diff --git a/nova/tests/unit/virt/libvirt/test_imagebackend.py b/nova/tests/unit/virt/libvirt/test_imagebackend.py
index 4739a59322cc..cfb6a7db3c6f 100644
--- a/nova/tests/unit/virt/libvirt/test_imagebackend.py
+++ b/nova/tests/unit/virt/libvirt/test_imagebackend.py
@@ -1363,9 +1363,33 @@ class BackendTestCase(test.NoDBTestCase):
     def test_image_raw(self):
         self._test_image('raw', imagebackend.Raw, imagebackend.Raw)
 
+    def test_image_raw_preallocate_images(self):
+        flags = ('space', 'Space', 'SPACE')
+        for f in flags:
+            self.flags(preallocate_images=f)
+            raw = imagebackend.Raw(self.INSTANCE, 'fake_disk', '/tmp/xyz')
+            self.assertTrue(raw.preallocate)
+
+    def test_image_raw_preallocate_images_bad_conf(self):
+        self.flags(preallocate_images='space1')
+        raw = imagebackend.Raw(self.INSTANCE, 'fake_disk', '/tmp/xyz')
+        self.assertFalse(raw.preallocate)
+
     def test_image_qcow2(self):
         self._test_image('qcow2', imagebackend.Qcow2, imagebackend.Qcow2)
 
+    def test_image_qcow2_preallocate_images(self):
+        flags = ('space', 'Space', 'SPACE')
+        for f in flags:
+            self.flags(preallocate_images=f)
+            qcow = imagebackend.Qcow2(self.INSTANCE, 'fake_disk', '/tmp/xyz')
+            self.assertTrue(qcow.preallocate)
+
+    def test_image_qcow2_preallocate_images_bad_conf(self):
+        self.flags(preallocate_images='space1')
+        qcow = imagebackend.Qcow2(self.INSTANCE, 'fake_disk', '/tmp/xyz')
+        self.assertFalse(qcow.preallocate)
+
     def test_image_lvm(self):
         self.flags(images_volume_group='FakeVG', group='libvirt')
         self._test_image('lvm', imagebackend.Lvm, imagebackend.Lvm)
diff --git a/nova/virt/libvirt/imagebackend.py b/nova/virt/libvirt/imagebackend.py
index b85b1e8c7418..266f677b6a6f 100644
--- a/nova/virt/libvirt/imagebackend.py
+++ b/nova/virt/libvirt/imagebackend.py
@@ -23,6 +23,7 @@ from oslo_config import cfg
 from oslo_log import log as logging
 from oslo_serialization import jsonutils
 from oslo_utils import excutils
+from oslo_utils import strutils
 from oslo_utils import units
 import six
 
@@ -383,7 +384,8 @@ class Raw(Image):
         self.path = (path or
                      os.path.join(libvirt_utils.get_instance_path(instance),
                                   disk_name))
-        self.preallocate = CONF.preallocate_images != 'none'
+        self.preallocate = (
+            strutils.to_slug(CONF.preallocate_images) == 'space')
         self.disk_info_path = os.path.join(os.path.dirname(self.path),
                                            'disk.info')
         self.correct_format()
@@ -455,7 +457,8 @@ class Qcow2(Image):
         self.path = (path or
                      os.path.join(libvirt_utils.get_instance_path(instance),
                                   disk_name))
-        self.preallocate = CONF.preallocate_images != 'none'
+        self.preallocate = (
+            strutils.to_slug(CONF.preallocate_images) == 'space')
         self.disk_info_path = os.path.join(os.path.dirname(self.path),
                                            'disk.info')
         self.resolve_driver_format()