实验隐藏参数"_allow_resetlogs_corruption"的使用

实验环境:OEL 5.7 + Oracle 10.2.0.5

Tips:该参数仅在特殊恢复场景下使用,需要在专业Oracle工程师指导下进行操作。

1.隐藏参数说明

查询隐藏参数"_allow_resetlogs_corruption"及说明:

set linesize 333
col name for a35
col description for a66
col value for a30
SELECT i.ksppinm name,
i.ksppdesc description,
CV.ksppstvl VALUE
FROM sys.x$ksppi i, sys.x$ksppcv CV
WHERE i.inst_id = USERENV ('Instance')
AND CV.inst_id = USERENV ('Instance')
AND i.indx = CV.indx
AND i.ksppinm LIKE '%&keyword%'
ORDER BY 1; Enter value for keyword: allow_resetlog
old 8: AND i.ksppinm LIKE '%&keyword%'
new 8: AND i.ksppinm LIKE '%allow_resetlog%' NAME DESCRIPTION VALUE
----------------------------------- ------------------------------------------------------------------ ------------------------------
_allow_resetlogs_corruption allow resetlogs even if it will cause corruption FALSE

通过这个隐藏参数非常规恢复的库,原则建议还是要重建库的。其实在alert日志中也会看到有这样的建议:

Wed Dec 26 00:00:41 CST 2018
alter database open resetlogs
Wed Dec 26 00:00:41 CST 2018
RESETLOGS is being done without consistancy checks. This may result
in a corrupted database. The database should be recreated.

2.故障场景再现

模拟常规开库失败的场景:

SQL> select checkpoint_change# from v$datafile_header;

CHECKPOINT_CHANGE#
------------------
10013731555
10014045643
10014045643
10014045643
10014045643
10014045643
10014045643
10014045643
10014045643 9 rows selected. SQL> alter database open;
alter database open
*
ERROR at line 1:
ORA-01589: must use RESETLOGS or NORESETLOGS option for database open SQL> alter database open resetlogs;
alter database open resetlogs
*
ERROR at line 1:
ORA-01194: file 1 needs more recovery to be consistent
ORA-01110: data file 1: '+ZHAOJINGYU/jy/datafile/system.256.839673875'

说明:这个环境是模拟数据文件1丢失,最终从备份restore出来一个旧的文件,但由于种种原因,总之没有后续的归档去做recover,导致无法追平。

此时就可尝试使用_allow_resetlogs_corruption隐藏参数强制开库:

SQL> alter system set "_allow_resetlogs_corruption" = true scope=spfile;
SQL> shutdown immediate
SQL> startup mount
SQL> alter database open resetlogs;

此时再去查询数据文件头的SCN已经一致:

SQL> select checkpoint_change# from v$datafile_header;

  CHECKPOINT_CHANGE#
--------------------
10014022016
10014022016
10014022016
10014022016
10014022016
10014022016
10014022016
10014022016
10014022016 9 rows selected.

注意处理完毕后及时改回这个隐藏参数为false:

alter system set "_allow_resetlogs_corruption" = false scope=spfile;

其他注意事项:如果开库遇到ORA-600 [2662]类错误,可以参考之前随笔:

最终通过推进SCN的手段来解决ORA-600 [2662]类问题。

其实这个场景其实可能会遇到各种问题,都属于非常规恢复范畴,后续我会计划去继续测试验证一些常见场景及解决方案。

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