UNDO表空间在有热备没有事物交易的时候有数据块损坏的恢复
第一，先看一下确保undo表空间里没有交易的事物
SQL>select XID USN,XIDSLOT,XIDSQN fromv$transaction;
no rows selected
第二，确认当前所用的回退表空间
SQL>show parameter undo
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
undo_management string AUTO
undo_retention integer 900
undo_tablespace string UNDOTBS1
第三查看表空间对应的数据文件
SQL>select tablespace_name,file_name from dba_data_Files
TABLESPACE_NAME FILE_NAME
------------------------------ ----------------------------------------
USERS /u01/tiger/oradata/orcl2/users01.dbf
SYSAUX /u01/tiger/oradata/orcl2/sysaux01.dbf
SYSTEM /u01/tiger/oradata/orcl2/system01.dbf
UNDOTBS2 /u01/tiger/oradata/orcl2/undotbs02.dbf
TIGER /u01/tiger/oradata/orcl2/tiger.dbf
第四查看数据文件的状态
SQL> selectfile#,name,status,recover,error from v$datafile_header;
FILE# NAME STATUS REC ERROR
---------- ---------------------------------------- ------- --- --------------------
1 /u01/tiger/oradata/orcl2/system01.dbf ONLINE NO
3 /u01/tiger/oradata/orcl2/sysaux01.dbf ONLINE NO
4 /u01/tiger/oradata/orcl2/users01.dbf ONLINE NO
5 /u01/tiger/oradata/orcl2/undotbs02.dbf ONLINE NO
7 /u01/tiger/oradata/orcl2/tiger.dbf ONLINE NO
第五，热备份undo表空间
SQL> alter tablespace undotbs2 beginbackup;
Tablespace altered.
SQL> !cp/u01/tiger/oradata/orcl2/undotbs02.dbf /bk
SQL> alter tablespace undotbs2 endbackup;
Tablespace altered.
如果在执行这条命令
SQL> alter tablespace undotbs2 beginbackup;
报错：
ERROR at line 1:
ORA-01123: cannot start online backup; media recovery notenabled
这说明了你的数据库不是归档模式，非归档的时候是不能进行热备的，只要把数据库改成归档模式即可。
第六，破坏数据块UNDO
QL> !cp /etc/passwd/u01/tiger/oradata/orcl2/undotbs02.dbf
SQL> selectfile#,name,status,recover,error from v$datafile_header;
FILE# NAME STATUS REC ERROR
---------- ---------------------------------------- ------- --- --------------------
1 /u01/tiger/oradata/orcl2/system01.dbf ONLINE NO
3 /u01/tiger/oradata/orcl2/sysaux01.dbf ONLINE NO
4 /u01/tiger/oradata/orcl2/users01.dbf ONLINE NO
5 /u01/tiger/oradata/orcl2/undotbs02.dbf ONLINE CANNOTREAD HEADER
7 /u01/tiger/oradata/orcl2/tiger.dbf ONLINE NO
第七，使UNDO文件离线会触发写文件操作就会报错系统将自动UNDOoffline
SQL> alter database datafile 5 offline;
alter database datafile 5 offline
*
ERROR at line 1:
SQL> /
FILE# NAME STATUS REC ERROR
-------------------------------------------------- ------- --- --------------------
1 /u01/tiger/oradata/orcl2/system01.dbf ONLINE NO
3 /u01/tiger/oradata/orcl2/sy