxxxxxx>]

Petersson, Mats wrote:
>  
>
>   
> > 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.
> >     
> The default kernel build in current Xen releases (since 3.0.4 or before)
> is to build one kernel that works for both. Yes, there are some
> differences between a XenU and Xen0 config, in that one has the
> "privileged" option set, and there are some differences in drivers added
> to the kernel (particularly, XenU doesn't normally have ANY support for
> ANY hardware drivers). But you should ALWAYS be able to sue the Dom0
> kernel for DomU, even if the other way around doesn't necessarily hold
> true. 
>
>   
> > 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.
> >     
> Yes, if you are loading dozens or more of guest kernels, the size of the
> actual kernel will matter. I'm not sure how much the difference is tho'.
>   
And if you call RedHat, Oracle, or McAfee for support with kernel 
related issues on RHEL 4, and tell them "I'm running a 2.6.18 kernel", 
they're going to have good reason to throw p their hands and say "revert 
to the published kernel, then we can help you". They might not: premium 
corporate support is pretty good, but I'd be tempted to do that as the 
support person.
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