Re: [Xen-users] Is upgrade DomU can broke my XEN box ?

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

> >   I'm running for servers with RHEL 5.1 XEN on it. On each of them, I'm
> > running 2 big Para-Virt RHEL 5.1 also.
> >
> >   I would like to know if someday, RedHat decide to upgrade the kernel
> > and make a RHEL 5.2 for example. If I upgrade DomU to 5.2 with a new Xen
> > Kernel, it will broke all my Para-Virt RHEL 5.1 machines ?

Newer Xens should generally always be able to run older domUs OK.  This is a 
guarantee provided by Xen - Xen keeps older interfaces around for 
compatibility reasons.

There are limitations if you start moving between PAE / non-PAE / 64-bit but 
it doesn't sound like you're looking at that right now.

So if you upgrade RHEL in dom0, then the domUs will still keep working even if 
you don't upgrade them.

> >   If I do the opposite, keep RHEL 5.1 XEN on DomU and upgrade the Xen
> > kernel inside a Para-Virt machine, is my machine will also broken ?

1) Changes within a single domU shouldn't break the whole machine, they should 
be restricted to within that domU.

2) XenLinux kernels can optionally retain compatibility with older Xens.  I 
would be very surprised if RHEL did not include this, so yes I'd expect you 
could run a RHEL in a domU that is newer than the RHEL in dom0.

A small caveat is that if you run a newer XenLinux in a domU than the Xen / 
XenLinux in dom0 then it's possible some newer features won't be available 
(depending on whether the dom0 version supports them).  But the domain should 
still run OK.

> >   In clear, how are XEN "relationships" between guest & host ? Are they
> > easily breakable? What are the common rules in that area?

Newer dom0 kernel / Xen should always run older domU kernels.  Newer domU 
kernels will often run on older Xen, although there may be reduced 
functionality.

HTH,
Cheers,
Mark


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