[libvirt-users] [RFC] per-device metadata

[Daniel P. Berrange]

On Thu, Mar 16, 2017 at 01:46:38PM +0100, Peter Krempa wrote:
> 
> For techincal discussions on design it's better to post to the
> development list.
> 
> On Mon, Feb 27, 2017 at 16:41:28 +0100, Francesco Romani wrote:
> > Hi,
> 
> Hi,
> 
> > 
> > 
> > Currently libvirt supports metadata in the domain XML. This is very
> > convenient for data related to the VM, but it is a little awkward for
> > devices. Let's pretend I want to have extradata (say, a specific port
> > for a virtual switch) related to a device (say, a NIC). Nowadays I can
> > store that data in the metadata section, but I need some kind of mapping
> > to correlate this piece of information to the specific device.
> > 
> > I can use the device alias, but this is not available when the device is
> > created. This is also more complex when doing hotplug/hotunplug, because
> > I need to do update device and update metadata; if either fails, the
> > entire operation must be considered failed.
> > 
> > It would be nice to be able to attach metadata to the device, and this
> > is what I'm asking for/proposing in this mail.
> > 
> > Would it be possible in a future libvirt release?
> 
> In general I don't have a problem with this and if it would make the
> life of mgmt tools easier I don't see a reason why not.
> 
> There's one problem though. For setter (or getter) APIs for the metadata
> you need a way how to uniquely identify a device on which you want to
> operate. There's no such way currently in use in libvirt so you need to
> come up with something to do it.

Once you have a reliable identifier for a device, it is not compelling
to store metadata against each device as you can reliably identify the
device from data in the existing metadata block. I don't find the
atomicity argument very compelling, because even if hidden behind a
single libvirt API, it is still fundamentally multiple operations in
libvirt which are just as likely to fail as if the app did multiple
libvirt API calls as we're not able to provide any kind of transactional
guarantees here.

IOW, we could effectively solve this by assigning the device alias
names at XML define time, instead of VM launch time. We historically
didn't do this since we wanted to cope with old QEMU which didn't
allow custom device aliases. We now mandate a new QEMU that supports
-device, so that's no longer a problem. So I'd prefer to see us just
define device aliases persistently and let apps use those to track
metadata in the global metadata section.

Regards,
Daniel
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