Docs and functional test for max_local_block_devices

We get questions about if it's possible to configure nova to
always use cinder for root volumes even if the user isn't
explicitly booting from volume. This change adds a FAQ entry
about that to the block device mappings docs and also suggests
a way to force users to use volume-backed servers using the
max_local_block_devices option. The config option help for that
option is also cleaned up since --image is a CLI option but we
should stick to REST API parameters in descriptions and the
default mentioned in the help already gets rendered by oslo.config.
Finally, a simple functional test is added to assert the workaround
mentioned in the docs.

Change-Id: I3e77796b051e8d007fefe2eea06d38e8265e8272

doc/source/user/block-device-mapping.rst

@@ -220,3 +220,21 @@ mapping is valid before accepting a boot request.
 .. [3] This is a feature that the EC2 API offers as well and has been in Nova
    for a long time, although it has been broken in several releases. More info
    can be found on `this bug <https://launchpad.net/bugs/1370250>`
+
+
+FAQs
+----
+
+1. Is it possible to configure nova to automatically use cinder to back all
+   root disks with volumes?
+
+   No, there is nothing automatic within nova that converts a
+   non-boot-from-volume request to convert the image to a root volume.
+   Several ideas have been discussed over time which are captured in the
+   spec for `volume-backed flavors`_. However, if you wish to force users
+   to always create volume-backed servers, you can configure the API service
+   by setting :oslo.config:option:`max_local_block_devices` to 0. This will
+   result in any non-boot-from-volume server create request to fail with a
+   400 response.
+
+.. _volume-backed flavors: https://review.opendev.org/511965/

nova/conf/compute.py

@@ -82,21 +82,20 @@ Possible values:
 Maximum number of devices that will result in a local image being
 created on the hypervisor node.
 
-A negative number means unlimited. Setting max_local_block_devices
+A negative number means unlimited. Setting ``max_local_block_devices``
 to 0 means that any request that attempts to create a local disk
 will fail. This option is meant to limit the number of local discs
-(so root local disc that is the result of --image being used, and
-any other ephemeral and swap disks). 0 does not mean that images
-will be automatically converted to volumes and boot instances from
-volumes - it just means that all requests that attempt to create a
-local disk will fail.
+(so root local disc that is the result of ``imageRef`` being used when
+creating a server, and any other ephemeral and swap disks). 0 does not
+mean that images will be automatically converted to volumes and boot
+instances from volumes - it just means that all requests that attempt
+to create a local disk will fail.
 
 Possible values:
 
 * 0: Creating a local disk is not allowed.
 * Negative number: Allows unlimited number of local discs.
 * Positive number: Allows only these many number of local discs.
-                       (Default value is 3).
 """),
     cfg.ListOpt('compute_monitors',
         default=[],

nova/tests/functional/test_boot_from_volume.py

@@ -10,9 +10,12 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
+import six
+
 from nova import context
 from nova import objects
 from nova.tests import fixtures as nova_fixtures
+from nova.tests.functional.api import client as api_client
 from nova.tests.functional import integrated_helpers
 from nova.tests.functional import test_servers
 
@@ -145,3 +148,17 @@ class BootFromVolumeTest(integrated_helpers.InstanceHelperMixin,
         self._verify_instance_flavor_not_zero(server_id)
         # Check that request spec has not been saved with 0 root_gb
         self._verify_request_spec_flavor_not_zero(server_id)
+
+    def test_max_local_block_devices_0_force_bfv(self):
+        """Tests that when the API is configured with max_local_block_devices=0
+        a user cannot boot from image, they must boot from volume.
+        """
+        self.flags(max_local_block_devices=0)
+        server = self._build_minimal_create_server_request(
+            self.admin_api, 'test_max_local_block_devices_0_force_bfv')
+        ex = self.assertRaises(api_client.OpenStackApiException,
+                               self.admin_api.post_server,
+                               {'server': server})
+        self.assertEqual(400, ex.response.status_code)
+        self.assertIn('You specified more local devices than the limit allows',
+                      six.text_type(ex))