diff --git a/src/bin/pg_ctl/t/004_logrotate.pl b/src/bin/pg_ctl/t/004_logrotate.pl
index bc39abd23e..2f381e556e 100644
--- a/src/bin/pg_ctl/t/004_logrotate.pl
+++ b/src/bin/pg_ctl/t/004_logrotate.pl
@@ -12,6 +12,8 @@ $node->init();
 $node->append_conf(
 	'postgresql.conf', qq(
 logging_collector = on
+# these ensure stability of test results:
+log_rotation_age = 0
 lc_messages = 'C'
 ));
 
diff --git a/src/test/kerberos/t/001_auth.pl b/src/test/kerberos/t/001_auth.pl
index e58e69b497..4b03ff561d 100644
--- a/src/test/kerberos/t/001_auth.pl
+++ b/src/test/kerberos/t/001_auth.pl
@@ -163,10 +163,16 @@ note "setting up PostgreSQL instance";
 
 my $node = get_new_node('node');
 $node->init;
-$node->append_conf('postgresql.conf', "listen_addresses = '$hostaddr'");
-$node->append_conf('postgresql.conf', "krb_server_keyfile = '$keytab'");
-$node->append_conf('postgresql.conf', "logging_collector = on");
-$node->append_conf('postgresql.conf', "log_connections = on");
+$node->append_conf(
+	'postgresql.conf', qq{
+listen_addresses = '$hostaddr'
+krb_server_keyfile = '$keytab'
+logging_collector = on
+log_connections = on
+# these ensure stability of test results:
+log_rotation_age = 0
+lc_messages = 'C'
+});
 $node->start;
 
 $node->safe_psql('postgres', 'CREATE USER test1;');