diff --git a/doc/src/sgml/pgupgrade.sgml b/doc/src/sgml/pgupgrade.sgml
new file mode 100644
index 30f4544..cffcebb
*** a/doc/src/sgml/pgupgrade.sgml
--- b/doc/src/sgml/pgupgrade.sgml
*************** psql --username postgres --file script.s
*** 518,523 ****
--- 518,529 ----
    </para>
  
    <para>
+    A Log-Shipping Standby Server (<xref linkend="warm-standby">) cannot
+    be upgraded because the server must allow writes.  The simplest way
+    is to upgrade the primary and use rsync to rebuild the standbys.
+   </para>
+ 
+   <para>
     If you want to use link mode and you do not want your old cluster
     to be modified when the new cluster is started, make a copy of the
     old cluster and upgrade that in link mode. To make a valid copy