If I’m misunderstanding this - or plain doing something wrong, please feel free to correct me 
I’m testing Rockstor as an alternative to ZFSonLinux - which, despite what the zfs zealots will tell you, has a lot of drawbacks for home use (which I won’t get into here.)
I have everything installed on a KVM VM, with 6 20GB Virtual Disks for storage in a ‘RAID10’ setup. I created 5 “shares” which I believe are subvols?? I moved 12GB of data to one of the “shares” and when I look on the dashboard - it tells me there is 0% used… If this is normal behaviour - well - there is no point in even bothering with Rockstor which is a shame. a Storage appliance - by default - must show the amount of space used in a given share, no??
Anyhow, I browsed the forums and didn’t see anything about this - so either I’m doing something wrong (likely) or nobody has any idea how much space they are using in their data-shares (incredibly unlikely)