ODA -- creating External Redundancy Diskgroup , ORA-15018 and ORA-15072 // _disable_appliance_check parameter & appliance.mode attirbute

Recently did a POC with an ODA X6 machine. It was a "ODA X6-2 M" and the disk capacity made available with the standard deployment was not sufficient to store a big reporting database.

So , I decided to reconfigure the ASM Diskgroups manually to gain some extra space in the ODA storage.

What I did was the following;

I dropped te RECO diskgroup and planned to use the 2 free disks (as the result of dropping RECO diskgroup) to recreate a new disk group named DATA2 using external redundancy.
When I dropped RECO , 2 NVMe disks(partitions) become available . (RECO was build on top of 2 disks-normal redundancy diskgroup , not counting the quorum..)
So I tried to create a new diskgroup named DATA2 with external redundancy using these 2 NVMe disks.
I used "asmca" for doing this.
asmca could see the disks as candidates (actually FORMERS), but it could not create a external redundancy group and encountered the following errors;

ORA-15018: diskgroup cannot be created

ORA-15072: command requires at least 1 failure groups, discovered only 0

Then I used sqlplus /as sysasm to do the same thing, but sqlplus encountered the same errors above, as well.

These error were not expected, so I thought that the configuration of this machine was somewhat incompatible with the external redundancy setting. (it was an appliance, an enginnered system...)

Note that, I knew that external redundancy was not supported with enginnered systems, but I tried to find the reason behind these errors.

Disks were seen, discovered but could not be used to create external redundancy disk groups...

Anyways, I could proceed my POC by scattering the db files to the RECO and DATA diskgroups and POC was succeded. 

However, I was still curious about  the reason behind those errors.
Since the POC was over, I had no ODA machine to test it and find the cause and solution for it.

Then I raised this question to the Oracle Community. The answer came from Viacheslav Leichinsky. 

"When the hidden parameter _disable_appliance_check is set to TRUE and ASM attribute 'appliance.mode' is set to FALSE, the external diskgroup can be created in ODA environments."

Example:

ALTER DISKGROUP DATA SET ATTRIBUTE 'appliance.mode'='FALSE';

SQL> CREATE DISKGROUP REDA EXTERNAL REDUNDANCY DISK
'/dev/mapper/SSD_E0_S12_133243434p1' NAME SSD_E0_S12_133243434P1
ATTRIBUTE 'compatible.asm'='12.1.0.2', 'compatible.rdbms'='12.1.0.2','sector_size'='512','AU_SIZE'='4M','compatible.advm'='12.1.0.2';
Diskgroup created

I found this story interesting, that's why I m sharing it with you.
It might come in handy one day.