通过 RMAN DUPLICATE...FROM ACTIVE DATABASE创建dataguard(for oracle 11g)
     oracle 10g可以通过基于备份的rman DUPLICATE实现dataguard，通过步骤需要对数据库进行备份，并在standby侧进行数据库的恢复。而到了11g，oracle推出了Duplicate From Active Database技术，不需要再对数据库进行rman备份恢复，一切动作都通过网络自动完成。
下面是具体的实现例子：
primary db：hrdbprim
standby db:standby(由于是三个节点的rac，实例名为standby1）

一、primary侧的环境准备：

1，确保数据库归档状态

SQL> select log_mode from v$database;

LOG_MODE
------------
ARCHIVELOG

2，Enable force logging

SQL> ALTER DATABASE FORCE LOGGING;

Database altered.

3，生成standby redolog

SQL> alter database add standby logfile '/oracle/app/oracle/oradata/hrdbprim/redo11.log' size 50m;

Database altered.

4，修改primary参数文件spfile，需要设置以下8个参数

SQL> alter system set LOG_ARCHIVE_CONFIG='DG_CONFIG=(hrdbprim,standby)';

System altered.

SQL> alter system set LOG_ARCHIVE_DEST_1='LOCATION=/oracle/app/oracle/oradata/hrdbprim/ VALID_FOR=(ALL_LOGFILES,ALL_ROLES) DB_UNIQUE_NAME=hrdbprim';

System altered.

SQL> alter system set LOG_ARCHIVE_DEST_2='SERVICE=standby LGWR ASYNC VALID_FOR=(ONLINE_LOGFILES,PRIMARY_ROLE) DB_UNIQUE_NAME=standby';

System altered.

SQL> alter system set LOG_ARCHIVE_DEST_STATE_1=ENABLE;

System altered.

SQL> alter system set FAL_SERVER=standby;

System altered.

SQL> alter system set FAL_CLIENT=standby;

System altered.

SQL> alter system set DB_FILE_NAME_CONVERT='/oracle/app/oracle/oradata/hrdbprim/','+DATA/standby/datafile/' scope=spfile;

System altered.

SQL> alter system set LOG_FILE_NAME_CONVERT='/oracle/app/oracle/oradata/hrdbprim/','+DATA/standby/onlinelog/' scope=spfile;

System altered.

二、修改sql*net相关文件，确保网络环境准备,确保互相tnsping通

listener.ora
SID_LIST_LISTENER =
  (SID_LIST =
    (SID_DESC =
     (GLOBAL_DBNAME = standby)
     (ORACLE_HOME = /oracle/app/oracle/product/11.2.0/db_1)
     (SID_NAME = standby3)
    )
   )

LISTENER =
  (DESCRIPTION =
    (ADDRESS = (PROTOCOL = TCP)(HOST = 10.4.124.235)(PORT = 1521))
  )

tnsnames.ora

hrdbprim =
  (DESCRIPTION =
    (ADDRESS = (PROTOCOL = TCP)(HOST = 10.4.124.239)(PORT = 1521))
    (CONNECT_DATA =
      (SERVER = DEDICATED)
      (SERVICE_NAME = hrdbprim)
    )
  )
standby =
  (DESCRIPTION =
    (ADDRESS = (PROTOCOL = TCP)(HOST = 10.4.124.235)(PORT = 1521))
    (CONNECT_DATA =
      (SERVER = DEDICATED)
      (SERVICE_NAME = standby)(UR=A)
    )
  )
三、创建standby数据库

1，password密码文件；既可以从primary复制改名，也可以重新生成一个，主要要保持sys口令一致，这里采用复制方式
sftp ...
mv orapwhrdbprim orapwstandby
2,新建pfile文件，注意pfile要放在$ORACLE_HOME/dbs目录,否则启动时需要指定pfile文件,注意启动时必须使用pfile文件启动，否者无法复制
vi initstandby3.ora

DB_NAME=standby
DB_UNIQUE_NAME=standby
*.audit_file_dest='/oracle/app/oracle/admin/standby/adump'
*.control_files='+DATA/standby/controlfile/control01.ctl','+FRA/standby/controlfile/control02.ctl'
*.db_create_file_dest='+DATA'
*.db_block_size=8192
*.db_recovery_file_dest='+FRA'
*.db_recovery_file_dest_size=10G

3，创建相关目录，用来放datafile和trace file
mkdir -p /oracle/app/oracle/ad