No idea. Sometime when it's not being hammered you can ask them ... it's not my instance.
And I don't know what my instance is running on ... I have a tech co-mod who deals with it. We have two rather grunty servers running the the cloud, that's all I know.
If you're genuinely interested, email me and I'll find out for you. Details in my profile.
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From their "about" page:
Hello World! Welcome to mastodon@bauhaus, our small niche in the fediverse.
This server originated @maschinenraum, a small hackspace in Weimar. It is currently administrated and moderated by a small team via @admin.
We run our own server in a local data centre just down the road, so there's no cloud here – however, that also means if it breaks, it breaks. And it will happen every now and then, and there are backups, and it's mostly fine, but don't expect 100% uptime – 99% will have to do.
We're part of the Chaos family surrounding the Chaos Computer Club, so you can meet us at events like the C3 and get to know your admins!