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

To: "Petersson, Mats" <Mats.Petersson@xxxxxxx>
Subject: Re: [Xen-users] Xen a couple of questions
From: Nico Kadel-Garcia <nkadel@xxxxxxxxx>
Date: Wed, 06 Jun 2007 10:16:10 +0100
Cc: Octavian Teodorescu <octav@xxxxxxxxxxxxxxxx>, Mark Williamson <mark.williamson@xxxxxxxxxxxx>, xen-users@xxxxxxxxxxxxxxxxxxx
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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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