----- Original Message -----
On Wed, Nov 23, 2016 at 07:59:40AM -0500, 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
> 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
> * 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
> * 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 
> And using the XML that is received as an input in VDSM 
>  https://gerrit.ovirt.org/#/c/64473/
>  https://gerrit.ovirt.org/#/c/65182/
I should start by saying that I love libvirt's domxml standard. Unlike
Vdsm's API, it is a real *standard* for defining VMs. In this regards,
you are suggesting a positive step.
However, Engine is much more complex than Vdsm. It is also our
single-point-of-failure, and where CPU is the most scarce. I am worried
that in the foreseeable future it would only make Engine bigger, without
reducing the size and complexity of Vdsm.
Before taking this move, we must map what Vdsm does, because that logic
would have to be copied into Engine. Few things pop up to mind:
- pci addresses. would Vdsm report back the libvirt-assigned addresses
in XML format, or would it keep parsing them?
Ideally, VDSM will report back the devices in XML format.
The engine will then add the unmanaged devices and update the pci addresses.
Need to put some more thoughts into this, though.
- hot plug. Device xml should be generated by Engine, much like in the
vm cteate flow
Good point, I didn't think of hot plugs - right, they could be changed as well later
- network rewiring. Vdsm uses the "dummy bridge" to implement a vNIC
that is connected no-where. Engine would need to care about what was
up until now a vdsm-side implementation detail.
Right, I almost finished to copy the creation of the network interfaces to the engine.
This knowledge that you refer to will only be in the module that creates the XML, it
doesn't seem to be much of an issue.
- storage path. this was mentioned above, but actually, the paths are
the same on all hosts. We inteded to have an abstraction layer there,
but we never ever used it. All volumes sit under
Basically, Engine can hard-code this in the domxml, and no one would
But I see that LUN and cinder disks are represented differently (not as PDIV) - I'll
- OvS. Recently, we have changed how VMs can be connected to their
network. It is possible (albeit not recommended yet!) to connect a VM
to an OvS instead of Linux bridges. This is done without Engine really
caring, or knowing how the domxml is modified.
Yeah, I saw that. The only complication I see at this point is that for OvS we create more
elements than only the 'source' element.
I believe that we could use a place-holder that contains the network name and replace it
with the tags that are needed for SR-IOV, OvS and ordinary interfaces, no?
This seems to be the only thing that is difficult to generate on the engine's side
(related to the network interfaces).
- minor tweaks. exposing a new feature into Engine's UI is hard. Over
the years, few tweaks have been pushed as custom properties.
there are not many (I see now only sndbuf, queues, viodiskcache,
vhost) but the implementation should make sure they are not forgotten.
Maybe, Vdsm should consider Engine's domxml only as a "recomendation"
and modify it based on its hooks and custom properties. This can
surprise Engine, a defies the pupose of having xml-building logic moved
away from Vdsm.