currently upgrading a 9.1 server to 10.0. I have run into problems with GELI-based jails.
Starting jails: cannot start jail "mygelijail":
jail: mygelijail: mount: /usr/jails/mygelijail/proc: No such file or directory
/etc/rc.d/jail: WARNING: Per-jail configuration via jail_* variables is obsolete. Please consider to migrate to /etc/jail.conf.
Error: Could not start mygelijail.
You need to start it by hand.
I can manually attach the GELI image and mount it, there is a /proc directory. It did work flawlessly with 9.1.