diff --git a/mysql/manifests/init.pp b/mysql/manifests/init.pp
index cea20a5..22ae5a4 100644
--- a/mysql/manifests/init.pp
+++ b/mysql/manifests/init.pp
@@ -12,6 +12,11 @@ class mysql::client {
 
 # Install MySQL server
 #
+# === Global variables
+#
+#   $mysql_datadir:
+#       Directory where MySQL databases are stored.
+#
 class mysql::server {
 
     package { "mysql-server":