-J doesn't do anything since

https://github.com/freenas/freenas/commit/3d1adc8dae43d923e6225f998a4b7e99ae25650d
but confuses people (e.g. http://forums.freenas.org/threads/guide-how-to-build-freenas-part-2-install-and-build-freenas.50/page-5#post-83467).