Btrfs devel - Jun 2014 - Means for userspace to know the used space (not allocated space) in a device of a multi-device filesystem

btrfs filesystem show displays allocated space, but how can userspace
know how much space is used on each device?

In #btrfs channel on freenode.net, it was answered that the
TREE_SEARCH ioctl can be used to inspect the data structures, and that
btrfs-gui does this (thanks @darkling): http://carfax.org.uk/btrfs-gui

However, it would be good if third-party applications had means to get
this information from btrfs, without reimplementing what btrfs-gui
does.

For example, partitioning tools would be able to show an accurate
number, rather than 0 or an estimate.

Please, implement something in btrfs that returns this information.

Thanks
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