[Libvir] RFC: get/set properties

Daniel P. Berrange berrange at redhat.com
Wed Apr 16 13:39:57 UTC 2008

On Wed, Apr 16, 2008 at 12:13:09PM +0100, John Levon wrote:
> On Wed, Apr 16, 2008 at 03:58:16AM -0400, Daniel Veillard wrote:
> > > One thing we have in mind is driver/software version numbers.  For 
> > > example, the control tools may change the domain configuration based on 
> > > whether a certain driver has the support for a new feature.  If we 
> > > create the domain's xml with the driver information, we can make this 
> > > decision quickly, on the fly, in dom0.
> > 
> > Taking the specific example of driver support I think it's far better to
> > express that as part of the device description of the domain.
> There's a wider usage case though. Management tools can often need to
> track assets, and the UUID is not necessarily sufficient for that. Tools
> want to be able to associate textual descriptions of the asset, or
> define ownership. All of these are higher-level values that don't
> necessarily have any meaning to the v12n technology itself. More
> crucially they can be specific to the management tool, and lack any
> reasonable generalization.

This kind of information doesn't really belong in the domain XML. The domain
XML is describing an instantiation of a VM on a particular machine. 

If you have a application managing a bunch of VMs & hosts across the data 
center you're not going to be relying on the hosts running the VMs as your
master storage repository for information about a VM. You have to expect 
any host in the data center is expendible and can go away at any time. The 
management app will have its own storage database of some form where it keeps
its master copy of all configs so it can re-deploy the VM to another host upon
failure of a physical host. This is the logical place for asset / ownership 
data and any other metadata of this kind, along with the master copy of the 
VM configuration (which may or may not be stored in the libvirt XML format).
The VM configuration need only be pushed out to individual hosts when a VM 
is deployed / run, and does not need to include asset / ownership metadata.

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