The things started to go from bad to worse when work_mem parameter has not been set up correctly.
Some queries that requires a large amount of memory to process started to use disk.
Once the autoscaling kicked in, even if we would have realized about it, wouldn't have helped as you are locked out of the system.
The auto-scaling event was triggered earlier than our alerts for low disk available.
What I don't know yet, is that we have been moving services for multiple days, and the service that required the work_mem parameter was in production in that AWS for 48 hours before started to use disk rather than memory to process the SQL Queries.
