diff --git a/nova/tests/virt/libvirt/test_libvirt.py b/nova/tests/virt/libvirt/test_libvirt.py
index 775e6f97835e..830fa4ae98d9 100644
--- a/nova/tests/virt/libvirt/test_libvirt.py
+++ b/nova/tests/virt/libvirt/test_libvirt.py
@@ -3787,7 +3787,7 @@ class LibvirtConnTestCase(test.TestCase):
         self.mox.ReplayAll()
         conn._chown_disk_config_for_instance(instance)
 
-    def test_create_image_plain(self):
+    def _test_create_image_plain(self, os_type='', filename='', mkfs=False):
         gotFiles = []
 
         def fake_image(self, instance, name, image_type=''):
@@ -3823,11 +3823,15 @@ class LibvirtConnTestCase(test.TestCase):
         instance_ref = self.test_instance
         instance_ref['image_ref'] = 1
         instance = db.instance_create(self.context, instance_ref)
+        instance['os_type'] = os_type
 
         conn = libvirt_driver.LibvirtDriver(fake.FakeVirtAPI(), False)
         self.stubs.Set(conn, 'to_xml', fake_none)
         self.stubs.Set(conn, '_create_domain_and_network', fake_none)
         self.stubs.Set(conn, 'get_info', fake_get_info)
+        if mkfs:
+            self.stubs.Set(nova.virt.disk.api, '_MKFS_COMMAND',
+                       {os_type: 'mkfs.ext3 --label %(fs_label)s %(target)s'})
 
         image_meta = {'id': instance['image_ref']}
         disk_info = blockinfo.get_disk_info(CONF.libvirt.virt_type,
@@ -3842,11 +3846,31 @@ class LibvirtConnTestCase(test.TestCase):
         wantFiles = [
             {'filename': '356a192b7913b04c54574d18c28d46e6395428ab',
              'size': 10 * unit.Gi},
-            {'filename': 'ephemeral_20_default',
+            {'filename': filename,
              'size': 20 * unit.Gi},
             ]
         self.assertEqual(gotFiles, wantFiles)
 
+    def test_create_image_plain_os_type_blank(self):
+        self._test_create_image_plain(os_type='',
+                                      filename='ephemeral_20_default',
+                                      mkfs=False)
+
+    def test_create_image_plain_os_type_none(self):
+        self._test_create_image_plain(os_type=None,
+                                      filename='ephemeral_20_default',
+                                      mkfs=False)
+
+    def test_create_image_plain_os_type_set_no_fs(self):
+        self._test_create_image_plain(os_type='test',
+                                      filename='ephemeral_20_default',
+                                      mkfs=False)
+
+    def test_create_image_plain_os_type_set_with_fs(self):
+        self._test_create_image_plain(os_type='test',
+                                      filename='ephemeral_20_test',
+                                      mkfs=True)
+
     def test_create_image_with_swap(self):
         gotFiles = []
 
diff --git a/nova/virt/disk/api.py b/nova/virt/disk/api.py
index 0e375ae04d3d..3a8da221ca82 100644
--- a/nova/virt/disk/api.py
+++ b/nova/virt/disk/api.py
@@ -95,6 +95,10 @@ for s in CONF.virt_mkfs:
         _DEFAULT_MKFS_COMMAND = mkfs_command
 
 
+def get_fs_type_for_os_type(os_type):
+    return os_type if _MKFS_COMMAND.get(os_type) else 'default'
+
+
 def mkfs(os_type, fs_label, target, run_as_root=True):
     """Format a file or block device using
        a user provided command for each os type.
diff --git a/nova/virt/libvirt/driver.py b/nova/virt/libvirt/driver.py
index e352375c2eb6..c733603934b4 100644
--- a/nova/virt/libvirt/driver.py
+++ b/nova/virt/libvirt/driver.py
@@ -2466,9 +2466,8 @@ class LibvirtDriver(driver.ComputeDriver):
                                 project_id=instance['project_id'])
 
         # Lookup the filesystem type if required
-        os_type_with_default = instance['os_type']
-        if not os_type_with_default:
-            os_type_with_default = 'default'
+        os_type_with_default = disk.get_fs_type_for_os_type(
+                                                          instance['os_type'])
 
         ephemeral_gb = instance['ephemeral_gb']
         if 'disk.local' in disk_mapping: