Matthew Dillon sent out notice of his recent journaling work, which now actually works for mirroring a partition. He sent another update, with more details.

For those readers who didn’t read the original description of journaling, this is different than the usual “fast reboot” version of journaling that other operating systems have. This scheme is intended to allow for rollback to arbitrary spots in disk history, or mirroring of data to other drives or other network locations.