Dan Blazejewski posted on Tue, 16 Feb 2016 15:20:12 -0500 as excerpted:

A little background: I started using BTRFS over a year ago, in RAID 1
with mixed size drives. A few months ago, I started replacing the disks
with 4 TB drives, and eventually switched over to RAID 6. I am currently
running a 6x4TB RAID6 drive configuration, which should give me ~14.5 TB
usable, but I'm only getting around 11.

The weird thing is that It seems to completely fill 4/6 of the disks,
while leaving lots of space free on 2 of the disks. I've tried full
filesystem balances, yet the problem continues.

# btrfs fi show

Label: none  uuid: 78733087-d597-4301-8efa-8e1df800b108
        Total devices 6 FS bytes used 11.59TiB
        devid    1 size 3.64TiB used 3.64TiB path /dev/sdd
        devid    2 size 3.64TiB used 3.64TiB path /dev/sdg
        devid    3 size 3.64TiB used 3.64TiB path /dev/sdf
        devid    5 size 3.64TiB used 2.92TiB path /dev/sda
        devid    6 size 3.64TiB used 1.48TiB path /dev/sdb
        devid    7 size 3.64TiB used 3.64TiB path /dev/sdc

btrfs-progs v4.2.3



# btrfs fi df /mnt/data

Data, RAID6: total=11.67TiB, used=11.58TiB
System, RAID6: total=64.00MiB, used=1.70MiB
Metadata, RAID6: total=15.58GiB, used=13.89GiB
GlobalReserve, single: total=512.00MiB, used=0.00B

# btrfs fi usage /mnt/data

WARNING: RAID56 detected, not implemented

Your btrfs-progs is old and I don't see any indication of kernel version 
at all, but I'll guess it's old as well.  Particularly for raid56 mode, 
which still isn't to the maturity level of the rest of btrfs, using 
current kernel and btrfs-progs is *very* strongly recommended.

Among other things, current userspace 4.4 btrfs fi usage should support 
raid56 mode properly, now.  Also, with newer userspace and kernel, btrfs 
balance supports the stripes= filter, which appears to be what you're 
looking for, to rebalance to full-width stripes anything that's not yet 
full width, thereby evening out your usage.

A full balance /should/ do it as well, I believe, but with raid56 support 
still not yet at the maturity level of btrfs in general, it's likely your 
version is old and buggy in that regard.

-- 
Duncan - List replies preferred.   No HTML msgs.
"Every nonfree program has a lord, a master --
and if you use the program, he is your master."  Richard Stallman