Re: understanding mount_union

[Eric Haszlakiewicz <erh%nimenees.com@localhost>]

On May 13, 2014 6:59:56 PM EDT, Patrick Welche <prlw1%cam.ac.uk@localhost> 
wrote:
>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?

Yep.

>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

I would expect that too, and in fact that's what I see on my system (NetBSD 
6.1.3).

Is there still no file in /var/upper after unmounting the union mount?
Maybe something is getting cached, or it's broken in -current?

># touch /var/upper/another
># ls /var/upper
>another
># ls /var/lower
>another file
>
>I would not have expected to see another in /var/lower

Of course you would, since the kernel is causing anything in /var/upper to 
appear at the /var/lower mount point.
However, I think it might be "not recommended" to go mucking around in 
/var/upper while it's in use as part of a union mount.
On the other hand, if it's present in /var/lower after unmounting, then 
something is very wrong.

Eric