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> If I need more nines of uptime than a single system could provide (and think about the MTBF numbers for the various parts in a computer, a single machine is not going to give you even three nines of uptime).

Three nines is 8 hours and 45 minutes. In my experience with quality hardware, at a facility with onsite spares, that gives you two to three hardware faults, assuming your software and network is perfect, which I think is a fair assumption :)

Definitely the hardest problem when you switch to a distributed database is you have to contemplate failure. When your SPOF database is down, it's easy to handle, nothing works. When your distributed database partially fails, chances are you have a weird UX problem.