[ovirt-devel] Heads-up: moving Libvirt xml creation to the engine

[Arik Hadas]

----- Original Message -----
> On Wed, Nov 23, 2016 at 2:59 PM, Arik Hadas <ahadas at redhat.com> wrote:
> > Hi All,
> >
> > We are working on something that is expected to have a big impact, hence
> > this heads-up.
> > First, we want you to be aware of this change and provide your feedback to
> > make it as good as possible.
> > Second, until the proposed mechanism is fully merged there will be a chase
> > to cover all features unless new features are also implemented with the
> > new mechanism. So please, if you are working on something that
> > adds/changes something in the Libvirt's domain xml, do it with this new
> > mechanism as well (first version would be merged soon).
> >
> > * Goal
> > Creating Libvirt XML in the engine rather than in VDSM.
> > ** Today's flow
> > Engine: VM business entity -> VM properties map
> > VDSM:   VM properties map  -> Libvirt XML
> > ** Desired flow
> > Engine: VM business entity -> Libvirt XML
> >
> > * Potential Benefits
> > 1. Reduce the number of conversions from 2 to 1, reducing chances for
> > mistakes in the process.
> > 2. Reduce the amount of code in VDSM.
> > 3. Make VM related changes easier - today many of these changes need to be
> > reviewed in 2 projects, this will eliminate the one that tends to take
> > longer.
> > 4. Prevent shortcuts in the form of VDSM-only changes that should be better
> > reflected in the engine.
> > 5. Not to re-generate the XML on each rerun attempt of VM run/migration.
> > 6. Future - not to re-generate the XML on each attempt to auto-start HA VM
> > when using vm-leases (need to make sure we're using the up-to-date VM
> > configuration though).
> > 7. We already found improvements and cleanups that could be made while
> > touching this area (e.g., remove the boot order from devices in the
> > database).
> >
> > * Challenges
> > 1. Not to move host-specific information to the engine. For example, path
> > to storage domain or sockets of channels.
> >    The solution is to use place-holders that will be replaced by VDSM.
> > 2. Backward compatibility.
> > 3. The more challenging part is the other direction - that will be the next
> > phase.
> >
> > * Status
> > As a first step, we began with producing the Libvirt XML in the engine by
> > converting the VM properties map to XML in the engine [1]
> > And using the XML that is received as an input in VDSM [2]
> 
> Thanks for the heads up!
> 
> How does this, if at all, affect:
> 1. vdsm hooks

VDSM hooks will remain unaffected - they will get the same XML.
The only difference is that this XML will be one that was produced by the engine after the replacement of its place-holders rather than one that is produced by VDSM from the properties map.

> 2. The interaction of engine, hosted-engine HA agent, engine vm
> configuration saved in shared storage, etc.? Will this require a
> change in ha-agent and the saved configuration?

In the foreseeable future VDSM should support creating the VM from VM properties map as well - so legacy code will keep working, including the creation of the hosted-engine.
However, at some point the creation of the hosted-engine would also need to change I assume.
The rest, other than the creation flow, will also remain unaffected.

> 
> Best,
> 
> >
> >
> > [1] https://gerrit.ovirt.org/#/c/64473/
> > [2] https://gerrit.ovirt.org/#/c/65182/
> >
> > Regards,
> > Arik
> > _______________________________________________
> > Devel mailing list
> > Devel at ovirt.org
> > http://lists.ovirt.org/mailman/listinfo/devel
> 
> 
> 
> --
> Didi
>