On Tue, 22 Aug 2017 17:45:37 +0200
Ulli Horlacher [off-list ref] wrote:

In perl I have now:

$root = $volume;
while (`btrfs subvolume show "$root" 2>/dev/null` !~ /toplevel subvolume/) {
  $root = dirname($root);
  last if $root eq '/';
}

If you are okay with rolling your own solutions like this, take a look at
"btrfs filesystem usage <path>". It will print the blockdevice used for
mounting the base FS. From that you can find the mountpoint via /proc/mounts.

Performance-wise it seems to work instantly on an almost full 2TB FS.

-- 
With respect,
Roman