DG架构图如下：
计划，切换之后的架构图：
DG切换：
主备切换：
这里所有的数据库数据文件、日志文件的路径是一致的
【旧主库】主库primarydb切换为备库standby3
主库检查switchover_status列的状态，是否支持switchover操作：如果该列值为"TO STANDBY"则表示primary 数据库支持转换为standby 角色
prod> select switchover_status from v$database;
SWITCHOVER_STATUS
--------------------
TO STANDBY
SQL> alter database commit to switchover to physical standby ;
SQL> select instance_name,status from v$instance;
SQL> shutdown immediate
【新建备库】新建备库standby3切换为主库
检查v$database视图的switchover_status列的状态，是否支持switchover操作：
SQL> select switchover_status from v$database;
SWITCHOVER_STATUS
--------------------
TO PRIMARY
SQL> alter database commit to switchover to primary ; --------此时此库可以处于mount模式或open read only模式
SQL> alter database open; -----至此该库转换成为Primary库
SQL> alter system switch logfile; 切日志
SQL> select max(sequence#) from v$archived_log;
新主库spfile修改：
alter system set LOG_ARCHIVE_CONFIG='DG_CONFIG=(primarydb,standby1,standby2,standby3,cascade)'
alter system set log_archive_dest_1 = 'location=/data/app/oracle/arch valid_for=(ALL_LOGFILES,ALL_ROLES) DB_UNIQUE_NAME=primarydb';
alter system set log_archive_dest_2 = 'SERVICE=standby1 SYNC VALID_FOR=(ONLINE_LOGFILES,PRIMARY_ROLE) DB_UNIQUE_NAME=standby1';
alter system set log_archive_dest_3 = 'SERVICE=standby2 SYNC VALID_FOR=(ONLINE_LOGFILES,PRIMARY_ROLE) DB_UNIQUE_NAME=standby2';
alter system set log_archive_dest_4 = 'SERVICE=standby3 SYNC VALID_FOR=(ONLINE_LOGFILES,PRIMARY_ROLE) DB_UNIQUE_NAME=standby3';
alter system set log_archive_dest_5 = 'SERVICE=cascade SYNC VALID_FOR=(ONLINE_LOGFILES,PRIMARY_ROLE) DB_UNIQUE_NAME=cascade';
alter system set FAL_CLIENT='primarydb';
alter system set FAL_SERVER='standby1','standby2','standby3','cascade';
在主备库创建静态listener和tnsnames.ora,远程连接测试
将新主库密码文件拷贝到各个新备库，并改名为orapwinstance_name
备库1：
修改密码文件cd $ORACLE_HOME/dbs mv orapwprimarydb orapwstandby1
alter system set FAL_SERVER=primarydb;
alter system set LOG_ARCHIVE_CONFIG='DG_CONFIG=(primarydb,standby1)'
alter system set log_archive_dest_2 = 'SERVICE=primarydb SYNC VALID_FOR=(ONLINE_LOGFILES,PRIMARY_ROLE) DB_UNIQUE_NAME=primarydb';
备库2：
修改密码文件cd $ORACLE_HOME/dbs mv orapwprimarydb orapwstandby2
alter system set FAL_SERVER=primarydb;
alter system set LOG_ARCHIVE_CONFIG='DG_CONFIG=(primarydb,standby2)'
alter system set log_archive_dest_2 = 'SERVICE=primarydb SYNC VALID_FOR=(ONLINE_LOGFILES,PRIMARY_ROLE) DB_UNIQUE_NAME=primarydb';
备库3：
备库3为先前主库：修改密码文件cd $ORACLE_HOME/dbs mv orapwprimarydb orapwstandby3
启动数据库到mount
SQL>startup mount
开启备库为应用日志模式状态
alter database recover managed standby database using current logfile disconnect from session;
alter system set FAL_SERVER=primarydb;
alter system set LOG_ARCHIVE_CONFIG='DG_CONFIG=(primarydb,standby3)'
alter system set log_archive_dest_2 = 'SERVICE=primarydb SYNC VALID_FOR=(ONLINE_LOGFILES,PRIMARY_ROLE) DB_UNIQUE_NAME=primarydb';
#指向级联备库的参数LOG_ARCHIVE_DEST_3='SERVICE=cascade SYNC VALID_FOR=(STANDBY_LOGFILES,STANDBY_ROLE) DB_UNIQUE_NAME=cascade’；
#因为之前的DG架构为一主三备库，所以应将不需要的归档日志路径log_archive_dest_state_3设置为defer
alter system set log_archive_dest_state_3=defer;
备库4：cascade
修改密码文件cd $ORACLE_HOME/dbs mv orapwprimarydb orapwcascade
alter system set FAL_SERVER=primarydb;
alter system set LOG_ARCHIVE_CONFIG='DG_CONFIG=(primarydb,cascade)'
alter system set log_archive_dest_2 = 'SERVICE=primarydb SYNC VALID_FOR=(ONLINE_LOGFILES,PRIMARY_ROLE) DB_UNIQUE_NAME=primarydb';
采坑：
一：主备库日志缺口：
FAL[client]: Failed to request gap sequence
GAP - thread 1 sequence 10894-10894
DBID 1016818008 branch 918555866
FAL[client]: All defined FAL servers have been attempted.
------------------------------------------------------------
Check that the CONTROL_FILE_RECORD_KEEP_TIME initialization
parameter is defined to a value that's sufficiently large
enough to maintain adequate log switch information to resolve
archivelog gaps.
------------------------------------------------------------
这里的解决方式是：
在主库拷贝相应的日志到备库，然后在备库注册：
alter database register logfile '/data/app/oracle/arch/1_3120_948067048.dbf';
二：主库可以远程连接备库，但是主库报错ORA16191
Error 1017 received logging on to the standby
------------------------------------------------------------
Check that the primary and standby are using a password file
and remote_login_passwordfile is set to SHARED or EXCLUSIVE,
and that the SYS password is same in the password files.
returning error ORA-16191
------------------------------------------------------------
密码文件需要在主库生成，然后拷贝之备库，然后在备库重命名。注意orapw文件的权限。
三：主库不能往新建的备库传日志
log_archive_dest_state_3默认是enable，但是也需要重新配置一下
alter system set log_archive_dest_state_3=enable;