I am trying to figure out what kind of hardware I need to run a phpBB 3 server for a fairly large community. When I say hardware, I mean CPU, RAM, and IO specs. When I say fairly large community, I mean a community of about 30,000 people that averages, let's say, 1,000 posts per day, with about 100 users active per day.
I have seen people discussing having 1 GB RAM for a phpBB 2 community of 5,000 users, but that is, of course, a bit old. Any ideas?
100 users active is not... game ending... If you want consistent performance take your budget, buy the biggest you can get and grow into it. (This is generally bad advice but I think it applies here...).
Generally when speccing server hardware one does bench marks and finds the hardware they need and buys that hardware at the cheapest price they can find. If you don't have that ability... it's probably best just to throw top dollar at hardware and then if that is not enough work on optimizations like caching / etc.
It's impossible to answer your question directly because:
The important things are:
1) Your users habits
2) The effect those habits have on server load / capacity / etc (EG: Reading a page from cache is easier on the server than uploading 50MB).