Oracle Database Appliance – Safely usable ASM diskgroup size

Not long ago I got a warning from Oracle Enterprise Manager that the REDO diskgroup on one of our ODA’s exceeded the warning threshold of 75%. Looking at the number of database instances and configured online redo log sizes I couldn’t understand how this was possible while the ODA documentation states that diskgroup +REDO size is 97.3 GB ( 4 * 73 GB / 3 – high redudancy is used on the ODA diskgroups).

I know the divide by 3 is a rough calculation but I only had 39 GB on redo logs in the REDO diskgroup. Withing ASM the diskgroup showed that 158 GB was still free (FREE_MB column of v$asm_diskgroup) so after triple-mirrored leaving 52 GB, but then the column REQUIRED_MIRROR_FREE_MB came into sight which showed that 140 GB was required for mirroring.
ASM documentation is clear about the calculation of the required size for mirroring: The value is the total raw space for all of the disks in the two largest failure groups.

This means for the ODA that the actual “safely” usable REDO diskgroup size can be calculated with: total_mb – required_mirrror_free_mb = 280016 – 140008 = 140008 MB (raw size)  after tripple-mirrorring leaves (140008/3) 45.6 GB which differs by 51.7 GB with the value that Oracle notes in its documentation.

So here is a table with the “safely” usable diskgroup sizes:

Actual diskgroup size (safely usable)

diskgroup internal backup external backup
+DATA 1.41 TB 2.81 TB
+RECO 1.87 TB 0.46 TB
+REDO 45.6 GB 45.6 GB

Oracle documentation size

diskgroup internal backup external backup
+DATA 1.6 TB 3.2 TB
+RECO 2.4 TB 0.8 TB
+REDO 97.3 GB 97.3 GB

Of course you can use more space, effectively resulting in a negative value for USABLE_FILE_MB, but the question if you want this to happen!

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