That VDO thing looks cool. Though it seems you have to be careful about setting the compression ratio and dedup ratio such that the actually used space doesn’t get larger than the available space. In connection with UrBackup one can probably monitor the acutally used space and run cleanups with appropriate sizes if necessary.
In connection with XFS reflinking Xfs reflink support in soon to come v2.2.1? that would give the same features as btrfs except for the data integrity/scrubbing.