最近对ASM进行重新规划后，在alert中频繁发现以下信息，很是烦人：

[@more@]Thread 4 advanced to log sequence 3499 (LGWR switch)
Current log# 56 seq# 3499 mem# 0: +DBSYS/dwrac/redo01_56.log
Wed Apr 13 05:29:59 CST 2011
SUCCESS: diskgroup ARCH was mounted
Wed Apr 13 05:31:59 CST 2011
SUCCESS: diskgroup ARCH was dismounted
Wed Apr 13 05:51:56 CST 2011
Thread 4 advanced to log sequence 3500 (LGWR switch)
Current log# 57 seq# 3500 mem# 0: +DBSYS/dwrac/redo01_57.log
Wed Apr 13 05:51:56 CST 2011
SUCCESS: diskgroup ARCH was mounted
Wed Apr 13 05:52:20 CST 2011
SUCCESS: diskgroup ARCH was dismounted
Wed Apr 13 06:50:30 CST 2011
Thread 4 advanced to log sequence 3501 (LGWR switch)
Current log# 58 seq# 3501 mem# 0: +DBSYS/dwrac/redo01_58.log
Wed Apr 13 06:50:30 CST 2011
SUCCESS: diskgroup ARCH was mounted
Wed Apr 13 06:50:49 CST 2011
SUCCESS: diskgroup ARCH was dismounted
Wed Apr 13 08:42:03 CST 2011
Thread 4 advanced to log sequence 3502 (LGWR switch)
Current log# 59 seq# 3502 mem# 0: +DBSYS/dwrac/redo01_59.log

这是因为一个bug：4465015导致的。不过Oracle也说这是一个正常的行为：
当关闭某个disk group上的最后一个文件时，Oracle会将这个diskgroup dismount；
当需要读取某个disk group某一个文件时，Oracle再把它mount。

在这个案例中，归档日志放在一个独立的disk group中，而归档日志是隔段时间才会产生一个，所以会频繁在alert日志中看到这个信息。
这个信息是正常的，但是如果你觉得比较烦，可以把控制文件、redo的一个副本或者放一个很小的数据文件到这个disk group上，依赖CKPT、LGWR、DWR几个进程分别需要频繁访问以上文件来避免。

如：
SQL> alter tablespace admin add datafile '+ARCH/dwrac/datafile/admin02.dbf' size 10m;

Tablespace altered

ITPub博客

alert中频繁出现diskgroup ARCH was disounted/mounted

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