归档模式，是将redo 日志组中将要被覆盖的文件，redo日志中存放操作的dml，拷贝一份形成归档日志文件。
归档模式，我认为对于数据库的安全和数据的完整性而言，至关重要。
oracle数据库的备份时间与当前时间，总有间隔时间，如果数据库在当前时间宕机，那么没有归档，间隔时间的dml操作将不会被保存，数据也会丢失。开启归档，所有的操作都会被记录，通过跑归档日志，可以恢复到当前的状态。
查看数据库是否开启归档
SQL> archive log list;
Database log mode No Archive Mode --无归档
Automatic archival Disabled
Archive destination USE_DB_RECOVERY_FILE_DEST --默认归档路径
Oldest online log sequence 41 --旧的在线redo log 序列号
Current log sequence 43 --当前日志序列号

修改归档模式，应关闭数据库，开启到mount状态
SQL> shutdown immediate
Database closed.
Database dismounted.
ORACLE instance shut down.

SQL> startup mount
ORACLE instance started.

Total System Global Area 2042241024 bytes
Fixed Size 1337548 bytes
Variable Size 1174406964 bytes
Database Buffers 855638016 bytes
Redo Buffers 10858496 bytes
ORA-01013: user requested cancel of current operation

当时数据库报了个错误，但重新启库却没有问题！！！

SQL> shut immediate
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup mount;
ORACLE instance started.

Total System Global Area 2042241024 bytes
Fixed Size 1337548 bytes
Variable Size 1174406964 bytes
Database Buffers 855638016 bytes
Redo Buffers 10858496 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 USE_DB_RECOVERY_FILE_DEST
Oldest online log sequence 41
Next log sequence to archive 43 --这里多了个下个归档日志序列
Current log sequence 43

--修改系统的归档日志存放路径

SQL> alter system set log_archive_dest_1='location=/arch';
System altered.

SQL> archive log list
Database log mode Archive Mode
Automatic archival Enabled
Archive destination /arch --归档路径改成自定义
Oldest online log sequence 41
Next log sequence to archive 43
Current log sequence 43

SQL> alter system switch logfile; --手动切换日志文件
System altered.

查看路径下生成归档日志文件
[oracle@TEST arch]$ ls -lrth
-rw-r----- 1 oracle oinstall 43M Jan 9 2017 1_52_931998347.dbf

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Oracle归档模式

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