mount -o union doesn't allow changes to sub directories?

[Kostik Belousov]

On Wed, Mar 23, 2011 at 04:48:21PM +1100, Lawrence Stewart wrote:
> Hi again,
> 
> If I run something like:
> 
> mount -o union /dev/md0 /etc
> 
> where md0 is a memory backed fs device, and /etc is fully populated but
> mounted RO off a CD, I can touch a new file in /etc, but can't in any
> subdir of etc (fails with "Read-only filesystem" reported).
> 
> As far as I can tell this is a bug, but wanted to check if I'm
> misunderstanding how the "-o union" mount option is supposed to work.

No, this is how the union mount work. Union is performed only
on the mount point directory, for the names that are absent in
the directory.
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