Today I was monitoring Fopnu's disk usage and I've found it absolutely destroys disk performance. It appears to constantly rehash files at the moment. On my server I share from two 3TB drives and the constant reading made them impossible to be used for writes. I cannot download anything beyond a trickle while Fopnu is hashing.
As soon as it's done hashing my 4-5TB of content, it straight away begins again from the start. I believe this to be a bug of course, not intended behaviour.