Oracle Data Guard_ 备库以只读或读写方式打开访问

• Opening the physical standby database read-only may lengthen the time it takes to recover from a failure or outage, because the database must be restarted after a failover.

  以只读方式打开物理备库可以延长失败或中断恢复的时间，因为数据库必须在故障切换后重新启动。

•  

  As long as the physical standby database has not been opened read-only since the last time it was started, a restart is unnecessary after failover, thus increasing system availability.

  只要物理备库自从上次启动没有以只读方式打开，在故障切换后，重启是必须的，那么就会增加系统的可用性。

•  

• While a standby database is open for read-only or read/write access, it does not apply redo data received from the primary database, thus it is not kept transactionally consistent with the primary database.

  当备库是以只读方式打开或者读写方式打开，就不会应用来自主库的重做数据，那么就不能与主库保持一致性的事务。

•  

  When a physical standby database is open, redo data from the primary database is received by the standby database, but the log files are not applied. At some point, you need to resume Redo Apply on the standby database, and apply the archived redo log files to resynchronize the standby database with the primary database. Because of the additional time required to apply any accumulated archived redo log files, having a standby database open for read-only access can increase the time required to complete failovers or switchovers.

  当物理备库是打开时，来自主库的重做数据被备库接收，但是不会被应用。在有点伤，你需要在备库上重新应用重做数据，应用归档日志来同步备库和主库之间的数据。因为应用任何所需的额外时间累积的归档重做日志文件，因为需要申请任何累计归档重做日志文件，有一个备用数据库只读访问打开会增加完成故障切换或换药所需的额外时间。

8.2.2 Opening a Physical Standby Database for Read-Only Access

You can alternate between having a physical standby database open for read-only access and performing Redo Apply using the following procedures.

你可以使用以下过程在物理备库只读方式打开访问和执行重做应用来轮流切换。

To open a standby database for read-only access when it is currently shut down:

当备库是关闭状态，直接使用STARTUP命令来以只读方式打开备库：

Start, mount, and open the database for read-only access using the following statement:

To open a standby database for read-only access when it is currently performing Redo Apply:

当目前正在执行重做应用，以只读方式打开备库来访问：（首先取消重做应用，然后再打开数据库）

1. Cancel Redo Apply:

   SQL> ALTER DATABASE RECOVER MANAGED STANDBY DATABASE CANCEL;

2. Open the database for read-only access:

   SQL> ALTER DATABASE OPEN;
   
   
   ######################################################################################
   SYS@PRODSTD>alter database open;

   
   

   Database altered.

   
   

   SYS@PRODSTD>select OPEN_MODE from v$database;

   
   

   OPEN_MODE

   ----------

   READ ONLY

   
   

   SYS@PRODSTD>
   ------------------------------------------------------------------------------------------------------------------
   Sat Mar 29 13:22:29 2014

   alter database open

   Sat Mar 29 13:22:29 2014

   SMON: enabling cache recovery

   Sat Mar 29 13:22:30 2014

   Database Characterset is AL32UTF8

   replication_dependency_tracking turned off (no async multimaster replication found)

   Physical standby database opened for read only access.

   Completed: alter database open
   
   
   ########################################################################################

You do not need to shut down the instance to open it for read-only access.

To change the standby database from being open for read-only access to performing Redo Apply:

切换备库从只读访问到重做应用：

1. Terminate all active user sessions on the standby database.

   1.终止备库上所有活跃的用户会话。

2. Restart Redo Apply. To start Redo Apply, issue the following statement:

   2.重启重做应用，发出以下语句可以启用重做应用：

   SQL>

   ALTER DATABASE RECOVER MANAGED STANDBY DATABASE

     2> DISCONNECT FROM SESSION;

   To enable real-time apply, include the USING CURRENT LOGFILE clause:

   实时应用，需要包含USING CURRENT LOGFILE子句：

   SQL> ALTER DATABASE RECOVER MANAGED STANDBY DATABASE

     2> USING CURRENT LOGFILE;

You do not need to shut down the instance to start either of these apply modes.

你不需要关闭数据库然后再启用这些应用模式。