简单理解oracle的SCN就是自己的时间功能,好比linux系统自己的时间一样,oracle它也有自己的一套时间。 在你干净的关闭数据库时shutdown immediate或者使用alter system checkpoint都会把SCN的值写入4个位置,其中有3个位于controlfile内,还有1个位于datafi
简单理解oracle的SCN就是自己的时间功能,好比linux系统自己的时间一样,oracle它也有自己的一套时间。
在你干净的关闭数据库时shutdown immediate或者使用alter system checkpoint都会把SCN的值写入4个位置,其中有3个位于controlfile内,还有1个位于datafile header内
controlfile里面的三个SCN分别是:1、system checkpoint SCN 2、datafile checkpoint SCN 3、stop SCN
---system checkpoint SCN 查询
SQL>select checkpoint_change# from v$database;
---datafile checkpoint SCN查询
SQL>select name,checkpoint_change# from v$datafile where name like '%user%';
---stop SCN查询
SQL>select name,last_change# from from v$datafile where name like '%user%';
其中这个stop SCN在数据库正常使用的时候是NUll值
datafile header里面的SCN是:start SCN
---start SCN 查询
SQL>select name,checkpoint_change# from v$datafile_header were name like '%user%';
1、 Clean shutdown 时
当clean shutdown 时,checkpoint会进行,并且此时datafile的stop scn和控制文件里的start scn会相同。 等到open数据库时,Oracle检查datafile header中的start scn和存于control file中的datafile的scn是否相同, 如果相同,接着检查datafile header的start scn和存于control file中的stop scn是否相同,如果仍然相同,数据库就会正常开启,否则就需要recovery。
等到数据库开启后,储存在control file中的stop scn就会恢复为NULL值,此时表示datafile是open在正常模式下了。
2、 非正常shutdown
如果不正常SHUTDOWN (shutdown abort),则mount数据库后,会发现stop scn并不是等于其它位置的scn, 而是等于NULL,这表示Oracle在shutdown时没有进行checkpoint,下次开机必须进行instance crash recovery(实例恢复)。
注意一点:
(1)启动数据库时,如果发现STOP SCN = NULL,表示需要进行instancecrash recovery;
(2)启动数据库时,如果发现有datafile header的START SCN 不等于储存于CONTROLFILE的DATAFILE SCN,表示需要进行Media recovery

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