diff --git a/provider/openstack/init.go b/provider/openstack/init.go
index <HASH>..<HASH> 100644
--- a/provider/openstack/init.go
+++ b/provider/openstack/init.go
@@ -11,12 +11,17 @@ import (
 const (
 	providerType = "openstack"
 
-	rootDiskSourceVolume = "volume"
-	rootDiskSourceLocal  = "local"
 	// Default root disk size when root-disk-source is volume.
 	defaultRootDiskSize = 30 * 1024 // 30 GiB
 )
 
+const (
+	// BlockDeviceMapping source volume type for cinder block device.
+	rootDiskSourceVolume = "volume"
+	// BlockDeviceMapping source volume type for local block device.
+	rootDiskSourceLocal = "local"
+)
+
 func init() {
 	environs.RegisterProvider(providerType, providerInstance)
Add comments to constants for BlockDeviceMapping source types.