$ lsblk
NAME   MAJ:MIN RM   SIZE RO TYPE MOUNTPOINT
sda      8:0    0 931.5G  0 disk
├─sda1   8:1    0    28G  0 part /
├─sda2   8:2    0     1K  0 part
├─sda5   8:5    0    11G  0 part [SWAP]
└─sda6   8:6    0 892.6G  0 part /home

$ sudo btrfs subvolume snapshot / Downloads/test
Create a snapshot of '/' in 'Downloads/test'
ERROR: cannot snapshot '/': Invalid cross-device link

$ sudo mount -t overlay -o lowerdir=/ overlay Downloads/test
mount: /home/ontoki/Downloads/test: wrong fs type, bad option, bad superblock on overlay, missing codepage or helper program, or other error.