if we have not enough space on /backup (sym for /var/backup_store/snaps) (higher than 80%?)
AND we have enough space on / (lower then 80%?)

-> ADD NEW SPACE:

DISK SIZE -> 100G+ID
BUT maximal (freespace of / - freespace of /var/backup_store/snaps) -10G *90%

cd /var/backup_store/imgs
fallocate -l 105G 5.img
losetup -f 5.img
losetup | grep 5.img => find out /dev/loop?
btrfs device add /dev/loop5 /var/backup_store/snaps/