OverlayFS with copy_down?

Hi All,

I'd like to use OverlayFS for a project. This allows one to stack multiple filesystems together.

However what I'd like to do is periodically copy data from the higher layers to the lower layers. OverlayFS does not support this whilst mounted so I just wanted to ask if there is any other stackable filesystems that will allow me to do this? Unmounting to do the copy-down is not practical in this case.

I'm aware of bcache but I'm looking for a simple file-level based system rather than block.

Thanks,