January 14, 2019

Enterprise Manager 13c - 'Tablespace Space Used %' metric sends false alarms

Situation is ...

Metric 'Tablespace Space Used %' is used to monitor the free space of the tablespaces. Sometimes You'll get an alarm that the Tablespace has reached it's critical value. But, the tablespace's data files are auto extensible and far away from the max size and the files are located on an ACFS which knows also auto-extensibility - or better said - an auto-resize feature. There is also plenty of space left on the underlying ASM. Nevertheless, FS free space is currently near to 100% and the current free space in the tablespace is lower than the critical value. Obviously a false alarm, isn't it?

Cause ...

for that behaviour is, that Oracle has changed the method to calculate a tablespace's free space for metric 'Tablespace Space Used %' with release 13c. New is that the metric takes the available disk space into account for its calculations. And this is correct , because no more space left on the disk means finally that a tablespace cannot auto-extend anymore. But with ACFS and the auto resize functionality, this shouldn't be a problem at all - ACFS will auto resize the FS if necessary.

Unfortunately, Oracle Support has no fix for this false alarm at this very moment. MOS says that view DBA_TABLESPACE_USAGE_METRICS has a problem to calculate the metric's base information correctly. Well, that might be a possible reason, but IMHO, the values of my particular database / tablespaces / datafiles are fine.

Solution / Workaround

Simply set the value of the auto resize parameter for the ACFS filesystem in question to an appropriate value other than the default of 20G, to avoid the false alarm problem (until the ASM runs full and ACFS cannot auto resize):
"acfsutil size -a <Size>G <FSName>"

Example:
acfsutil size -a 100G /u02/oradata/TESTDB
acfsutil size: ACFS-03642: successfully updated auto-resize settings


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