No available diskspace - but admin page reports 1 tb free

I had this filled up disc deleted about 2TB then had disk full messages. There is a thread which also describes this issue and the solution but i cannot find it

Thanks to mdgm for the actual solution

You have to do a balance. This is possible through Frontview" System -Settings- Volume Settings

I did it through SSH as I did not seem to fix it this way and this is my cheat sheet
1) Get % space used on NAS

	# df - h data

Result similar to /devNNN X.XTb Y.YTb ZZZGb A%
2) Get space used on NAS devices

	# btrfs fi show

optional

	# btrfs fi df /data

3) run balance

	# btrfs balance start /data -dusage=%

where % is A% from step 1, only use number do not put in % sign
window will hang till finished
4) follow progress in second instance of putty with

	# btrfs balance status -v  /data

where -v is optional

5) when finished check using step 2

It seem that the space despite being "Free" is still allocated as used, a balance "frees" up this space again