Re: Optional switching off cow in overlayfs

[Jan Olszak <j.olszak@samsung.com>]

On 06/23/2015 03:23 PM, David Howells wrote:
> Jan Olszak <j.olszak@samsung.com> wrote:
>
>> Why not just say: "rename causes copy" and leave everything as it is now.
> That's another option - but your whole proposal was that we specifically don't
> copy up these files IIRC.
>

Yes, so let's modify the proposal:

Overlayfs checks xattr "trusted.overlay.cow" to switch COW off whenever 
it's possible.

1. This would be an optimization/convenience option, not a security option
2. By default overlayfs uses COW like it does now
3. trusted.overlay.cow=="n" means:
- Unlink, open, setattr etc. use the original inode from lowerdir
- Unmodifying operations also use inode from lowerdir
- Other operations (rename) may copy the file if it's required by the 
implementation of overlayfs

Thanks,
Jan