+ORADATA01/ORCL/onlinelog/group_9.329.731786323
Mon Apr 08 16:00:05 GMT+03:00 2011SUCCESS: diskgroup FRA was mounted
Mon Apr 08 16:00:18 GMT+03:00 2011SUCCESS: diskgroup FRA was dismounted
Mon Apr 08 16:15:06 GMT+03:00 2011Thread 2 advanced to log sequence 51398 (LGWR switch)
  Current log# 10 seq# 51398 mem# 0: +ORADATA01/ORCL/onlinelog/group_10.330.731! 786327
Mon Apr 08 16:15:06 GMT+03:00 2011SUCCESS: diskgroup FRA was mounted
Mon Apr 08 16:15:11 GMT+03:00 2011SUCCESS: diskgroup FRA was dismounted
Mon Apr 08 16:30:06 GMT+03:00 2011Thread 2 advanced to log sequence 51399 (LGWR switch)
  Current log# 8 seq# 51399 mem# 0: +ORADATA01/ORCL/onlinelog/group_8.328.731786319
Mon Apr 08 16:30:06 GMT+03:00 2011SUCCESS: diskgroup FRA was mounted
Mon Apr 08 16:30:12 GMT+03:00 2011SUCCESS: diskgroup FRA was dismounted
Mon Apr 08 16:45:06 GMT+03:00 2011Thread 2 advanced to log sequence 51400 (LGWR switch)
  Current log# 9 seq# 51400 mem# 0: +ORADATA01/ORCL/onlinelog/group_9.329.731786323
Mon Apr 08 16:45:10 GMT+03! :00 2011SUCCESS: diskgroup FRA was mounted
Mon Apr 08 16:45:15 GMT+03:00 2011SUCCESS: diskgroup FRA was dismounted

After some search, I found that its a bug 4465015 which is mention in Metalink note

Why FRA Diskgroup Gets Mounted/Dismounted? [ID 603204.1]

Here is the explanation of that message:

This is the expected behavior. The messages you see are not errors (that is why they are prefixed with “SUCCESS”).
The database instance is creating and then closing archive logs one at a time in the FRA disk group.
Whenever a database closes its last file in a disk group, it dismounts the disk group. Whenever a database accesses a disk group when it does not have any other files open in the disk group, it mounts the disk group.

If you do not want to see the log messages frequently, you can put a mirrored control-file, or mirrored online redo, or dummy online tablespace data file on the diskgroup.
This message will not be output frequently in this case. Because CKPT keeps opening a control-file, LGWR keeps opening an online redo, DBW keeps opening an online tablespace data file. So, the diskgroup is not dismounted until the database shutdown.

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