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[Erno Kuvaja <jokke@xxxxxx>]

** Also affects: cinder
   Importance: Undecided
       Status: New

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https://bugs.launchpad.net/bugs/1818851

Title:
  Snapshots of volume backed instances default to qcow2

Status in Cinder:
  New
Status in Glance:
  New

Bug description:
  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://www.mail-archive.com/openstack@xxxxxxxxxxxxxxxxxxx/msg20935.html
  [2] http://lists.openstack.org/pipermail/openstack-discuss/2019-March/003383.html

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