diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index a31130810083a57cec6b2a478997ab98eca82d33..79e438ca01cbfce5add3dd988cd26eb6c43fd3a6 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -2288,8 +2288,7 @@ SET ENABLE_SEQSCAN TO OFF;
         is measured in terms of number of write transactions occurring on the
         primary server, it is difficult to predict just how much additional
         grace time will be made available to standby queries.
-        This parameter can only be set in the <filename>postgresql.conf</>
-        file or on the server command line.
+        This parameter can only be set at server start.
        </para>
        <para>
         You should also consider setting <varname>hot_standby_feedback</>
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index c37c1d9a5fb42c1b4c19b7cf54718f751b695633..25722fac68d02e4155030e631e41ae95a52c7efc 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -1880,7 +1880,12 @@ static struct config_int ConfigureNamesInt[] =
 	},
 
 	{
-		{"vacuum_defer_cleanup_age", PGC_SIGHUP, REPLICATION_MASTER,
+		/*
+		 * Setting this to a higher value without restarting postmaster
+		 * can cause various bugs in TOAST object removal, CLUSTER and
+		 * possibly other places, so this must be changed only on restart.
+		 */
+		{"vacuum_defer_cleanup_age", PGC_POSTMASTER, REPLICATION_MASTER,
 			gettext_noop("Number of transactions by which VACUUM and HOT cleanup should be deferred, if any."),
 			NULL
 		},