Snapshots of volume backed instances default to qcow2

This report applies to a production Ocata cloud and also to a Rocky lab environment.
I'm not entirely sure which component is responsible exactly but the inconsistency is strongly related to glance.

This report results from a mailing list thread I started in October 2018 [1], I only could put the pieces together step by step as some of the behavior was not reproducible, so this took a while, and my conclusion is summed up in [2]. I'll recap here:

The backend for nova, cinder and glance is ceph.

1. Creating an instance from a (regular) glance image with 'disk_format = qcow2' results in a "flat" nova instance with no parent data. Although a snapshot seems to create a "raw" image it's actually "qcow2", resulting in more flat images if instances are launched from this new image.

2. Creating a snapshot of a volume based nova instance (raw format) unexpectedly
results in a glance image with 'disk_format = qcow2'.
But launching a new instance from that snapshot does *not* result in a flat image as I
would have expected because of 1. Instead the new instance is a cow-clone of the original volume, which in fact is expected because of the ceph backend.

So the workflow seems fine, but I find it very confusing (obviously) that the volume based snapshot is declared as a qcow2 image, that doesn't seem right.

Steps to reproduce (part I):

1. Create qcow2 image.
2. Launch instance from qcow2 image.
3. Instance has no parent data, it's a flat rbd image without 'parent data'.

Steps to reproduce (part II):

1. Create raw image.
2. Launch volume backed instance from raw image.
3. Create snapshot of instance, resulting in a glance image with 'disk_format = qcow2'.
4. Launch instance from that image, results in a new copy-on-write clone (volume).

Expected result:
Snapshot of a volume backed instance should also have 'disk_format=raw'.
The consequences of the image property 'disk_format' should always be the same, no matter how the image was created.

Actual result:
Depending on the image creation the same property 'disk_format' has different effects on the ceph backend and the resulting usability.

[1] https://<email address hidden>/msg20935.html
[2] http://lists.openstack.org/pipermail/openstack-discuss/2019-March/003383.html