Beginning with the Train release, untyped volumes (that is, volumes
with no volume-type) have been disallowed. To facilitate this, a
__DEFAULT__ volume-type was included as part of the Train
database migration. Since the Train release, handling of the default
volume-type has been improved:

• The default_volume_type configuration option is required to have a
  value. The default value is __DEFAULT__.
• A request to delete the currently configured default_volume_type
  will fail. (You can delete that volume-type, but you cannot do it while
  it is the value of the configuration option.)
• There must always be at least one volume-type defined in a Cinder
  installation. This is enforced by the type-delete call.
• If the default_volume_type is misconfigured (that is, if the value
  refers to a non-existent volume-type), requests that rely on the default
  volume-type (for example, a volume-create request that does not specify
  a volume-type) will result in a HTTP 500 response.

Default types per project

We have overriden the existing Cinder default Volume Type on a per
project basis to make it easier to manage complex deployments.

With the introduction of this new default volume type support, we’ll
now have 2 different default volume types. From more specific to more
generic these are:

• Per project
• Defined in cinder.conf (defaults to __DEFAULT__
  type)

So when a user creates a new volume that has no defined volume type
(explicit or in the source), Cinder will look for the appropriate
default first by checking if there’s one defined in the DB for the
specific project and use it, if there isn’t one, it will continue like
it does today, using the default type from cinder.conf.

Administrators and users must still be careful with the normal Cinder
behavior when creating volumes, as Cinder will still only resort to
using the default volume type if the user doesn’t select one on the
request or if there’s no volume type in the source, which means that
Cinder will not use any of those defaults if we:

• Create a volume providing a volume type
• Create a volume from a snapshot
• Clone a volume
• Create a volume from an image that has cinder_img_volume_type
  defined in its metadata.

There is a new set of commands in the python-cinderclient to match
the new REST API endpoints:

• Set default:
  cinder default-type-set <project-id> <type-name>
• Unset default:
  cinder default-type-unset <project-id>
• List defaults:
  cinder default-type-list [--project <project-id>]

By default the policy restricting access to set, unset, get or list
all project default volume type is set to admins only.