[CentOS-virt] moving from Xen to KVM

Pasi Kärkkäinen pasik at iki.fi
Thu Mar 18 20:34:31 UTC 2010

On Wed, Mar 17, 2010 at 03:49:40PM -0700, Grant McWilliams wrote:
>    On Wed, Mar 17, 2010 at 6:35 AM, Dennis J. <[1]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,
>      Â  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.

Do you have some benchmarks showing Xen HVM is slower than KVM? 
I believe so far Xen HVM has actually been faster than KVM.

>    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).

Lately Xen has gotten a couple of different types of memory sharing implementations aswell.
At least one of them will ship in upcoming Xen 4.0.0.

-- Pasi

>    Grant McWilliams
>    Some people, when confronted with a problem, think "I know, I'll use
>    Windows."
>    Now they have two problems.
> References
>    Visible links
>    1. mailto:dennisml at conversis.de

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