To chime in here on this question specifically:
One architecture that I’m thinking won’t be uncommon for folks with medium sized systems would be one with the Historian separate from a main server, for a lot of the reasons you’re describing. This provides the benefits of QuestDB, doesn’t require a SQL database, and takes advantage of store & forward and the Gateway Network’s ability to stream historical data.
In that image, the Ignition on the left would be a standard Gateway with full visualization and device connections, and would have a Gateway Network connection to a dedicated Ignition Historian server / VM / container (pictured on the right), which would be using the Internal Historian - QuestDB. You could have several Ignition Gateways all share the same Ignition Historian server.
I think this architecture will increase in popularity after we ship with redundancy support for the Internal Historian - QuestDB, which we’re aiming to have out within a few months from initial 8.3.0 release. The initial release won’t have redundancy support, so standard IT tools for file backups are the main method of resiliency until we do release that support.
We’re doing a lot of work on our Historian Guide for 8.3, so we’ll try to package information like this and other related things in a nice way for the whole community.