understanding mount_union

[Patrick Welche <prlw1%cam.ac.uk@localhost>]

I thought that one use of mount_union was to be able to layer read/write
filesystem over a read/only filesystem, such that writing would
create a file in the upper filesystem, and reading would read an unchanged
file from the lower one. Is this correct?

A quick experiment on -current/amd64:

# mkdir /var/upper
# mkdir /var/lower
# touch /var/lower/file
# mount_union /var/upper /var/lower
# mount
...
<above>:/var/upper on /var/lower type union (local)
# ls /var/upper
# ls /var/lower
file

I would have expected to see file in /var/upper

# touch /var/upper/another
# ls /var/upper
another
# ls /var/lower
another file

I would not have expected to see another in /var/lower

How are union mounts meant to work?

Cheers,

Patrick