在DG的Primary DB 的监听日志中出现如下信息：

WARNING: Subscription for node down event still pending

metalink的解释如下：

Applies to:

Symptoms

You are receiving the following warning messages in the listener.log file constantly:

'WARNING: Subscription for node down event still pending' 

Changes

This may be a new installation or a recent upgrade to 10g or newer.

Cause

These messages are related to the Oracle TNS Listener's default subscription to the Oracle Notification Service (ONS). In a non-RAC environment it is recommended to disable this subscription.   This feature was introduced in Oracle 10g.

Solution

Set the following parameter in the listener.ora:

SUBSCRIBE_FOR_NODE_DOWN_EVENT_=OFF

Where should be replaced with the actual listener name configured in the
LISTENER.ORA file.

SUBSCRIBE_FOR_NODE_DOWN_EVENT_ parameter is to be placed by itself on an empty line.

It will be necessary to restart or reload the listener following the addition of this parameter.

This will prevent the messages from being written to the log file and may also prevent the TNS
Listener from hanging periodically. 

Please Note: Setting SUBSCRIBE_FOR_NODE_DOWN_ to OFF disables a necessary RAC functionality. The above workaround is recommended only for non-RAC environments. 
The issue may be present in all 10g and newer installations.

Note:  The use of this undocumented parameter may cause problems with the use of the Net Manager (NetMgr) configuration utility.