HT: Hardlinking failed (unknown error) -- Invalid cross-device


Does anybody know this type of message :

27/08/16 09:25  	DEBUG  	HT: Hardlinking failed (unkown error) Source="/media/backup/urbackup_btrfs/SV401/160708-2217/Fichiers/Stats.xls" Destination="/media/backup/urbackup_btrfs/SV401/160827-0016/Fichiers/stats.xls" -- Invalid cross-device link (code: 18)

We got a lot of those messages for many clients backups.
Storage is BTRFS on Debian 8, and yesterday i’ve run a defrag on the volume :

ionice -c idle btrfs filesystem defragment -f -t 32M -r /media/backup/urbackup_btrfs/

Maybe something goes wrong with this …