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

[Arik Hadas]

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]


[1] https://gerrit.ovirt.org/#/c/64473/
[2] https://gerrit.ovirt.org/#/c/65182/

Regards,
Arik