Comment by tachion

Actually, you don't need another copy of userland. You merely need a dedicated space for the jailed OS (assuming we're talking OS-style jails, not process-style ones) to write the things OS writes to (devices, logs, etc.). You could get away with null mounting your host filesystem and then mounting writeable space on top of it. Once I've had read-only mounts for my jails, for funsies.