On Wed, Dec 05, 2012 at 04:05:00PM +0200, Yaniv Kaul wrote:
On 12/05/2012 03:55 PM, Laszlo Hornyak wrote:
The nice thing about hostModel (unlike hostPassthrough) is that once we created the VM we can migrate it to stronger hosts, and back to the original host. I suppose that it complicates the scheduler.
Yes with host-model you get the features that libvirt handles. In such cases the engine could decide, if you want this functionality. Well the scheduler architecture is just being reinvented.
For the host-passthrough, I think the AllowMigrateCPUHost configuration option would be a simple decision for the administrator: set it to true if all hosts are uniform.
If it is THAT simple, Engine could take this decision without human intervension.
If it is not set to true, then we will not allow migration of such VMs. That's not what I understood from libvirt's documentation. I You may be right, could you send an URL to that point of the documentation or copy-paste?
The link I followed from your feature page: http://libvirt.org/formatdomain.html#elementsCPU :
host-model The host-model mode is essentially a shortcut to copying host CPU definition from capabilities XML into domain XML. Since the CPU definition is copied just before starting a domain, exactly the same XML can be used on different hosts while still providing the best guest CPU each host supports. Neither match attribute nor any feature elements can be used in this mode. Specifying CPU model is not supported either, but model's fallback attribute may still be used. Libvirt does not model every aspect of each CPU so the guest CPU will not match the host CPU exactly. On the other hand, the ABI provided to the guest is reproducible. During migration, complete CPU model definition is transferred to the destination host so the migrated guest will see exactly the same CPU model even if the destination host contains more capable CPUs for the running instance of the guest; but shutting down and restarting the guest may present different hardware to the guest according to the capabilities of the new host. host-passthrough With this mode, the CPU visible to the guest should be exactly the same as the host CPU even in the aspects that libvirt does not understand. Though the downside of this mode is that the guest environment cannot be reproduced on different hardware. Thus, if you hit any bugs, you are on your own.
That's exactly where AllowMigrateCPUHost fits well: when a user ticks this for a cluster he says "yeah, I like to be on my own."