如果有一天你的主库宕机了，然后起不来了，或者机器能起来，但是你不敢确认数据库能起来，现在盘柜是没有损坏的，能够挂载到其他主机并能正常访问的，这时你应该先把主库上所有的联机日志做一个备份，然后再做其他动作。下面来做这个试验。
    Standby Database为物理备库，采用最大性能模式，并使用arch方式传递日志。
    现在在主库上建一张表，然后插入一些数据：
    create table test as select * from dba_objects;
    commit;
    select count(*) from test;
     COUNT(*)
    ----------
      5752
    然后模拟主机宕机，数据库crash掉：
    shutdown abort；
    因为插入的数据量比较小，还不至于引起归档，所以备库上是没有应用该日志的，也就没有这张表：
    select count(*) from test
                         *
    ERROR at line 1:
    ORA-00942: table or view does not exist
    这时你是不是想着去failover备库啊，别急，先检查一下备库是不是把所有的日志都应用了：
    select max(a.sequence#), max(b.sequence#) from v$archived_log a, v$log b where a.dest_id=2 and b.archived='YES';

    MAX(A.SEQUENCE#) MAX(B.SEQUENCE#)
    ---------------- ----------------
              62               66
    在这肯定是没有应用全的，如果failover了肯定会丢失一部分数据，所以要做恢复了：
    alter database recover managed standby database cancel;
    recover automatic standby database;
    ORA-00279: change 31825494112 generated at 09/11/2008 16:05:56 needed for thread 1
    ORA-00289: suggestion : /u02/orcl/arch/TEST/1_66.dbf
    ORA-00280: change 31825494112 for thread 1 is in sequence #66
    ORA-00278: log file '/u02/orcl/arch/TEST/1_66.dbf' no longer needed for this recovery
    ORA-00308: cannot open archived log '/u02/orcl/arch/TEST/1_66.dbf'
    ORA-27037: unable to obtain file status
    HP-UX Error: 2: No such file or directory
    Additional information: 3

    Specify log: {=suggested | filename | AUTO | CANCEL}
    /u02/orcl/arch/TEST/redo01.log
    ORA-00310: archived log contains sequence 65; sequence 66 required
    ORA-00334: archived log: '/u02/orcl/arch/TEST//redo01.log'
    提示要下一个归档日志的时候，手动指定第一个联机日志，报ORA-00310错误，看来这个不是想要的联机日志，指定下一个：
    recover automatic standby database;
    ORA-00279: change 31825494112 generated at 09/11/2008 16:05:56 needed for thread 1
    ORA-00289: suggestion : /u02/orcl/arch/TEST/1_66.dbf
    ORA-00280: change 31825494112 for thread 1 is in sequence #66
    ORA-00278: log file '/u02/orcl/arch/TEST/1_66.dbf' no longer needed for this recovery
    ORA-00308: cannot open archived log '/u02/orcl/arch/TEST/1_66.dbf'
    ORA-27037: unable to obtain file status
    HP-UX Error: 2: No such file or directory
    Additional information: 3

    Specify log: {=suggested | filename | AUTO | CANCEL}
    /u02/orcl/arch/TEST/redo02.log
    Log applied.
    Media recovery complete.
    日志被成功应用了，看来这个思路是正确的，打开数据库，看看数据是否正确：
    alter database open read only;
    select count(*) from test;
     COUNT(*)
    ----------
      5752
    数据被成功恢复，然后就可以转换为主库了。