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

Michal Privoznik mprivozn at redhat.com
Thu Oct 5 11:31:02 UTC 2017

On 10/05/2017 01:01 PM, Pavel Hrdina wrote:
> On Thu, Oct 05, 2017 at 10:40:01AM +0100, Daniel P. Berrange wrote:
>> On Thu, Oct 05, 2017 at 11:27:29AM +0200, Martin Kletzander 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 at 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.
>>> They are not the same thing.  Our alias is an ID we pass to QEMU and we
>>> are dependent on it in multiple ways, plus we generate it with some of
>>> our rules in mind.  Allowing users to modify this would add so many
>>> possible problems that I would not consider next two releases stable.  I
>>> would also not be confident enough to tell someone "just use the alias"
>>> because I see all the places where it's not treated as just another
>>> configurable parameter.  And there's no problem with that.  But there
>>> will be if we allow users to set an identifier that's heavily used for
>>> internal purposes.
>> I honestly don't see where the major instability is going to come from ? We
>> don't have code that cares about the format of the string used in the alias.
>> What matters is what we keep track of the aliases for any running guests, and
>> we do that via the state XML, so if libvirt changes the format of aliases it
>> generates internally it would not break upgrades.
> Actually we have some code that depends on the format of aliases.
> Yes, it can be easily fixed but we may miss something.
> In src/qemu/qemu_alias.c there is a lot of function that expect certain
> format of aliases of existing devices in order to generate a new alias
> for remaining devices and if they fail recognize the format they
> will report an error instead of assigning new alias.
> Another example what could go wrong is that in src/lxc/lxc_process.c
> while starting new LXC process we simple generate new alias for consoles
> regardless whether there was already one generated or not.

Worse. Imagine an user who gives a disk alias "vnet0". Now, when we
would generate the aliases for the rest of devices, say interfaces, we
would have to check all other devices and see if the alias we generated
is not already in use. Possibly by any other device.

Although, if we have the prefixed aliases, such clash would not be possible.


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