[Openstack-operators] User Survey usage of QEMU (as opposed to KVM) ?

Jared Wilkinson JWilkinson at ebsco.com
Tue May 3 19:06:56 UTC 2016

So forgive my lack of kvm/qemu knowledge but I couldn’t find anything on Google on this. If you deployed an instance of a different architecture than the physical CPU, wouldn’t qemu just emulate the processor (if you were in virt_type=kvm) mode, or would libvirt throw some error?


Jared Wilkinson | Infrastructure Engineer – Systems
jwilkinson at ebsco.com | (W) 205/981-4018 | (M) 205/259-9802
5724 US Highway 280 East, Birmingham, AL 35242, USA

On 5/3/16, 10:01 AM, "Daniel P. Berrange" <berrange at redhat.com> wrote:

>Hello Operators,
>One of the things that constantly puzzles me when reading the user
>survey results wrt hypervisor is the high number of respondants
>claiming to be using QEMU (as distinct from KVM).
>As a reminder, in Nova saying virt_type=qemu causes Nova to use
>plain QEMU with pure CPU emulation which is many many times slower
>to than native CPU performance, while virt_type=kvm causes Nova to
>use QEMU with KVM hardware CPU acceleration which is close to native
>IOW, virt_type=qemu is not something you'd ever really want to use
>unless you had no other options due to the terrible performance it
>would show. The only reasons to use QEMU are if you need non-native
>architecture support (ie running arm/ppc on x86_64 host), or if you
>can't do KVM due to hardware restrictions (ie ancient hardware, or
>running compute hosts inside virtual machines)
>Despite this, in the 2016 survey 10% claimed to be using QEMU in
>production & 3% in PoC and dev, in 2014 it was even higher at 15%
>in prod & 12% in PoC and 28% in dev.
>Personally my gut feeling says that QEMU usage ought to be in very
>low single figures, so I'm curious as to the apparent anomoly.
>I can think of a few reasons
> 1. Respondants are confused as to the difference between QEMU
>    and KVM, so are saying QEMU, despite fact they are using KVM.
> 2. Respondants are confused as to the difference between QEMU
>    and KVM, so have mistakenly configured their nova hosts to
>    use QEMU instead of KVM and suffering poor performance without
>    realizing their mistake.
> 3. There are more people than I expect who are running their
>    cloud compute hosts inside virtual machines, and thus are
>    unable to use KVM.
> 4. There are more people than I expect who are providing cloud
>    hosting for non-native architectures. eg ability to run an
>    arm7/ppc guest image on an x86_64 host and so genuinely must
>    use QEMU
>If items 1 / 2 are the cause, then by implication the user survey
>is likely under-reporting the (already huge) scale of the KVM usage.
>I can see 3. being a likely explanation for high usage of QEMU in a
>dev or PoC scenario, but it feels unlikely for a production deployment.
>While 4 is technically possible, Nova doesn't really do a very good
>job at mixed guest arch hosting - I'm pretty sure there are broken
>pieces waiting to bite people who try it.
>Does anyone have any thoughts on this topic ?
>Indeed, is there anyone here who genuinely use virt_type=qemu in a
>production deployment of OpenStack who might have other reasons that
>I've missed ?
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