At the moment, we are running on two dedicated PHP servers and a dedicated Master and Slave MySQL server.
We ran into limits today, and haven't really reassessed our infrastructure in a while. So we are looking into this, to try to make it more efficient.
We do use a CDN, use Varnish for the front page. Varnish on the forums is a little trickier, because it affects user counts which everyone loves.
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