I can run the shell command:
unionfs-fuse /changedata=RW:/immutedata=RO -o cow /data
This mounts the filesystem exactly how I want to. Now I need to incorporate it into the automounter so that it's remounted upon demand.
What I tried in /etc/auto.misc:
/data -fstype=fuse,cow /changedata=RW:/immutedata=RO
What automount --debug -f /etc/auto.master says when I do ls /data:
handle_packet: type = 5
handle_packet_missing_direct: token 19, name /data, request pid 6063
attempting to mount entry /data
lookup_mount: lookup(file): looking up /data
lookup_mount: lookup(file): /data -> -fstype=fuse,cow /changedata=RW:/immutedata=RO
parse_mount: parse(sun): expanded entry: -fstype=fuse,cow /changedata=RW:/immutedata=RO
parse_mount: parse(sun): gathered options: fstype=fuse,cow
parse_mount: parse(sun): dequote("/changedata=RW:/immutedata=RO") -> /changedata=RW:/immutedata=RO
parse_mapent: parse(sun): gathered options: fstype=fuse,cow
parse(sun): invalid location
dev_ioctl_send_fail: token = 19
failed to mount /data
Google reveals little. Man pages are rather empty on the incorporation of this filesystem.
Maybe I should just put it in /etc/fstab and ask users to remount?