[PVE-User] live migration amd - intel

[Stefan Reiter]

Hi!

On 2/26/20 3:35 PM, mj wrote:
> Hi,
> 
> Just to make sure I understand something. We have an identical 
> three-node hyperconverged pve cluster, on Intel Xeon CPU's.
> 
> Now we would like to expand, and we are investigating what path to choose.
> 
> The way we understand it is: if we would like to do live migrations 
> between pve hosts, the servers need to have similar CPUs, or otherwise 
> we need to virtualise the CPU as well. (so set CPU type to kvm64)
> 

Note that even kvm64 does not really work for cross-vendor 
live-migrations (i.e. AMD <-> Intel as mentioned in subject). This was 
recently discussed on the pve-devel list [0], as well as an older bug 
report [1].

[0] https://pve.proxmox.com/pipermail/pve-devel/2020-February/041750.html
[1] https://bugzilla.proxmox.com/show_bug.cgi?id=1660

> In case it matters: 90% of our VMs are debian 9/10 hosts.
> 
> While we could do change to kvm64, we currently use cpu type 'host', and 
> we wonder how much performance kvm64 would cost us, and what potential 
> other drawbacks this could have.
> 

Aside from the above, the performance impact of switching to kvm64 is 
largely dependent on the software you are running within your VM. Raw 
CPU performance will stay the same, but most hardware acceleration (e.g. 
AES for encryption or AVX) will not be available to the guest.

If the CPUs are from the same vendor, you can most likely enable the 
older generation CPU type for your VMs and still get away with 
live-migration. This way you can use _most_ acceleration features, and 
are just missing out on any new ones introduced in the newer CPU gen.

> Has anyone ever done much testing on this subject? Anyone with 
> interesting insights / knowledge / experiences to share on this subject?
> 
> MJ

Hope that helps,

Stefan