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Re: understanding mount_union

On May 13, 2014 6:59:56 PM EDT, Patrick Welche <> 
>I thought that one use of mount_union was to be able to layer
>filesystem over a read/only filesystem, such that writing would
>create a file in the upper filesystem, and reading would read an
>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
>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 

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
># 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.


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