On Thu, Oct 05, 2017 at 10:44:29AM +0200, Michal Privoznik wrote: > On 10/05/2017 10:10 AM, Daniel P. Berrange wrote: > > 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 <mpriv...@redhat.com> > >>>>>>> --- > >>>>>>> > >>>>>>> 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. > > Question is if we are confident enough that: > > a) apps will provide unique aliases (since we'll be putting user input > onto qemu cmd line) > > b) apps will provide only allowed characters in the alias (not every > character can be in id=, can it?)
We will have to validate both these points when looking at the XML. > But I think we still have not answered this question: what if we need to > change pattern by which we generate aliases in the future? On one hand, > an alias is just a string so the pattern should not matter. On the other > hand, that's not quite true. For instance, "pci.0" has a very special > meaning. IOW, if we now worry about users cutting off the branch they > are sitting on, this is like giving them flamethrower in fireworks > production hall. 'pci.0' is not an alias - 'pci' is the alias, the '0' is a bus number, so users only provide the first bit which has no special semantics other than needing to comply with a permitted set of characters and be unique. In terms of validation I think we should permit a-Z, 0-9 and -, upto a maximum of say 32 characters in length. Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :| -- libvir-list mailing list libvir-list@redhat.com https://www.redhat.com/mailman/listinfo/libvir-list
