快照DataGuard是11R2的一个功能,可以用于临时的测试后回到主从模式，测试坏境为LGWR ASYNC的最大性能模式。

一、设置snapshot standby

关闭日志应用进程MRP
ALTER DATABASE RECOVER MANAGED STANDBY DATABASE CANCEL;
关闭备数据库，启动到MOUNT阶段
SHUTDOWN IMMEDIATE;
STARTUP NOMOUNT;
设置快速回复区域，但是注意这里并不需要FLASHBACK DATABASE支持，但是实际上应该还是FLASHBACK DATABASE
（Ensure that a fast recovery area has been configured. It is not necessary for
flashback database to be enabled.）

SQL> alter system set db_recovery_file_dest_size=30g;

System altered.

SQL> alter system set db_recovery_file_dest='/home/oradba/flash_area';

System altered.
如果不设置会报错
ERROR at line 1:
ORA-38784: Cannot create restore point 'SNAPSHOT_STANDBY_REQUIRED_04/24/2015
10:30:47'.
ORA-38786: Recovery area is not enabled.

执行转换
ALTER DATABASE CONVERT TO SNAPSHOT STANDBY;

SQL> ALTER DATABASE CONVERT TO SNAPSHOT STANDBY;

Database altered.

SQL> archive log list
Database log mode? ? ? ? ? ? ? Archive Mode
Automatic archival? ? ? ? ? ? Enabled
Archive destination? ? ? ? ? ? /oradba/archive
Oldest online log sequence? ? 1
Next log sequence to archive? 1
Current log sequence? ? ? ? ? 1

完成后完全是一个新的数据库

List of Database Incarnations
DB Key? Inc Key DB Name? DB ID? ? ? ? ? ? STATUS? Reset SCN? Reset Time
------- ------- -------- ---------------- --- ---------- ----------
1? ? ? 1? ? ? TEST? ? 2151380215? ? ? PARENT? 1? ? ? ? ? 17-SEP-11
2? ? ? 2? ? ? TEST? ? 2151380215? ? ? PARENT? 995548? ? 18-JUL-14
3? ? ? 3? ? ? TEST? ? 2151380215? ? ? PARENT? 142690086? 10-NOV-14
4? ? ? 4? ? ? TEST? ? 2151380215? ? ? CURRENT 168265840? 24-APR-15

最后就是打开数据库可以使用了
SQL> alter database open;

Database altered.

SQL> select DATABASE_ROLE from v$database;

DATABASE_ROLE
----------------
SNAPSHOT STANDBY

注意：
1、snapshot standby 产生自身的归档日志，但是ALTER SYSTEM SWITCH LOGFILE 并不会马上归档，会等待5分钟归档1个日志
2、snapshot standby 确实会接受来自主库的日志，同时STANDBY LOGFILE也会接受来自主库的日志