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On Wed, May 20, 2015 at 5:06 PM, Daniel P. Berrange <berrange at redhat.com> wrote:
> On Wed, May 20, 2015 at 12:01:37AM +0200, Flavio Percoco wrote:
>> On 19/05/15 17:19 +0100, Daniel P. Berrange wrote:
>> >In Nova we are attempting to model[1] the glance image metadata and
>> >properties using the Nova object model (now oslo.versionedobjects).
>> >
>> >The one item I'm stuck on understanding is the 'locations' field
>> >and more specifically the 'metadata' element in each location
>> >entry
>> >
>> >
>> >In the file glance/api/v2/images.py I can see this description
>> >of the data format:
>> >
>> >       'locations': {
>> >           'type': 'array',
>> >           'items': {
>> >               'type': 'object',
>> >               'properties': {
>> >                   'url': {
>> >                       'type': 'string',
>> >                       'maxLength': 255,
>> >                   },
>> >                   'metadata': {
>> >                       'type': 'object',
>> >                   },
>> >               },
>> >               'required': ['url', 'metadata'],
>> >           },
>> >           'description': _('A set of URLs to access the image file kept in '
>> >                            'external store'),
>> >
>> >
>> >As you can see here, 'metadata' is just said to be of type 'object'.
>> >
>> >Is there somewhere that actually describes what is valid contents
>> >for this field ? Is it sufficient to assume the metadata will only
>> >ever be a dict of strings, or can the metadata be a complex type
>> >with arbitrarily nested data structures ?
>>
>> It's just arbitrary metadata for now, we don't have a specific format.
>> I'm curious to know if there are folks using this field. We do (did)
>> have a use case for it.
>
> Yep, I'd be curious to understand just what it is used for in practice ?
> Is the data to be stored in there determined by python code, or by the
> local administrator or both ?
>
>
> Regards,
> Daniel
> --
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Yes, It determined by python code in nova as a part of image download
plugin, and administrator needs to prepare it based on particular
deployment environment as well. Current a usage is to accelerate image
download from nfs store to nova compute node, now there is only one
particular plugin in nova upstream tree [0]. (from the logic in
_file_system_lookup(), i think a predefined 'id' is needed in the
metadata of the location entry).

[0] https://github.com/openstack/nova/blob/master/nova/image/download/file.py#L150

zhiyan