[openstack-dev] [Nova][Glance] Nova + Glance_v2 = Love

Sam Matzek matzeksam at gmail.com
Tue Dec 29 13:41:54 UTC 2015

On Thu, Dec 24, 2015 at 7:49 AM, Mikhail Fedosin <mfedosin at mirantis.com> wrote:
> Hello, it's another topic about glance v2 adoption in Nova, but it's
> different from the others. I want to declare that there is a set of commits,
> that make Nova version agnostic and allow to work with both glance apis. The
> idea of the solution is to determine the current api version in the
> beginning and make appropriate requests after.
> (https://review.openstack.org/#/c/228578/,
> https://review.openstack.org/#/c/238309/,
> https://review.openstack.org/#/c/259097/)
> Indeed, it requires some additional (and painful) work, but now all tempest
> tests pass in Jenkins.
> Note: this thread is not about xenplugin, there is another topic, called
> 'Xenplugin + Glance_v2 = Hate'
> Here the main issues we faced and how we've solved them:
> 1. "changes-since" filter for image-list is not supported in v2 api. Steve
> Lewis made a great job and implemented a set of filters with comparison
> operators https://review.openstack.org/#/c/197388/. Filtering by
> 'changes-since' is completely similar to 'gte:updated_at'.
> 2. Filtering by custom properties must have prefix 'property-'. In v2 it's
> not required.
> 3. V2 states that all custom properties must be image attributes, but Nova
> assumes that they are included in 'properties' dictionary. It's fixed with
> glanceclient's method 'is_base_property(prop_name)', that returns False in
> case of custom property.
> 4. is_public=True/False is visibility="public"/"private" respectively.
> 5. Deleting custom image properties in Nova is performed with 'purge_props'
> flag. If it's set to True, then all prop names, that are not included in
> updated data, will be removed. In case of v2 we have to explicitly specify
> prop names in the list param 'remove_props'. To implement this behaviour, if
> 'purge_props' is set, we make additional 'get' request to determine the
> existing properties and put in 'remove_prop' list only those, that are not
> in updated_data.
> 6. My favourite:
> There is an ability to activate an empty image by just providing 'size = 0':
> https://review.openstack.org/#/c/9715/, in this case image will be a
> collection of metadata. Glance v2 doesn't support this "feature" and that's
> why we have to implement a very dirty workaround:
>     * v2 requires, that disk_format and container-format must be set before
> the activation. if these params are not provided to 'create' method then we
> hardcode it to 'qcow2' and 'bare'.
>     * we call 'upload' method with empty data (data = '') to activate image.
> Me (and the rest glance team) think that this image activation with
> zero-size is inconsistent and we won't implement it in v2. So, I'm going to
> ask if Nova really needs this feature and maybe it's possible to make it
> work without it.

Nova uses this functionality when it creates snapshots of volume
backed instances, that is, instances that only have Cinder volumes
attached and do not have an ephemeral disk.
In this case Nova API creates Cinder snapshots for the Cinder volumes
and builds the block_device_mapping property with block devices that
reference the Cinder snapshots.  Nova activates this image with size=0
because this image does not have a disk and simply refers to the
collection of Cinder snapshots that collectively comprise the binary
image.  Callers of Glance outside of Nova may also use the APIs to
create "block device mapping" images as well that contain references
to Cinder volumes to attach, blank volumes to create, snapshots to
create volumes from, etc during the server creation.  Not having the
ability to create these images with V2 is a loss of function.

The callers could supply 1 byte of junk data, like a space character,
but that would result in a junk image being stored in Glance's default
backing store for the image.  It would also give the impression that a
real disk image exists for the image in a backing store which could be
fetched, which is incorrect.

If I remember correctly Glance V2 lets you transition an image from
queued to active state with size = 0 if the image is using an external
backing store such as http.  The store is then called to fetch the
size.  Would it be possible to allow Glance to consider images with
size = 0 and the block_device_mapping property to be "externally
sourced" and allow the transition?

> In conclusion, I want to congratulate you with this huge progress and say
> there is a big chance that we can deprecate glance v1 in Mitaka cycle.
> Hooray!
> And Merry Christmas!
> --
> Best regards,
> Mikhail Fedosin
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