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Re: [Xen-users] Separate kernel on domU's

On Feb 27, 2012 3:55 PM, "eva" <evammg@xxxxxxxxx> wrote:
> On 24 February 2012 22:55, Luke S. Crawford <lsc@xxxxxxxxx> wrote:
> > On Wed, Feb 22, 2012 at 12:28:51PM +0100, eva wrote:
> >> Hello,
> >>
> >> I am still learning about Xen.. I am trying to setup Xen hypervisor
> >> for the first time. I was reading the howto here:
> >>
> >> http://www.howtoforge.com/paravirtualization-with-xen-4.0-on-debian-squeeze-amd64
> >>
> >> and I stopped here:
> >>
> >> "(To use the default Ubuntu kernel instead of Debian's Xen kernel in
> >> the guest, you can also comment out theÂkernelÂandÂinitrdÂlines in
> >> /etc/xen-tools/xen-tools.conf.)"
> >
> [...]
> Thanks, your answer was very useful to me.
> I thought in xen kernels work separately, as it's shown in this
> representation....
> https://community.emc.com/servlet/JiveServlet/showImage/38-3466-30315/Xen.png
> but now it seems that you can set it up to share a few things. At this
> point I was quite surprised.
> I just want to understand how xen works.. so why would you share the
> kernel or parts of the kernel with another dom? Must be a good reason
> to do it, I think.
> Using the same initrd looks fine to me, but I using other parts of the
> kernel.. it looks like that you must really know what you doing or you
> will end up with a mess after an upgrade, for example.
> So in what cases would you do it and why?
> Fajar tried to explain it a little, but I still don't fully understand it ..
> Thank you

Usually, domU's use a different kernel from dom0. But multiple domU's might share a single kernel image, stored in dom0.

When starting a domU configured to use a kernel stored in dom0, dom0 will simply tell Xen to "boot using this file, and pass these parameters to the kernel".

Benefits: Don't have to update domU kernels one by one, domU root can't overwrite the kernel in dom0, domU's can be forced to use a specific kernel, no need for a separate /boot partition, etc.

Drawback: a buggy kernel shared by multiple domU's will impact many VMs at the same time.

But it its perfectly acceptable also for domU's to have their own kernels; this allows Windows VMs, for example. In this situation, dom0 will tell Xen: This is the virtual hard disk. Please boot it from the MBR.


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