[ovirt-users] Mixing CPU types

[Matthew Trent]

Sure would be a nice feature, though! It would simplify things for those of us who build out of re-purposed Windows servers (still a lot of life left in them for Linux applications!) and end up with a mix of CPUs. For most of my VMs I don't need the latest and greatest CPU features, but being able to consolidate my hosts into fewer clusters would be really useful.

Like you said, oVirt already allows mixing CPU generations by selecting the lowest common denominator CPU type -- as long as you stick to Intel or AMD. It's a similar scenario, and the KVM stuff seems well established. Just expose some additional "generic" CPU types, per the KVM docs, and allow mixed clusters...

--
Matthew Trent
Network Engineer
Lewis County IT Services
360.740.1247 - Helpdesk
360.740.3343 - Direct line

________________________________________
From: Martin Polednik <mpolednik at redhat.com>
Sent: Wednesday, February 3, 2016 11:43 PM
To: Matthew Trent
Cc: Yaniv Kaul; users at ovirt.org
Subject: Re: [ovirt-users] Mixing CPU types

----- Original Message -----
> From: "Matthew Trent" <Matthew.Trent at lewiscountywa.gov>
> To: "Yaniv Kaul" <ykaul at redhat.com>
> Cc: users at ovirt.org
> Sent: Wednesday, February 3, 2016 8:10:35 PM
> Subject: Re: [ovirt-users] Mixing CPU types
>
> ?Actually it works great! It's been supported by KVM for a while now. I had
> one Proxmox cluster with a mix of AMD and Intel and migrated KVM VM's
> between them frequently. OpenStack (KVM) and VMWare support this as well.
>
>
> The trick is to expose a virtual CPU with a common set of features to the
> VMs. KVM handles the rest. From the KVM FAQ:
>
> http://www.linux-kvm.org/page/FAQ#Does_KVM_support_live_migration_from_an_AMD_host_to_an_Intel_host_and_back.3F

That is pretty much what libvirt does. The sandy-bridge, haswell and other
cpu's that we expose are not really a CPUs, but sets of flags (called
baseline CPUs in libvirt's terminology).

Afaik there should be no issue running cluster with both CPU vendors,
the problem is lack of testing on devel side as we mostly run
single vendor clusters everywhere.

> "Does KVM support live migration from an AMD host to an Intel host and back?
>
>
> Yes. There may be issues on 32-bit Intel hosts which don't support NX (or
> XD), but for 64-bit hosts back and forth migration should work well.
> Migration of 32-bit guests should work between 32-bit hosts and 64-bit
> hosts. If one of your hosts does not support NX, you may consider disabling
> NX when starting the guest on a NX-capable system. You can do it by passing
> "-cpu qemu64,-nx" parameter to the guest."
>
> --
> Matthew Trent
> Network Engineer
> Lewis County IT Services
> 360.740.1247 - Helpdesk
> 360.740.3343 - Direct line