对于已经建立的rac dataguard 环境，standby rac和主库对应拥有两个节点并且可以应在一个节点用日志！在此情况下 启用 ADG 特性 1. 日志已经在rac3上应用，所以只需在节点rac3上取消日志应用，注意：对于节点rac4 不用做任何操作！
rac3> alter database recover managed standby database cancel;
2. Since database added to cluster to startup default on mount mode with
打开数据库至 read only 模式，需要注意的是向集群添加节点的时候一般是以mount状态启动的，对于备库要以read only 方式打开！
rac3> alter database open read only;
rac3> select open_mode from v$database;
对节点rac3操作完之后，节点rac4依然是mounted状态
rac4> select open_mode from v$database;
rac4> alter database open read only;
rac4> select open_mode from v$database;
rac3> alter database recover managed standby database using current logfile disconnect;
rac3> select open_mode from v$database;
rac4> select open_mode from v$database;
二 如果配置的dataguard broker，并且已经应用日志了，按照如下方法进行： DGMGRL> edit database rac set state='APPLY-OFF';
DGMGRL> show database rac
Intended State: APPLY-OFF
2. 进行非dataguard broker的第二步！
alter database open read only;
DGMGRL> edit database rac set state='APPLY-ON';
rac3> alter database recover managed standby database cancel;
open_mode 将由 READ ONLY WITH APPLY 变为 READ ONLY
rac3> select open_mode from v$database;
2. 关闭并重启standby 至mount状态！
srvctl stop database -d rac
srvctl start database -d rac
rac3> select open_mode from v$database;
rac3> alter database recover managed standby database using current logfile disconnect;