[Engine-devel] oVirt upstream meeting : VM Version

[Itamar Heim]

On 02/05/2012 02:57 PM, Miki Kenneth wrote:
...

>>>> Isn't the VM version derived from the version of the cluster on
>>>> which it was last edited?
>>>> For example: you've created a VM on a cluster v3.0. When it is
>>>> running on a v3.2 cluster, is there any reason to change its
>>>> version?
>>>> When it is edited, then perhaps yes - because it may have
>>>> changed/added properties/features that are only applicable to
>>>> v3.2.
>>>> But until then - let it stay in the same version as it was
>>>> created.
>>>> (btw, how does this map, if at all, to the '-m' qemu command line
>>>> switch?)
>>>> Y.
>>>>
>>>
>>> Currently we do not persist the VM version at all, it is derived
>>> from
>>> the cluster version the VM belongs to (that's why I suggested to
>>> save it
>>> as part of the OVF so we can be aware of the VM version when
>>> exporting/importing a VM etc.).
>>>
>>> The VM does not have to be edited to be influenced by the cluster
>>> version. For example if you start a VM on 3.1 cluster you get the
>>> stable
>>> device address feature with no manual editing.
>>>
>>> Livnat
>>>
> However, I do agree with Yaniv that changing the VM version "under the hood" is a bit problematic. Version is a parameter associated with create/update operation, and less with Run command.

but the engine currently has no logic to detect the need to increase the 
emulated machine to support feature X.
the engine currently does not save this parameter at VM level.
it will also need to compare it to the list of supported emulated 
machines at the cluster, and prevent running the VM if there isn't a match.
it also increases the matrix of possible emulated machines being run on 
different versions of hypervisor to N*cluster_levels, instead of just 
the number of cluster levels.
plus, if a cluster is increased to a new version of hosts which doesn't 
support an older emulated machine level - user will need to upgrade all 
VMs one by one?
(or will engine block upgrading cluster level if the new cluster level 
doesn't have an emulated machine in use by one of the virtual machines)
it also means engine needs to handle validation logic for this field 
when exporting/importing (point of this discussion), as well as just 
moving a VM between clusters.

so before introducing all this logic - were issues observed where 
changing the cluster level (i.e., -M at host level) resulted in 
problematic changes at guest level worth all of these?