Re: [Qemu-devel] Should we auto-generate IDs?

[Jeff Cody <jcody@redhat.com>]

On Thu, Aug 27, 2015 at 01:27:40PM -0600, Eric Blake wrote:
> On 08/27/2015 01:08 PM, Jeff Cody wrote:
> > On Thu, Aug 27, 2015 at 05:02:17PM +0100, Daniel P. Berrange wrote:
> >>
> >> Just to be clear - libvirt will *never* use an auto-generated device
> >> IDs feature. It is way more complicated to let QEMU assign device IDs
> >> and then auto-detect them from some 'info device-list' output, than
> >> to just specify IDs upfront at device/object creation time which
> >> it already does[1]. There is simply no benefit to auto-generating device
> >> IDs for a mgmt app like libvirt, and plenty of downside.  Auto-generated
> >> IDs will only be of interest to humans talking to the monitor directly
> >> without a mgmt app involved.
> >>
> >> Regards,
> >> Daniel
> >>
> >> [1] we don't provide IDs for qcow2 image backing file chain, but that's
> >>     part of a bigger story that's being dealt with in this area.
> > 
> > This is the part that interests me the most :)
> > 
> > Do you think in dealing with image backing file chains, libvirt would
> > ever make use of QEMU auto-generated node-names (either in the
> > current feature set, or with future features)?  I'm not sure if your
> > above statement is specific to device ID, or extends to node-names as
> > well.
> 
> I have a patch series that I posted for libvirt that used a hack to try
> and take advantage of auto-generated names (basically, you can't use
> allocation watermark events on qcow2-over-block-devices without proper
> node names).  Had we turned on auto node names in 2.4 (the release that
> added the event), then it might be in libvirt now.  But now that we have
> to support 2.4 without auto node names, it's in libvirt's long-term
> interest to avoid technical debt and directly supply node names for all
> BDS, rather than relying on node names, and rather than cheating by
> waiting for qemu 2.5.
> 
> So my current task in libvirt is to try and fix things to supply node
> names everywhere, then rewrite my allocation watermark event series atop
> that change, at which point relying on auto names will not be necessary
> for libvirt.
> 

OK, thanks. That certainly makes it less urgent, at least on my end.
Seems like the only time we'd need it for libvirt is if in the future
we created BDSs as a byproduct of an operation, and wanted to have a
node-name so that we could present it to libvirt afterwards.

Thanks,
Jeff