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Re: [Fedora-xen] Upgrade FC5 Xen -> FC6



On Thu, Nov 09, 2006 at 07:47:09PM -0500, Kanwar Ranbir Sandhu wrote:
> On Thu, 2006-11-09 at 16:03 +0000, Daniel P. Berrange wrote:
> > The solution is fairly simple though - the latest FC5 errata also provides a 
> > 'kernel-xen' RPM which is PAE enabled. So before upgrading your host, just 
> > make sure you have done 'yum install kernel-xen' in each of your guests.
> > This will let your FC5 guests contonue working on an FC6 host - you can then
> > upgrade your guests to FC6 at will.
> 
> I don't understand this.  Why does kernel-xen have to be installed in
> the guest?  Doesn't the guest just use the xen kernel running in dom0?
> I'm obviously missing something here.

It doesn't *have* to be installed in the guest - that's just way we deal 
with it in Fedora. The overriding strategy for Xen in Fedora is that we
want to make installing, upgrading, managing the operating system in a
guest a close to what you do on bare metal as possible[1] - so all your
existing Linux knowledge still applies - this makes it easier for people
who don't know Xen to get started.

Basically the way it works is thus:

 - In the guest OS a regular rpm install of the kernel-xen is done.
   So a regular grub config is written to the boot sector of the 
   virtual disk pointing to the kernel inside the image.
 - When booting the guest from Dom0, we run 'pygrub' which pulls
   the grub config from the DomU filesystem & uses that to extract
   the guest kernel to a temporay file /var/lib/xen
 - The DomU is then booted off the temp file in /var/lib/xen

So, yes for the actual boot process the kernel is located in Dom0.

The other nice thing about this is that the DomU it self contained - you
can move the filesystem image anywhere & no have to worry about carting
around a separate kernel image file to go with it.

Then again, some people may like to have a single kernel image in the Dom0
whcih they use to  boot all their guest images. If you want to manage your
guests that way, simply add the 'kernel=/foo/bar/wizz' parameter to the
guest config, and remove the pygrub config option.

Dan.

[1] Other examples....
      - using regular Anaconda for installing the DomU rather than
        fragile install into a chroot shell scripts other distro use.
      - adding the paravirt framebuffer to Xen so you have a fully
        graphical console 
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