SYSTEM表空间是Oracle创建数据库时候自动创建的，每个Oracle数据库都会有SYSTEM表空间，而且SYSTEM表空间总是要保持在联机模式下并且不可重命名，因为其包含了数据库运行所要求的基本信息，如：数据字典、联机求助机制、所有回退段、临时段和自举段、所有的用户数据库实体、其它ORACLE软件产品要求的表等等。

    常用system表空间备份还原方法

    1.系统热备

    1. SQL> alter   tablespace   system   begin   backup;                                                         
    3. Tablespace altered.
    4. [oracle@instance-ej1hspdt backup]$ cp  /opt/oracle/oradata/ORCLCDB/system01.dbf   /backup/
    6. SQL> alter  tablespace  system   end   backup;                                                             
    8. Tablespace altered. 
    9. [oracle@instance-ej1hspdt backup]$ rm -rf  /opt/oracle/oradata/ORCLCDB/system01.dbf 
    10. [oracle@instance-ej1hspdt backup]$ sqlplus / as sysdba   
    11. SQL> shutdown abort; 
    12. SQL> quit                                                                                                  
    13. Disconnected                                                                                               
    14. [oracle@instance-ej1hspdt backup]$ cp /backup/system01.dbf /opt/oracle/oradata/ORCLCDB/                    
    15. [oracle@instance-ej1hspdt backup]$ sqlplus / as sysdba   
    16. SQL> startup  mount;                                                                                       
    17. ORACLE instance started.                                                                                   
    19. Total System Global Area  838857584 bytes                                                                  
    20. Fixed Size                  8901488 bytes                                                                  
    21. Variable Size             700448768 bytes                                                                  
    22. Database Buffers          121634816 bytes                                                                  
    23. Redo Buffers                7872512 bytes                                                                  
    24. Database mounted.                                                                                          
    25. SQL> recovery   database;                                                                                  
    26. SP2-0734: unknown command beginning "recovery  ..." - rest of line ignored.                                
    27. SQL> recover   database;                                                                                   
    28. Media recovery complete. 
    29. SQL> alter  database  open;                                                                                
    31. Database altered.

    2.rman备份恢复

    1. RMAN> backup  tablespace   system;                                                                                                                              
    3. Starting backup at 28-JUL-19                                                                                                                                    
    4. using channel ORA_DISK_1                                                                                                                                        
    5. channel ORA_DISK_1: starting datafile copy                                                                                                                      
    6. input datafile file number=00001 name=/opt/oracle/oradata/ORCLCDB/system01.dbf                                                                                  
    7. output file name=/opt/oracle/product/18c/dbhome_1/dbs/data_D-ORCLCDB_I-2782375137_TS-SYSTEM_FNO-1_3bu7qi0p tag=TAG20190728T205656 RECID=15 STAMP=1014843429     
    8. channel ORA_DISK_1: datafile copy complete, elapsed time: 00:00:15                                                                                              
    9. Finished backup at 28-JUL-19                                                                                                                                    
    11. Starting Control File and SPFILE Autobackup at 28-JUL-19                                                                                                        
    12. piece handle=/opt/oracle/product/18c/dbhome_1/dbs/c-2782375137-20190728-00 comment=NONE                                                                         
    13. Finished Control File and SPFILE Autobackup at 28-JUL-19 
    15. [oracle@instance-ej1hspdt backup]$ rm /opt/oracle/oradata/ORCLCDB/system01.dbf       
    16. SQL> shutdown  abort;                                                                                                                                           
    17. ORACLE instance shut down.  
    20. SQL> startup  mount;                                                                                                                                            
    21. ORACLE instance started.                                                                                                                                        
    23. Total System Global Area  838857584 bytes                                                                                                                       
    24. Fixed Size                  8901488 bytes                                                                                                                       
    25. Variable Size             700448768 bytes                                                                                                                       
    26. Database Buffers          121634816 bytes                                                                                                                       
    27. Redo Buffers                7872512 bytes                                                                                                                       
    28. Database mounted. 
    29. [oracle@instance-ej1hspdt backup]$ rman  target /                                                                                                               
    31. Recovery Manager: Release 18.0.0.0.0 - Production on Sun Jul 28 21:00:50 2019                                                                                   
    32. Version 18.3.0.0.0                                                                                                                                              
    34. Copyright (c) 1982, 2018, Oracle and/or its affiliates.  All rights reserved.                                                                                   
    36. connected to target database: ORCLCDB (DBID=2782375137, not open)                                                                                               
    38. RMAN> restore   tablespace   system;                                                                                                                            
    40. Starting restore at 28-JUL-19                                                                                                                                   
    41. using target database control file instead of recovery catalog                                                                                                  
    42. allocated channel: ORA_DISK_1                                                                                                                                   
    43. channel ORA_DISK_1: SID=24 device type=DISK                                                                                                                     
    45. channel ORA_DISK_1: restoring datafile 00001                                                                                                                    
    46. input datafile copy RECID=15 STAMP=1014843429 file name=/opt/oracle/product/18c/dbhome_1/dbs/data_D-ORCLCDB_I-2782375137_TS-SYSTEM_FNO-1_3bu7qi0p               
    47. destination for restore of datafile 00001: /opt/oracle/oradata/ORCLCDB/system01.dbf                                                                             
    48. channel ORA_DISK_1: copied datafile copy of datafile 00001, elapsed time: 00:00:15                                                                              
    49. output file name=/opt/oracle/oradata/ORCLCDB/system01.dbf RECID=0 STAMP=0                                                                                       
    50. Finished restore at 28-JUL-19 
    52. RMAN> quit                                                                                                                                                      
    55. Recovery Manager complete.                                                                                                                                      
    56. [oracle@instance-ej1hspdt backup]$ sqlplus / as sysdba                                                                                                          
    58. SQL*Plus: Release 18.0.0.0.0 - Production on Sun Jul 28 21:01:57 2019                                                                                           
    59. Version 18.3.0.0.0                                                                                                                                              
    61. Copyright (c) 1982, 2018, Oracle.  All rights reserved.                                                                                                         
    64. Connected to:                                                                                                                                                   
    65. Oracle Database 18c Enterprise Edition Release 18.0.0.0.0 - Production                                                                                          
    66. Version 18.3.0.0.0                                                                                                                                              
    68. SQL> recover   database;                                                                                                                                        
    69. Media recovery complete.                                                                                                                                        
    70. SQL> alter  database  open;                                                                                                                                     
    72. Database altered.