Re: [Xen-users] live migration between heterogeneous cpu

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

On Fri, 2015-09-18 at 17:28 +0200, matteo nunziati wrote:
> hello,
> 
> quick Q&A redirects me here [1]. 
> 
> I'm going to build a micro-cluster for internal services in our company.
> services run on both windows and linux. it is a fact that the cluster
> will grow up slowly and cpu diversity will be very probable. 
> 
> While I do not know if an intel-amd mix will be present, for sure we
> already have started using different xeon E* families from either v2 or
> v3 generations (namely an E3-1220v2 will be "pooled" with either a E3
> -1220v3 or a E5-2609v3).
> 
> the question is: can I live migrate guests in this kind of scenario? 

Within processors from a single manufacturer the thing to watch out for is
processors with features which others in the pool do not have and which a
guest might therefore be exposed to on boot and be upset about losing after
a migration.

The way to make this work is to "level" the CPUs, by hiding features on the
newer processors from guests.

You can do this on a per guest bases from the xl cfg file using the cpuid
directive or for the whole host from the hypervisor command line using the
various cpuid_mask identifiers.

Unfortunately actually figuring the masks to use is tricky. I suppose you
could start by comparing the flags shown in /proc/cpuinfo in dom0 across
the different machines and then eradicating the differences by taking each
one in turn, figuring out which cpuid bit it corresponds to, and setting
the host mask to get rid of it. Pretty tedious though.

> 
> To be honest, live migration is not mandatory, but considering that some
> dependable services run on windows server, move the machine in offline
> migration from 1 cpu to another is not something I really like to do as
> windows OS itself will start complain about the move. Therefore I _guess_
> that avoiding windows complains goes in parallel to live migration
> capabilities as this means that VM is not aware of the underling HW
> (maybe I'm wrong).
> 
> regards,
> MN
> 
> [1] http://www.xenproject.org/help/questions-and-answers/live-migrate-bet
> ween-different-cpu.html
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