Xen users - Jan 2008 - Is upgrade DomU can broke my XEN box ?

Hello,

  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 ?

  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 ?

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


  thanks by advance,


   LMJ

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linuxmasterjedi@free.fr wrote:
> Hello,
>
>   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 ?
>   
RedHat publishes minor kernel and software upgrades on a frequent basis, 
as bugs and security issues get revealed. RedHat, like SuSE and other 
commercial vendors, guarantees cross-compatibility and support for 
particular release''s software as long as you stay within that release. 
So othe kernel updates should be compatible with the Xen release at RedHat.

This can drive you nuts if you need to jump up a significant version to 
get necessary features. But staying within the RHEL 5.x published 
packages should work well.
>   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 ?
>   
See above. RedHat is selling Xen support, so they should be good about 
not breaking clients who have RHEL 5.x on the server and RHEL 5.x on the 
client, even as one is updated and not the other.

>   In clear, how are XEN "relationships" between guest & host
? Are they easily
> breakable? What are the common rules in that area?
>
>
>   thanks by advance,
>   
It Depends(tm). If you manually download the very latest Xen source 
tarball and lay it on top of the RHEL published RPM''s, you''re
going to
break things.
 If you''re using fully virtualized DomU''s, they should be
fairly immune
from Dom0 changes, barring some major Xen upgrade that seriously changes 
the drivers expected by the DomU.

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> >   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


-- 
Push Me Pull You - Distributed SCM tool (http://www.cl.cam.ac.uk/pmpu/)

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