We have an on-premise portal installation. Recently we had a couple of instances of running out of disk space, the database went into read-only mode, and we could not place the Postgresql database back into read/write mode. By rolling back to a backup we were able to restart the database in read-write mode.
We have decided to move to a highly available portal but as an interim step we want to investigate using SQL Server (our enterprise DBMS) instead of Postgresql as our portal DBMS. Is this possible?
There is no current supported or documented workflow to accomplish replacing Postgresql with SQL in regards to Portal's underlying database. The underlying database is intended to function without direct access. If you think users would benefit from having this option, you could possibly log a suggestion under the Ideas page?
You could also look in to different monitoring options regarding disk space. ArcGIS Monitor could be configured to email you if low disk space is detected. Performance monitor and other tools could most likely accomplish something similar.
I assume it was the ArcGIS Data Store that went into read-only vs the portal database, because there is no provision for the portal database to go into read-only automatically. There is a concept of read-only for Enterprise, but this is admin-initiated. The ArcGIS Data Store will go into read-only when disk space is running out, which I assume is what you saw:
There's a utility for setting the Data Store to read-write so you don't need to roll back to a backup, but monitoring disk space, (as well as other parts of the software) should certainly be something you should look into.