@martin @hoper There's nothing magical :) it's basically having 2 btrfs partitionned drives, making a snapshot of a subvolume and sending that read-only snapshot from one to the other disk. That prevent us from the use a of a tool like rsync as there's an option to send only the increment when you already have subvolumes on the other drive. Based on : https://btrfs.wiki.kernel.org/index.php/Incremental_Backup