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

[Lawrence Stewart]

Hi Jeremy,

On 03/23/11 17:16, Jeremy Chadwick wrote:
> 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.
> 
> Please see mount_unionfs(8), specifically the explanations of "lower"
> and "upper" layers, along with EROFS (errno 30, "Read-only filesystem").

I don't think 'mount -o union' is the same thing as doing a unionfs
mount. Earlier today I was playing with unionfs and it worked perfectly
for the scenario I describe above. 'mount -o union' seems to do
something similar, but subtly different to unionfs.

The key difference from the UI perspective is that unionfs can be used
to mount a directory on top of another directory, whereas using 'mount
-o union' requires you to mount a device with an fs on it on top of a
directory.

The semantics of 'mount -o union' are far more useful for my situation
that unionfs, so I'm keen to try and see if I can get it working
properly, which I suspect will involve a kernel patch.

Cheers,
Lawrence