I always see the following messages in Alertlog , was curious to know the reason behind it…
Wed Jun 25 06:35:22 2014 SUCCESS: diskgroup DATA2 was mounted SUCCESS: diskgroup DATA2 was dismounted SUCCESS: diskgroup DATA2 was mounted SUCCESS: diskgroup DATA2 was dismounted SUCCESS: diskgroup DATA2 was mounted SUCCESS: diskgroup DATA2 was dismounted SUCCESS: diskgroup DATA2 was mounted SUCCESS: diskgroup DATA2 was dismounted Wed Jun 25 06:35:22 2014 Starting control autobackup Control autobackup written to DISK device handle '/backup/BAK/OMSMIT/cfc-453770213-20140625-01.ctl' Wed Jun 25 06:35:38 2014 SUCCESS: diskgroup DATA2 was mounted SUCCESS: diskgroup DATA2 was dismounted SUCCESS: diskgroup DATA2 was mounted SUCCESS: diskgroup DATA2 was dismounted Wed Jun 25 09:34:53 2014
According to metalink note : Why Archive, FRA Diskgroup Gets Mounted / Dismounted ? (Doc ID 603204.1)
This is 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 FLASHDG_NEW 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.
This alerts will be only seen for diskgroups that holds flash recovery area.
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.
Hope so u will find this post very useful 🙂
Cheers
Regards,
Adityanath
Categories: Administration
Adityanath's Oracle Blog 
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