Oracle数据库有联机重做日志，这个日志是记录对数据库所做的修改，比如插入，删除，更新数据等，对这些操作都会记录在联机重做日志里。一般数据库至少要有2个联机重做日志组。当一个联机重做日志组被写满的时候，就会发生日志切换，这时联机重做日志组2成为当前使用的日志，当联机重做日志组2写满的时候，又会发生日志切换，去写联机重做日志组1，就这样反复进行。
如果数据库处于非归档模式,联机日志在切换时就会丢弃. 而在归档模式下，当发生日志切换的时候，被切换的日志会进行归档。比如，当前在使用联机重做日志1，当1写满的时候，发生日志切换，开始写联机重做日志2，这时联机重做日志1的内容会被拷贝到另外一个指定的目录下。这个目录叫做归档目录，拷贝的文件叫归档重做日志。
数据库使用归档方式运行时才可以进行灾难性恢复。
1.归档日志模式和非归档日志模式的区别
非归档模式只能做冷备份,并且恢复时只能做完全备份.最近一次完全备份到系统出错期间的数据不能恢复.
归档模式可以做热备份,并且可以做增量备份,可以做部分恢复.
用ARCHIVE LOG LIST 可以查看期模式状态时归档模式还是非归档模式
1.非归档模式切换至归档模式
[oracle@oracle11g ~]$ sqlplus "/as sysdba"
SQL*Plus: Release 11.2.0.3.0 Production on Sat May 17 16:33:17 2014
Copyright (c) 1982, 2011, Oracle. All rights reserved.
Connected to:
Oracle Database 11g Enterprise Edition Release 11.2.0.3.0 - 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options
SQL> select inst_id,instance_name,version,archiver,status from gv$instance;
INST_ID INSTANCE_NAME VERSION ARCHIVE STATUS
---------- ---------------- ----------------- ------- ------------
1 orcl 11.2.0.3.0 STOPPED OPEN
更改归档日志默认路径
SQL> alter system set log_archive_dest_1='location=/u01/oracle/orcl_arch' sid='orcl';
System altered.
SQL> show parameter log_archive
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
log_archive_config string
log_archive_dest string
log_archive_dest_1 string location=/u01/oracle/orcl_arch
log_archive_dest_10 string
log_archive_dest_11 string
log_archive_dest_12 string
log_archive_dest_13 string
log_archive_dest_14 string
log_archive_dest_15 string
log_archive_dest_16 string
log_archive_dest_17 string
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
log_archive_dest_18 string
log_archive_dest_19 string
log_archive_dest_2 string
log_archive_dest_20 string
log_archive_dest_21 string
log_archive_dest_22 string
log_archive_dest_23 string
log_archive_dest_24 string
log_archive_dest_25 string
log_archive_dest_26 string
log_archive_dest_27 string
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
log_archive_dest_28 string
log_archive_dest_29 string
log_archive_dest_3 string
log_archive_dest_30 string
log_archive_dest_31 string
log_archive_dest_4 string
log_archive_dest_5 string
log_archive_dest_6 string
log_archive_dest_7 string
log_archive_dest_8 string
log_archive_dest_9 string
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
log_archive_dest_state_1 string enable
log_archive_dest_state_10 string enable
log_archive_dest_state_11 string enable
log_archive_dest_state_12 string enable
log_archive_dest_state_13 string enable
log_archive_dest_state_14 string enable
log_archive_dest_state_15 string enable
log_archive_dest_state_16 string enable
log_archive_dest_state_17 string enable
log_archive_dest_state_18 string enable
log_archive_dest_state_19 string enable
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
log_archive_dest_state_2 string enable
log_archive_dest_state_20 string enable
log_archive_dest_state_21 string enable
log_archive_dest_state_22 string enable
log_archive_dest_state_23 string enable
log_archive_dest_state_24 string enable
log_archive_dest_state_25 string enable
log_archive_dest_state_26 string enable
log_archive_dest_state_27 string enable
log_archive_dest_state_28 string enable
log_archive_dest_state_29 string enable
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
log_archive_dest_state_3 string enable
log_archive_dest_state_30 string enable
log_archive_dest_state_31 string enable
log_archive_dest_state_4 string enable
log_archive_dest_state_5 string enable
log_archive_dest_state_6 string enable
log_archive_dest_state_7 string enable
log_archive_dest_state_8 string enable
log_archive_dest_state_9 string enable
log_archive_duplex_dest string
log_archive_format string %t_%s_%r.dbf
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
log_archive_local_first boolean TRUE
log_archive_max_processes integer 4
log_archive_min_succeed_dest integer 1
log_archive_start boolean FALSE
log_archive_trace integer 0
查看归档日志路径
SQL> archive log list
Database log mode No Archive Mode
Automatic archival Disabled
Archive destination /u01/oracle/orcl_arch
Oldest online log sequence 12
Current log sequence 14
SQL> startup mount;
ORACLE instance started.
Total System Global Area 1603411968 bytes
Fixed Size 2228784 bytes
Variable Size 1023413712 bytes
Database Buffers 570425344 bytes
Redo Buffers 7344128 bytes
Database mounted.
SQL> alter database archivelog;
Database altered.
SQL> alter database open;
Database altered.
SQL> archive log list;
Database log mode Archive Mode
Automatic archival Enabled
Archive destination /u01/oracle/orcl_arch
Oldest online log sequence 12
Next log sequence to archive 14
Current log sequence 14
一次完全备份,因为非归档日志模式下产生的备份日志对于归档模式已经不可用了
SQL> alter tablespace users begin backup;
Tablespace altered.
2.归档到非归档切换
SQL> shutdown immediate;
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup mount;
ORACLE instance started.
Total System Global Area 1603411968 bytes
Fixed Size 2228784 bytes
Variable Size 1023413712 bytes
Database Buffers 570425344 bytes
Redo Buffers 7344128 bytes
Database mounted.
SQL> alter database noarchivelog ;
Database altered.
SQL> alter database open;
Database altered
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