--=============================
-- SYSTEM 表空间管理及备份恢复
--=============================
SYSTEM表空间是Oracle数据库最重要的一个表空间,存放了一些DDL语言产生的信息以及PL/SQL包、视图、函数、过程等,称之为数据字典,
因此该表空间也具有其特殊性，下面描述SYSTEM表空间的相关特性及备份与恢复。
一、SYSTEM表空间的管理
1.建议不存放用户数据，避免用户错误导致系统表空间不可用
应当为系统设定缺省的默认表空间来避免用户创建时使用系统表空间
ALTER DATABASE DEFAULT TABLESPACE tablespace_name
SQL> col property_value format a30
SQL> select property_name,property_value from database_properties
2 where property_name like 'DEFAULT%';
PROPERTY_NAME PROPERTY_VALUE
------------------------------ ------------------------------
DEFAULT_TEMP_TABLESPACE TEMP
DEFAULT_PERMANENT_TABLESPACE USERS --此处应当为非SYSTEM表空间
DEFAULT_TBS_TYPE SMALLFILE
2.SYSTEM表空间特性
不能脱机offline
不能置为只读read only
不能重命名
不能删除
--演示不能脱机
SQL> alter tablespace system offline;
alter tablespace system offline
*
ERROR at line 1:
ORA-01541: system tablespace cannot be brought offline; shut down if necessary
SQL> alter database datafile 1 offline;
alter database datafile 1 offline
*
ERROR at line 1:
ORA-01541: system tablespace cannot be brought offline; shut down if necessary
--不能置为只读状态
SQL> alter tablespace system read only;
alter tablespace system read only
*
ERROR at line 1:
ORA-01643: system tablespace can not be made read only
--不能重命名
SQL> alter tablespace system rename to system2;
alter tablespace system rename to system2
*
ERROR at line 1:
ORA-00712: cannot rename system tablespace
--不能删除
SQL> drop tablespace system;
drop tablespace system
*
ERROR at line 1:
ORA-01550: cannot drop system tablespace
SQL> drop tablespace system including contents and datafiles;
drop tablespace system including contents and datafiles
*
ERROR at line 1:
ORA-01550: cannot drop system tablespac
总结：system表空间就一句话，普通表空间所具有的更名、删除、只读、脱机不为system表空间所拥有
3.空间管理
保证空间可用，一般存放单个数据文件。设置为自动扩展
如果SYSTEM表空间数据文件很大，可以考虑使用bigfile
使用下面的视图来获取表空间的相关状态，使用空间等等
dba_data_files
dba_tablespaces
dba_free_space
v$datafiles
v$tablespace--查看表空间的大小及已用大小
SQL> select tablespace_name,bytes/1024/1024 cur_size,user_bytes/1024/1024 as user_bytes,status,online_status
2 from dba_data_files;
TABLESPACE_NAME CUR_SIZE USER_BYTES STATUS ONLINE_
------------------------------ ---------- ---------- --------- -------
USERS 5 4.9375 AVAILABLE ONLINE
SYSAUX 250 249.9375 AVAILABLE ONLINE
UNDOTBS1 35 34.9375 AVAILABLE ONLINE
SYSTEM 500 499.9375 AVAILABLE SYSTEM
EXAMPLE 100 99.9375 AVAILABLE ONLINE
--查看表空间的剩余空间
SQL> select tablespace_name,sum(bytes/1024/1024)
2 from dba_free_space group by tablespace_name;
TABLESPACE_NAME SUM(BYTES/1024/1024)
------------------------------ --------------------
UNDOTBS1 11.6875
SYSAUX 2.125
USERS 2
SYSTEM 10.125
EXAMPLE 31.75
--查看已用空间的百分比
select a.tablespace_name, round((sizea-sizeb),2) as used_space, round(sizeb,2) ||' MB' as free_space,
round((sizea-sizeb)/sizea*100,2) ||'%' as used_percent
from
(select tablespace_name,bytes/1024/1024 sizea from dba_data_files) a
inner join
(select tablespace_name,sum(bytes/1024/1024) sizeb
from dba_free_space group by tablespace_name) b
on a.tablespace_name = b.tablespace_name
order by a.tablespace_name;
二、SYSTEM表空间的冷备与恢复
关于数据库的冷备份请参考：冷备份
关于如何切换归档模式请参考：Oracle 归档日志
因仅仅涉及SYSTEM表空间的备份与恢复，在此仅仅备份SYSTEM表空间
1.非归档模式下的备份与恢复
--查看归档模式
SQL> select log_mode from v$database;
LOG_MODE
------------
NOARCHIVELOG
--关闭数据库后备份system01.dbf文件到指定备份目录
SQL> shutdown immediate;
SQL> ho cp $ORACLE_BASE/oradata/orcl/system01.dbf $ORACLE_BASE/coolbak/
--基于非系统表空间创建表tb
SQL> startup
SQL> create table tb tablespace users as select * from scott.emp;
SQL> select count(1) from tb;
COUNT(1)
----------
14
--多次切换日志直到日志组被清空
SQL> alter system switch logfile;
--删除system01.dbf文件
SQL> ho rm $ORACLE_BASE/oradata/orcl/system01.dbf
--强制启动后出现错误提示
SQL> startup force;
ORACLE instance started.
Database mounted.
ORA-01157: cannot identify/lock data file 1 - see DBWR trace file
ORA-01110: data file 1: '/u01/app/oracle/oradata/orcl/system01.dbf'
--还原system表空间的数据文件
SQL> ho cp $ORACLE_BASE/coolbak/system01.dbf $ORACLE_BASE/oradata/orcl/
--基于放弃的恢复system01.dbf
SQL> recover database until cancel;
ORA-00279: change 677850 generated at 08/15/2010 14:00:36 needed for thread 1
ORA-00289: suggestion : /u01/app/oracle/flash_recovery_area/ORCL/archivelog/2010_08_15/o1_mf_1_10_%u_.arc
ORA-00280: change 677850 for thread 1 is in sequence #10
Specify log: {<RET>=suggested | filename | AUTO | CANCEL}
cancel --系统提示成功恢复需要使用RESETLOGS打开数据库
ORA-01547: warning: RECOVER succeeded but OPEN RESETLOGS would get error below
ORA-01194: file 2 needs more recovery to be consistent--提示数据文件需要执行一致性恢复,有些数据在undo表空间丢失
ORA-01110: data file 2: '/u01/app/oracle/oradata/orcl/undotbs01.dbf'
ORA-01112: media recovery not started
--由于没有备份datafile 2，因此启用隐藏参数_allow_resetlogs_corruption
SQL> alter system set "_allow_resetlogs_corruption" = true scope = spfile; --此参数为静态参数，需要设置scope
SQL> startup force;
ORACLE instance started.
Database mounted.
ORA-01589: must use RESETLOGS or NORESETLOGS option for database open
--使用resetlogs打开数据库时系统异常终止
SQL> alter database open resetlogs;
alter database open resetlogs
*
ERROR at line 1:
ORA-00603: ORACLE server session terminated by fatal error
SQL> exit; --退出
--重新登录后可以正常启动
[oracle@robinson ~]$ sqlplus / as sysdba
SQL> startup
ORACLE instance started.
Database mounted.
Database opened.
--关闭隐藏参数_allow_resetlogs_corruption
SQL> alter system reset "_allow_resetlogs_corruption" scope =spfile sid ='*';
--提交的事务因日志被清空，数据丢失
SQL> select count(1) from tb;
select count(1) from tb
*
ERROR at line 1:
ORA-00942: table or view does not exist
2.归档模式下的备份与恢复
--获得system表空间的路径后一致性关闭数据库并复制system表空间数据文件到备份路径
SQL> col name format a55
SQL> select name from v$datafile where file# = 1;
NAME
-------------------------------------------------------
/u01/app/oracle/oradata/orcl/system01.dbf
SQL> shutdown immediate;
SQL> ho cp $ORACLE_BASE/oradata/orcl/system01.dbf $ORACLE_BASE/coolbak/
