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Redo日志是分组的,一个库至少需要两组,默认是三组。每个组内的redo日志称为成员。默认情况下,每个组只有一个成员,这样没有冗余性,可能造成online redo log的丢失,要提高数据的可靠性,应该为两个组至少配置两个成员,交将这两个成员分配到不同的磁盘上。
Redo日志是轮流使用的,一个redo log满了,LGWR会切换到下一组redo log,这种操作称为log switch,做log switch的同时也会做checkpoint,相应的信息还会写入控制文件。
也可以手工执行log switch或checkpoint操作
代码如下SQL> alter system switch logfile;
System altered.
SQL> alter system checkpoint;
System altered.
查看系统的redo log信息:
代码如下SQL> select l.group#,l.sequence#,l.bytes,l.members,l.status,f.member from v$log l, v$logfile f where l.GROUP#=f.GROUP#
GROUP# SEQUENCE# BYTES MEMBERS STATUS MEMBER
———- ———- ———- ———- —————- ———————————-
1 43 104857600 1 INACTIVE /oradata/redo/redo01.log
2 44 104857600 1 INACTIVE /oradata/redo/redo02.log
3 45 104857600 1 CURRENT /oradata/redo/redo03.log
可以在线添加online redo log组:
代码如下SQL> alter database add logfile group 4 (‘/oradata/redo/redo04.log’) size 100m;
Database altered.
SQL> select member from v$logfile;
MEMBER
——————————————————————————–
/oradata/redo/redo03.log
/oradata/redo/redo02.log
/oradata/redo/redo01.log
/oradata/redo/redo04.log
可以在线添加online redo log组成员:
代码如下SQL> alter database add logfile member ‘/oradata/redo/redo0a.log’ to group 1;
Database altered.
sys@SKDHC>select member from v$logfile;
MEMBER
———————————-
/oradata/redo/redo01.log
/oradata/redo/redo02.log
/oradata/redo/redo03.log
/oradata/redo/redo0a.log
/oradata/redo/redo0b.log
/oradata/redo/redo0c.log
6 rows selected.
在线删除刚才添加的组和组成员:
代码如下SQL> alter database drop logfile group 4;
Database altered.
SQL> alter database drop logfile member ‘/oradata/redo/redo0c.log;
Database altered.
SQL> select member from v$logfile;
MEMBER
——————————————————————————–
/home/oracle/oradata/gldb/redo03.log
/home/oracle/oradata/gldb/redo02.log
/home/oracle/oradata/gldb/redo01.log