Real time volume free space

["Dan" <dan@xxxxxxxx>]

I've been trying to figure out how to determine the free space of a real
time volume.  It seems xfs_db might hold the key but no luck so far... 
Simply running df -h tells me the free space of the meta-data drive only
which basically doesn't change even after Terabyte data collections.

I created the filesystem like this:

mkfs.xfs -r rtdev=/dev/md0 /dev/sda

How would I tell how much space is left on /dev/md0?

Thank you,

Dan