'btrfs subvolume snapshot <src> <dest>' is a writable snapshot; with -r flag it's read only. A Btrfs snapshot is a subvolume with stuff already in it, basically

'btrfs subvolume snapshot <src> <dest>' is a writable snapshot; with -r flag it's read only. A Btrfs snapshot is a subvolume with stuff already in it, basically. In fact, all the inode numbers are initially identical in the original subvolume and the snapshot, and initially the snapshot is just a pointer. That's why they're cheap to create.