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RE: [Xen-users] Several XEN questions


> -----Original Message-----
> From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx 
> [mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of 
> Alexander Wilms
> Sent: 08 March 2006 23:48
> To: xen-users@xxxxxxxxxxxxxxxxxxx
> Subject: Re: [Xen-users] Several XEN questions
> Am Donnerstag, 9. März 2006 00:10 schrieb Fabiano Sidler:
> > I planned to play around a bit with XEN on my P3 machine. 
> Before I go 
> > on, I still have several questions that involve the planning phasis.
> >
> > -What about module loading in domU kernels?
> > I know that it won't success with hardware related drivers, 
> but is it 
> > possible to load "pure software drivers" like iptables?
> Yes sure, software modules are handled like in normal kernels.
> In xen-unstable there are also already patches to pass PCI 
> devices to domU's.
> (I'm sending this mail via a domU with a delegated physical NIC ;-) )
> >
> > Reboot the dom0 system while running domU systems?
> > I'm not sure how equal the dom0 system is to the domU ones, but as 
> > long as the hypervisor isn't stopped, can dom0 be rebooted without 
> > interrupt of the domU systems? Would it be possible on 
> > hardware-supported virtualisation versions of the 
> hypervisor, as with Vanderpool of Pacifica?
> Not possible, because the hypervisor need's userspace tools 
> like xend to run domU's. I guess VT et. al. won't change 
> that. general architecture.

Most importantly, the HVM (VT/SVM) guest would rely on qemu-dm, which lives in 
Dom0. And there's currently no way to reboot Dom0 without also reloading Xen, 
so the first problem would be to disconnect Dom0 from Xen. If you loose Xen, 
then NO DomU would work at all, obviously. 

There is work going on with kexec, which will allow you to replace the Linux 
kernel without rebooting. This MAY solve the problem [but I'm not sure how far 
along the kexec work has got]. Here's some info on kexec under Linux [I haven't 
read all of it, I just googled for kexec - there may be better material, I got 
MANY hits]: http://www-128.ibm.com/developerworks/linux/library/l-kexec.html

Whilst it would potentially be possible to do so, it would make much more sense 
to shuffle DomU to a second machine using the migration features in Xen, then 
reboot/replace Dom0 on the first machine. Admittedly, migration, I think, is 
still broken on HVM systems, but it's certainly meant to work at some future 
point in time. Migration will of course stop the domain for a brief moment, so 
if you want five nines service availability, you'll have to combine this with 
some other fail-over technology, like having two servers running the same DomU 
in parallell or some such. [In which case it may not be needed to move the DomU 
in the first place, of course]. 

If you view the Dom0 as part of the Operating System, and DomU's as 
"applications", then it becomes more clear.


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