Add transform_image_metadata request filter

Add a table-driven prefilter to transform image metadata into required
traits. This requires a new config option to make the filter optional.

Change-Id: I257ff81e23cdae6f2b62ec3d071b8f8f32d97781
Implements: blueprint image-metadata-prefiltering
Co-Authored-By: Stephen Finucane <sfinucan@redhat.com>

nova/conf/scheduler.py

@@ -198,7 +198,17 @@ flavor/image. If an aggregate is configured with a property with key
 and/or image metadata must also contain ``trait:$TRAIT_NAME=required`` to be
 eligible to be scheduled to hosts in that aggregate. More technical details
 at https://docs.openstack.org/nova/latest/reference/isolate-aggregates.html
-""")
+"""),
+    cfg.BoolOpt("image_metadata_prefilter",
+                default=False,
+                help="""
+This setting causes the scheduler to transform well known image metadata
+properties into placement required traits to filter host based on image
+metadata. This feature requires host support and is currently supported by the
+following compute drivers:
+
+- ``libvirt.LibvirtDriver`` (since Ussuri (21.0.0))
+"""),
 ]
 
 filter_scheduler_group = cfg.OptGroup(name="filter_scheduler",

nova/scheduler/request_filter.py

@@ -193,6 +193,47 @@ def require_image_type_support(ctxt, request_spec):
     return True
 
 
+@trace_request_filter
+def transform_image_metadata(ctxt, request_spec):
+    """Transform image metadata to required traits.
+
+    This will modify the request_spec to request hosts that support
+    virtualisation capabilities based on the image metadata properties.
+    """
+    if not CONF.scheduler.image_metadata_prefilter:
+        return False
+
+    prefix_map = {
+        'hw_cdrom_bus': 'COMPUTE_STORAGE_BUS',
+        'hw_disk_bus': 'COMPUTE_STORAGE_BUS',
+        'hw_video_model': 'COMPUTE_GRAPHICS_MODEL',
+        'hw_vif_model': 'COMPUTE_NET_VIF_MODEL',
+    }
+
+    trait_names = []
+
+    for key, prefix in prefix_map.items():
+        if key in request_spec.image.properties:
+            value = request_spec.image.properties.get(key).replace(
+                '-', '_').upper()
+            trait_name = f'{prefix}_{value}'
+            if not hasattr(os_traits, trait_name):
+                LOG.error(('Computed trait name %r is not valid; '
+                           'is os-traits up to date?'), trait_name)
+                return False
+
+            trait_names.append(trait_name)
+
+    for trait_name in trait_names:
+        LOG.debug(
+            'transform_image_metadata request filter added required '
+            'trait %s', trait_name
+        )
+        request_spec.root_required.add(trait_name)
+
+    return True
+
+
 @trace_request_filter
 def compute_status_filter(ctxt, request_spec):
     """Pre-filter compute node resource providers using COMPUTE_STATUS_DISABLED
@@ -215,6 +256,7 @@ ALL_REQUEST_FILTERS = [
     require_image_type_support,
     compute_status_filter,
     isolate_aggregates,
+    transform_image_metadata,
 ]
 
 

nova/tests/unit/scheduler/test_request_filter.py

@@ -12,11 +12,13 @@
 
 import mock
 import os_traits as ot
+
 from oslo_utils.fixture import uuidsentinel as uuids
 from oslo_utils import timeutils
 
 from nova import context as nova_context
 from nova import exception
+from nova.network import model as network_model
 from nova import objects
 from nova.scheduler import request_filter
 from nova import test
@@ -400,3 +402,36 @@ class TestRequestFilter(test.NoDBTestCase):
         log_lines = [c[0][0] for c in mock_log.debug.call_args_list]
         self.assertIn('added forbidden trait', log_lines[0])
         self.assertIn('took %.1f seconds', log_lines[1])
+
+    @mock.patch.object(request_filter, 'LOG', new=mock.Mock())
+    def test_transform_image_metadata(self):
+        self.flags(image_metadata_prefilter=True, group='scheduler')
+        properties = objects.ImageMetaProps(
+            hw_disk_bus=objects.fields.DiskBus.SATA,
+            hw_cdrom_bus=objects.fields.DiskBus.IDE,
+            hw_video_model=objects.fields.VideoModel.QXL,
+            hw_vif_model=network_model.VIF_MODEL_VIRTIO
+        )
+        reqspec = objects.RequestSpec(
+            image=objects.ImageMeta(properties=properties),
+            flavor=objects.Flavor(extra_specs={}),
+        )
+        self.assertTrue(
+            request_filter.transform_image_metadata(None, reqspec)
+        )
+        expected = {
+            'COMPUTE_GRAPHICS_MODEL_QXL',
+            'COMPUTE_NET_VIF_MODEL_VIRTIO',
+            'COMPUTE_STORAGE_BUS_IDE',
+            'COMPUTE_STORAGE_BUS_SATA',
+        }
+        self.assertEqual(expected, reqspec.root_required)
+
+    def test_transform_image_metadata__disabled(self):
+        self.flags(image_metadata_prefilter=False, group='scheduler')
+        reqspec = objects.RequestSpec(flavor=objects.Flavor(extra_specs={}))
+        # Assert that we completely skip the filter if disabled
+        self.assertFalse(
+            request_filter.transform_image_metadata(self.context, reqspec)
+        )
+        self.assertEqual(set(), reqspec.root_required)

releasenotes/notes/image-metadata-prefiltering-2921c1d38951f7a9.yaml

@@ -0,0 +1,11 @@
+---
+features:
+  - |
+    A new image metadata prefilter has been added to allow translation of
+    hypervisor-specific device model requests to standard traits. When this
+    feature is enabled, nova is able to utilize placement to select hosts that
+    are capable of emulating the requested devices, avoiding hosts that
+    could not support the request. This feature is currently supported by the
+    libvirt driver and can be enabled by configuring the
+    ``[scheduler]/image_metadata_prefilter`` to ``True`` in the controller
+    ``nova.conf``.