diff --git a/doc/src/sgml/pgupgrade.sgml b/doc/src/sgml/pgupgrade.sgml
index d3e26eec1d..ebb7a60de7 100644
--- a/doc/src/sgml/pgupgrade.sgml
+++ b/doc/src/sgml/pgupgrade.sgml
@@ -335,7 +335,8 @@ NET STOP pgsql-8.3  (<productname>PostgreSQL</> 8.3 and older used a different s
      copying), but you will not be able to access your old cluster
      once you start the new cluster after the upgrade.  Link mode also
      requires that the old and new cluster data directories be in the
-     same file system.  See <literal>pg_upgrade --help</> for a full
+     same file system.  (Tablespaces and <filename>pg_xlog</> can be on
+     different file systems.)  See <literal>pg_upgrade --help</> for a full
      list of options.
     </para>