Re: [PATCH 0/5] hybrid union filesystem prototype

[Miklos Szeredi]

On Mon, 30 Aug 2010, Neil Brown wrote:

> > You didn't mention one possibility: add limitations that prevent the
> > weird corner cases arising.  I believe this is the simplest option.
> 
> Val has been following that approach and asking if it is possible to make an
> NFS filesystem really-truly read-only. i.e. no changes.
> I don't believe it is.

Perhaps it doesn't matter.  The nasty cases can be prevented by just
disallowing local modification.  For the rest NFS will return ESTALE:
"though luck, why didn't you follow the rules?"

> > I think *notify will work correctly, every modificaton will be
> > notified on both the union fs and the upper fs.  But I haven't tested
> > this yet.
> 
> I tried.  It doesn't.
> To be precise:  directory changes like file creation (even creation of a file
> that already exists) get notified, but purely file-based event like modifying
> the contents of the file don't generate events back to the overlayfs
> directory.
> Because an open (for write) of a file is passed through to the upper
> filesystem, the notifications of modification through that open only go to the
> upper filesystem.

Ah, right.

> > > I think the way to fix this would involve the union-fs putting a
> > > notifier on the upper dir to match whatever is on the
> > > merged-dir.  However the filesystem currently isn't told when
> > > notifiers are attach to an inode.  I think it would be good to
> > > add an inode_operation which is called whenever the notifiers
> > > are changed.  This would also allow a networked filesystem to
> > > report notification if the protocol supported it.  Does that
> > > seem reasonable?

In that light this sounds entirely reasonable.

Thanks,
Miklos
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