Bah, you want to have the cake and eat it too. Doing a full disk backup naturally creates a backup of the full disk, including "free space" (which is never really free). Anything else would be an inaccurate backup and prone to error. And anyway, the point is that you should never have free space. Just like with RAM, any free space on the disk is wasted space. It's much better to make use of it by creating snapshots, so all the free space is filled with backup data (except for certain overhead space so you could delete the oldest snapshot when you need to write more to the disk). So if you want to have a space-efficient backup, then you can use send/receive. And if you want a truly full backup of the disk that you can reliably restore, then use dd and make a full copy (and you can even compress it afterwards and possibly save some extra space).
Originally Posted by Ericg