Re: [libvirt] [PATCH v2 3/3] conf: Allow users to define UUID for devices

On Wed, Oct 04, 2017 at 08:31:36AM +0200, Martin Kletzander wrote: > On Tue, Oct 03, 2017 at 03:10:48PM +0100, Daniel P. Berrange wrote: >> On Tue, Oct 03, 2017 at 04:03:20PM +0200, Martin Kletzander wrote: >>> On Tue, Oct 03, 2017 at 02:53:46PM +0100, Daniel P. Berrange wrote: >>>> On Tue, Oct 03, 2017 at 02:11:44PM +0200, Martin Kletzander wrote: >>>>> On Tue, Oct 03, 2017 at 12:58:59PM +0200, Michal Privoznik wrote: >>>>>> https://bugzilla.redhat.com/show_bug.cgi?id=1434451 >>>>>> >>>>>> It comes handy for management application to be able to have a >>>>>> per-device label so that it can uniquely identify devices it >>>>>> cares about. The advantage of this approach is that we don't have >>>>>> to generate aliases at define time (non trivial amount of work >>>>>> and problems). The only thing we do is parse the user supplied >>>>>> UUID and format it back. For instance: >>>>>> >>>>>> <disk type='block' device='disk'> >>>>>> <driver name='qemu' type='raw'/> >>>>>> <source dev='/dev/HostVG/QEMUGuest1'/> >>>>>> <target dev='hda' bus='ide'/> >>>>>> <uuid>1efaf08b-9317-4b0f-b227-912e4bd9f483</uuid> >>>>>> <address type='drive' controller='0' bus='0' target='0' unit='0'/> >>>>>> </disk> >>>>>> >>>>>> Signed-off-by: Michal Privoznik <mprivozn(a)redhat.com&gt; >>>>>> --- >>>>>> >>>>>> This is just a very basic implementation. If I get a green light on this, I can >>>>>> implement the feature further, i.e. allow device lookup on the UUID. For >>>>>> instance: >>>>>> >>>>>> virsh domiftune fedora $UUID $bandwidth >>>> >>>> <snip> >>>> >>>> I'm thinking that part of the problem we're having with agreeing how to >>>> deal with this RFE is that we're over-analysing semantics, by wondering >>>> whether its a unique name or UUID, its relation to alias, whether it has >>>> bearing on APIs. >>>> >>>> How about we change tack, and do what we did when we needed application >>>> specific information at the top level - just declare a general purpose >>>> <metadata> element and say it is a completely opaque blob. Libvirt will >>>> *never* do anything with it, other than to preserve it exactly as is. >>>> No API will ever use the metadata in any way. Libvirt will never try to >>>> guarantee uniqueness of metadata for each device. It can be JSON or a >>>> gziped microsoft word document for all we care. Entirely upto the app >>>> developer to decide what metadata is saved and guarantee uniqueness if >>>> they so desired. >>>> >>> >>> That is kind of what I was aiming for. But in order for it to be cleaner and >>> easier to use from user as well (and not only mgmt apps) I thought the metadata >>> would just be one identifier. If you want to store more metadata for the >>> device, then you can do all that in the domain metadata and just reference the >>> particular device using the identifier if mgmt app wants to do that. >> >> Yes that is certainly possible. The caveats are that we still need a unique >> identifier for the device, and the metadata update is not atomic wrt >> to device hotplug. >> > > Yes, well, our (libvirt) unique identifier is not going anywhere, so > that's OK, we'll be using what we have been until now. > >> The plus side of the global metadata is that we have APIs to update it >> on the fly already, and its fully namespaced to allow multiple independant >> data sets to be stored. >> > > Yes, exactly. > >> I don't think lack of atomicity is a big deal as you could order it so that >> you update metadata before doing the hotplug. Then worst case you have a >> device mentioned in metadata that doesn't exist, which is easy enough to >> detect. >> > > Right, if you want metadata for device, then you'll just update > metadata, hotplug device, and if it failed you update the metadata once > more. > > So are we on the same page? By that I mean agreeing on any sane user-supplied > identifier that we'll not guarantee uniqueness for, and neither will we use it > for anything for now? (We can deal with the issues regarding using it when > someone wants to actually implement it). Per my reply to the earlier patch series, I'm now inclined to say that we should - Allow the mgmt app to set the aliases upfront - Auto-fill missing aliases at XML define time it has some downsides, but all the other solutions we've discussed have their own downsides too. So on balance I think its not worth it to add a second identifier for each device, when we already have alias.