Hi all.  I'm trying to figuring out why my btrfs file system doesn't
show all the available space.  I currently have four 4TB drives set up
as a raid6 array, so I would expect to see a total available data size
slightly under 8TB (two drives for data + two drives for parity).  The
'btrfs fi df' command consistently shows a total size of around 3TB, and
says that space is almost completely full.  Here's my current system
information...

===
root@selene:~# uname -a
Linux selene.dhampton.net 3.19.0-32-generic #37~14.04.1-Ubuntu SMP Thu
Oct 22 09:41:40 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux
root@selene:~# btrfs --version
Btrfs v3.12
root@selene:~# btrfs fi show /video
Label: none  uuid: 74a4c4fa-9e83-465a-850d-cc089ecd00f6
	Total devices 4 FS bytes used 3.12TiB
	devid    1 size 3.64TiB used 1.58TiB path /dev/vdb
	devid    2 size 3.64TiB used 1.58TiB path /dev/vda
	devid    3 size 3.64TiB used 1.58TiB path /dev/vdc
	devid    4 size 3.64TiB used 1.58TiB path /dev/vdd

Btrfs v3.12
root@selene:~# btrfs fi df /video
Data, RAID6: total=3.15TiB, used=3.11TiB
System, RAID6: total=64.00MiB, used=352.00KiB
Metadata, RAID6: total=5.00GiB, used=3.73GiB
unknown, single: total=512.00MiB, used=1.07MiB
root@selene:~# df -h /video
Filesystem      Size  Used Avail Use% Mounted on
/dev/vda         15T  3.2T  8.3T  28% /video
===

I have tried issuing the command "btrfs filesystem resize
<id>:max /video" on each devid in the array, and also tried balancing
the array.  None of these commands changed the indication that the file
system is almost full.  I'm wondering if the problem is because this
file system began as a two drive raid1 array, and I later added the
other two drives and used the 'btrfs balance' command to convert to
raid6.  Any suggestions on what I can try to get the 'btrfs fi df'
command to show me more available space?  Did I forget a command when I
converted the raid1 array to raid6?

Alternatively, can I trust the numbers in the standard df command?  The
'used' number seems right but the 'avail' number seems high.

If i can provide any more information to help figure out what's
happening, please ask.

Thanks.

David