Oracle重做日志操作是为了记录数据的改变,提供数据库的恢复,下面就为您介绍重做日志的运行流程,希望对您能够有所帮助。 一、ORACLE引入重做日志目的:记录数据的改变,提供数据库的恢复。 1、日志文件需要分组,同一个重做日志组的每个成员所存的信息完全
Oracle重做日志操作是为了记录数据的改变,提供数据库的恢复,下面就为您介绍重做日志的运行流程,希望对您能够有所帮助。
一、ORACLE引入重做日志目的:记录数据的改变,提供数据库的恢复。
1、日志文件需要分组,同一个重做日志组的每个成员所存的信息完全相同。(当为当前联机日志组添加成员时,造成该组的成员间数据不完全一样,因此添加成员时应注意被添加组是否处于CURRENT状态,如果是则可Alter system swtich logfile 来改变当前使用的日志组)
2、每个组中的重做日志文件均被称为成员
3、每组最好有2、3个成员,分别放置不同的物理磁盘,这样安全性较高。
4、最少需要两个重做日志组
二、Oracle重做日志的运行流程:
Oracle重做日志按照有序循环的方式被使用,即当一组日志文件被填满后,循环覆盖下一组日志文件,不断循环。此时称为日志切换。检查点操作也在此时发生,检查点操作是用来实现同步的,它会写数据文件的头信息、控制文件。在归档模式下,当一组日志文件被填满后,ARCH(归档写进程)将这组复制到归档日志文件中。(在ARCH正在写的重做日志文件LGWR是不可以写的)
1、Oracle重做日志文件的定稿是依靠LGWR后台进程
2、.LGWR正在写的重做日志组称为:当前重做日志组。
3、重做日志文件------联机日志文件;归档日志文件------脱机日志文件
三、查询重做日志组:
1、日志组:
select group#, sequence#, members, bytes, status, archived from v$log;
Status列中各值的意义:
Current:当前组。
Inactive:实例恢复已不需要这组日志了
Activie:这组日志是活动的,但非当前组。例如正在归档。
Unused:此组从未被写过。是日志刚被添加到DB中的状态。
2、日志文件:
Select * from v$logfile;
Status列中各值的含义:
空白:此文件正在使用。
Stale:该文件内容是不完全的。
Invalid:该文件不可以被访问。例如刚建立
Deleted:该文件已不再有用。
四、创建与删除重做日志文件组:
1、添加删除重做日志文件组:
Alter database add logfile (‘d:\**.log’, ‘e:\**.log’ ) size15m;
(这样创建的2个重做日志文件都会是同一组)
Alter database drop logfile group 4;
删除指定组号的重做日志组(删除后实际文件并未删除,需手动删除)
删除过程中出现的各种限制条件如下:
current log group当前日志组不可删除,要删除当前日志组需要先对当前日志组进行切换,使用命令为alter system switch logfile;
active log group活动的日志组不可删除
no archived log group没有归档的日志组不可删除(前提是已运行在归档模式下)
2、添加/删除重做日志文件:
Alter database add logfile member‘d:\red004.log’ to group 1,‘d:\red005.log’ to group2;
Alter database drop logfile member ‘d:\red004.log’;
删除时遇到的限制条件:
不能删除当前组的成员,如要删除则:强制性切换重做日志的命令:alter system switch logfile;
active log group活动的日志成员不可删除
no archived log group没有归档的日志成员不可删除(前提是已运行在归档模式下)
at least one member per group当日志成员中只有一个成员
特例:
有的时候在删除重做日志文件redo log file时,会出现如下错误
ERROR位于第1行:
ORA-00362:要求输入成员以组成组4中的有效日志文件
ORA-01517:日志成员: 'C:\ORACLE\ORADATA\GXCNC\REDO041.LOG'
(之所以不能drop logfile member from group 2,是因为group 2的其他redo log file的状态不对。这种情况,应该作几次alter system switch logfile ,使group 2的其他redo log file的状态变为正常 就可以了。)

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