PGN Mirror. There will be dups, dirty data, errors, GM draws etc -- the data will probably need to be post-processed, filtered, deduped etc.
In the news:
The first thing to do is get a lot of game data. This proved more difficult than I thought it would be, but after some looking around online I found a git repository on GitHub from rozim that had plenty of games. I used this to compile a set of 3.46GB of data, which is about twice what Tom used in his test. The next step is to get all that data into our pipeline