[CentOS-virt] XEN and RH 6

Pasi Kärkkäinen pasik at iki.fi
Tue Nov 10 15:13:52 UTC 2009


On Tue, Nov 10, 2009 at 05:12:50PM +0200, Pasi Kärkkäinen wrote:
> On Tue, Nov 10, 2009 at 03:49:59PM +0100, Dennis J. wrote:
> > On 11/10/2009 03:35 PM, Grant McWilliams wrote:
> > >
> > >     Both Novell and Oracle having been deeply involved in Xen lately, both
> > >     are developing and supporting their own products based on Xen.
> > >
> > >     -- Pasi
> > >
> > >     ___
> > >
> > >
> > >
> > > I have no problem with a "better" solution than Xen because to be honest
> > > it's a pain sometimes but at this point virtually all enterprise VM
> > > deployments are either based on VMware ESX or Xen (Xenserver,
> > > VirtualIron, Amazon AWS, Oracle, Sun SVM, Redhat and Suse). This tide
> > > will change as KVM becomes more dominant in the VM space but I don't see
> > > that happening for a while. I'm also a bit skeptical as to how well a
> > > fully virtualized system (KVM) will run in comparison to a fully
> > > paravirtualized system (Xen PV). I have a system with 41 VMs on it and
> > > I'll be having 2 weeks of planned downtime in the near future. I'd like
> > > to see how these systems run under KVM.
> > 
> > I've been wondering about the definition of PV in the context of KVM/Xen. 
> > In the Linux on Linux case for Xen PV practically means that in the HVM 
> > case I have to access block devices using /dev/hda while in the PV case I 
> > can use the faster /dev/xvda. When using KVM which apparently only supports 
> > HVM I can still install a guest using the virtio drivers which seem to do 
> > the same as the paravirtualized devices on Xen.
> > 
> > So what is the KVM+virtio case if not paravirtualization?
> > 
> 
> KVM+virtio means you're using paravirtualized disk/net drivers on a
> fully virtualized guest.. where Qemu emulates full PC hardware with BIOS
> and all. So only the disk/net virtio drivers bypass Qemu emulation. 
> (Those are the most important and most used devices.)
> 
> Xen paravirtualized guests run natively on Xen, there's no need for
> emulation since the guest kernels are aware that they're being
> virtualized.. There's no Qemu emulating PC hardware with BIOS for PV guests.
> 

Oh, and Xen also has PV-on-HVM drivers for HVM fully virtualized guests
to bypass Qemu :)

-- Pasi



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