diff --git a/nova/block_device.py b/nova/block_device.py
index 360316631f..fbc87099db 100644
--- a/nova/block_device.py
+++ b/nova/block_device.py
@@ -453,6 +453,16 @@ def strip_prefix(device_name):
     return _pref.sub('', device_name)
 
 
+_nums = re.compile('\d+')
+
+
+def get_device_letter(device_name):
+    letter = strip_prefix(device_name)
+    # NOTE(vish): delete numbers in case we have something like
+    #             /dev/sda1
+    return _nums.sub('', letter)
+
+
 def instance_block_mapping(instance, bdms):
     root_device_name = instance['root_device_name']
     # NOTE(clayg): remove this when xenapi is setting default_root_device
diff --git a/nova/compute/api.py b/nova/compute/api.py
index 47ec3af7e6..c025616711 100644
--- a/nova/compute/api.py
+++ b/nova/compute/api.py
@@ -861,7 +861,9 @@ class API(base.Base):
             return {}
 
         for bdm in block_device_mapping:
-            if legacy_bdm and bdm.get('device_name') != 'vda':
+            if (legacy_bdm and
+                    block_device.get_device_letter(
+                       bdm.get('device_name', '')) != 'a'):
                 continue
             elif not legacy_bdm and bdm.get('boot_index') != 0:
                 continue
diff --git a/nova/compute/utils.py b/nova/compute/utils.py
index 1eea08449b..3b47dce3d0 100644
--- a/nova/compute/utils.py
+++ b/nova/compute/utils.py
@@ -15,7 +15,6 @@
 """Compute-related Utilities and helpers."""
 
 import itertools
-import re
 import string
 import traceback
 
@@ -164,10 +163,7 @@ def get_next_device_name(instance, device_name_list,
 
     used_letters = set()
     for device_path in device_name_list:
-        letter = block_device.strip_prefix(device_path)
-        # NOTE(vish): delete numbers in case we have something like
-        #             /dev/sda1
-        letter = re.sub("\d+", "", letter)
+        letter = block_device.get_device_letter(device_path)
         used_letters.add(letter)
 
     # NOTE(vish): remove this when xenapi is properly setting
diff --git a/nova/tests/test_block_device.py b/nova/tests/test_block_device.py
index c1b5f11926..2f0792a202 100644
--- a/nova/tests/test_block_device.py
+++ b/nova/tests/test_block_device.py
@@ -94,6 +94,15 @@ class BlockDeviceTestCase(test.NoDBTestCase):
         self.assertEqual(block_device.strip_prefix('xvda'), 'a')
         self.assertEqual(block_device.strip_prefix('vda'), 'a')
 
+    def test_get_device_letter(self):
+        self.assertEqual(block_device.get_device_letter(''), '')
+        self.assertEqual(block_device.get_device_letter('/dev/sda1'), 'a')
+        self.assertEqual(block_device.get_device_letter('/dev/xvdb'), 'b')
+        self.assertEqual(block_device.get_device_letter('/dev/d'), 'd')
+        self.assertEqual(block_device.get_device_letter('a'), 'a')
+        self.assertEqual(block_device.get_device_letter('sdb2'), 'b')
+        self.assertEqual(block_device.get_device_letter('vdc'), 'c')
+
     def test_volume_in_mapping(self):
         swap = {'device_name': '/dev/sdb',
                 'swap_size': 1}