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

[Itamar Heim]

On 11/23/2016 07:59 AM, Arik Hadas 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.

This would be interesting in several aspects:
- parsing it to learn what we expect.
- but not throwing an exception if a hook changed the expected 
result/device-type to something we don't know/expect in the engine...
- to make "external VMs" better integrated
- so we can at some point allow external manipulation/creation of a VM.

>
> * 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
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