> An important case is one in which you delete many files; with a unioning file system this will
> decrease the space needed to store differences. With a COW block device, used space will
It doesn't actually *have* to be the case. When linux adds the hooks needed by Flash drives to
notify the device about which blocks are free, then the COW block device can use those same hooks
to free space.
If I were a kernel developer I'd be very wary of the endless-tarpit of unionfs issues when a way to
make a COW block device probably efficient enough is right around the corner.