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

On 10/05/2017 11:13 AM, Daniel P. Berrange wrote: > 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 <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. >> >> 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. Okay. We can check that. But now, does it make sense to generate the aliases at define time? I mean, users could provide alias at define time, and we can fill in the missing ones when starting up the domain. Just like we're doing now.