Problem: If you setup a free freebsd webserver with two disks on same size the ufs2 filesystem fails the gmirror raid1 if the server lost power. The problem is that open files are destroyed.
Solution: If you setup a new freebsd system create for every partition a extra partition (for journal) install the gjournal AFTER GMIRROR!! and make some power failure tests. After powerfailure the system must reboot and rebuild its mirror and should run online after rebuild without any problem.