The total downtime was 2 bouts of about 3 minutes each, for each of the upgrades to first MariaDB 10.1 then 10.2.
The upgrade to PostgreSQL 10 from version 9 had no downtime, because we only use PostgreSQL for logging, and we’ve engineered our system such that a downtime for PostgreSQL doesn’t result in an outage of the Simplero application.
All went well. When MariaDB 10.3 and PostgreSQL 11 are available, we’ll probably upgrade to those sooner.