[CentOS-virt] XEN and RH 6

Dennis J. dennisml at conversis.de
Tue Nov 10 15:18:39 UTC 2009

On 11/10/2009 04:13 PM, Pasi Kärkkäinen wrote:
> 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 :)

Which I guess makes describing a guest as "fully virtualized" or 
"paravirtualized" rather pointless given that there now is just a degree of 
how paravirtualized a guest is depending on the drivers you use.


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