[CentOS-virt] moving from Xen to KVM

Grant McWilliams grantmasterflash at gmail.com
Wed Mar 17 23:49:40 UTC 2010

On Wed, Mar 17, 2010 at 6:35 AM, Dennis J. <dennisml at conversis.de> wrote:

> On 03/17/2010 02:15 PM, Hildebrand, Nils, 232 wrote:
> > Hi,
> > I have 31 DomUs up and running on a single Box - and have a strong
> feeling that even 60 will run flawless.
> > But: All of them are Para-Virtualized.
> >
> > I have no problem with disk IO-Bottlenecks since my DomUs are not
> Database-Servers - so there is mostly static information in the filesystems.
> The term "paravirtualization" is becoming quite dated. Even if you install
> a KVM guest without that option if you choose the virtio driver inside then
> you still end up with "paravirtualized" I/O. With the advent of things like
> nested page tables and SR-IOV the "fully virtualized=slow,
> paravirtualized=way faster" logic is no longer necessarily true at least
> not for every aspect of the system.
> Regards,
>   Dennis

In the Xen world paravirtualizing will be replaced by Hybrid virtualizing.
As hardware virt becomes faster (ie, not so slow) then Xen will change to
using HVM as the default and paravirtualize EVERYTHING else. This is not the
same thing as KVM which uses hardware virt for cpu, emulation for most
things except disk and network which are paravirtualized (if chosen). I look
forward to this as HVM in Xen is slower than KVM even though it's kind of
doing the same thing.  However, I don't think people have benchmarked either
enough to realize how much of a hit we're taking with virtio.

KVM has some neat tricks up their sleeve as well like shared memory, nesting
etc.. I may put up a KVM box just because I need nesting (for a classroom to
teach virtualization).

Grant McWilliams

Some people, when confronted with a problem, think "I know, I'll use
Now they have two problems.
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