If the Server doesn’t have enough memory for all its segments, does it offload some to disk? What’s the offload directory? Or should I turn on swap + mmap, and let the OS deal with it? (Sorry I couldn’t find any documentation for that.)

Pinot always offloads segments to disk and uses mmap (lets OS deal with it). Only place where it needs enough memory is in real-time consuming segments (even in that case, its only for inverted index)

Awesome. Thanks!