On Thu, Feb 13, 2014 at 9:28 AM, Miklos Szeredi <miklos@xxxxxxxxxx> wrote:
> On Thu, Feb 13, 2014 at 5:42 PM, David Howells <dhowells@xxxxxxxxxx> wrote:
>> Miklos Szeredi <miklos@xxxxxxxxxx> wrote:
>>
>>> Regarding whiteouts, I raised a couple of questions that nobody answered
>>> yet, so let me ask again.
>>>
>>> - If a filesystem containing whiteouts (fallthroughs, etc...) is mounted as
>>> not part of a union, how are these special entities represented to
>>> userspace?
>>
>> I would suggest that whiteouts appear as otherwise negative dentries and that
>> they don't appear in getdents().
>
> I'd argue that this is an administration nightmare. E.g. what if the
> a backup needs to be made of the rw layer?
>
> Will rmdir work normally in a directory containing whiteouts? Will
> the VFS take care of that, just like if it was part of a union? Or
> will it fail with ENOTEMPTY despite *appearing* empty?
>
> And zillion other problems related to the fact that things happen to a
> filesystem even when they do not appear to happen ("mv foo bar; mv bar
> foo" has side effects).
Are there any users of unions / overlays who will want to modify the
bottom layer after creating the top layer? I'm starting to think that
changing the bottom layer should require userspace to do a three-way
merge or something and explicitly decide what it wants to do.
--Andy
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