最近在想一个问题，当我在新安装的一个数据库上怎么来评估和监控这个数据库的log是否设置的合理那，应该怎么去优化log那。因为一个不良的log设置会是数据库性能大大受影响。默认情况下log是50mb，在绝大多数的系统中这个是不能满足的，设置过大又会导致自动恢复时间变长，怎么权衡这个问题那。尽量要保证每小时日志切换不能超过5次。

我个人观点就是看wait等待事件，如果有如下的等待事件就应该引起我们对log的注意了

 log file parallel write    log file sync      control file parallel write  

现在就怎么去处理这一类问题说下

alter database add logfile group 4 ('/u01/app/oracle/oradata/cks/redo04.log') size 500m;添加一个新的组，并且加大日志大小降低日志切换频率，将所有的日志组都改到500mb（这个500mb是更具我先有系统上的问题给出来的一个值，具体的还需要自己估量）

删除过去的日志

alter database drop logfile group 2;
到操作系统层删除对应的物理log文件

alter system switch logfile;
alter system checkpoint;

后期监控日志的切换频率可以用如下sql

SELECT 
to_char(first_time,'YYYY-MM-DD') day,  
to_char(sum(decode(to_char(first_time,'HH24'),'00',1,0)),'999') "00",  
to_char(sum(decode(to_char(first_time,'HH24'),'01',1,0)),'999') "01",  
to_char(sum(decode(to_char(first_time,'HH24'),'02',1,0)),'999') "02",  
to_char(sum(decode(to_char(first_time,'HH24'),'03',1,0)),'999') "03",  
to_char(sum(decode(to_char(first_time,'HH24'),'04',1,0)),'999') "04",  
to_char(sum(decode(to_char(first_time,'HH24'),'05',1,0)),'999') "05",  
to_char(sum(decode(to_char(first_time,'HH24'),'06',1,0)),'999') "06",  
to_char(sum(decode(to_char(first_time,'HH24'),'07',1,0)),'999') "07",  
to_char(sum(decode(to_char(first_time,'HH24'),'08',1,0)),'999') "08",  
to_char(sum(decode(to_char(first_time,'HH24'),'09',1,0)),'999') "09",  
to_char(sum(decode(to_char(first_time,'HH24'),'10',1,0)),'999') "10",  
to_char(sum(decode(to_char(first_time,'HH24'),'11',1,0)),'999') "11",  
to_char(sum(decode(to_char(first_time,'HH24'),'12',1,0)),'999') "12",  
to_char(sum(decode(to_char(first_time,'HH24'),'13',1,0)),'999') "13",  
to_char(sum(decode(to_char(first_time,'HH24'),'14',1,0)),'999') "14",  
to_char(sum(decode(to_char(first_time,'HH24'),'15',1,0)),'999') "15",  
to_char(sum(decode(to_char(first_time,'HH24'),'16',1,0)),'999') "16",  
to_char(sum(decode(to_char(first_time,'HH24'),'17',1,0)),'999') "17",  
to_char(sum(decode(to_char(first_time,'HH24'),'18',1,0)),'999') "18",  
to_char(sum(decode(to_char(first_time,'HH24'),'19',1,0)),'999') "19",  
to_char(sum(decode(to_char(first_time,'HH24'),'20',1,0)),'999') "20",  
to_char(sum(decode(to_char(first_time,'HH24'),'21',1,0)),'999') "21",  
to_char(sum(decode(to_char(first_time,'HH24'),'22',1,0)),'999') "22",  
to_char(sum(decode(to_char(first_time,'HH24'),'23',1,0)),'999') "23" 
from 
   v$log_history 
GROUP by to_char(first_time,'YYYY-MM-DD') 

每月每天的日志切换次数
select to_char(trunc(first_time), 'Mon') month,  
       to_char(trunc(first_time), 'YYYY-MM-DD:Day') day,  
       count(*) count  
from   v$log_history  
where  trunc(first_time) > last_day(sysdate - 100) + 1  
group  by trunc(first_time)  
order  by 1, 2;