RAID-1 refuses to balance large drive

[Brad Templeton <bradtem@gmail.com>]

I have a RAID 1, and was running a bit low, so replaced a 2TB drive with
a 6TB.  The other drives are a 3TB and a 4TB.    After switching the
drive, I did a balance and ... essentially nothing changed.  It did not
balance clusters over to the 6TB drive off of the other 2 drives.  I
found it odd, and wondered if it would do it as needed, but as time went
on, the filesys got full for real.

Making inquiries on the IRC channel, it was suggested perhaps the drives
were too full for a balance, but they had at least 50gb free I would
estimate, when I swapped.    As a test, I added a 4th drive, a spare
20gb partition and did a balance.  The balance did indeed balance the 3
small drives, so they now each have 6gb unallocated, but the big drive
remained unchanged.   The balance reported it operated on almost all the
clusters, though.

Linux kernel 4.2.0 (Ubuntu Wiley)

Label: 'butter'  uuid: a91755d4-87d8-4acd-ae08-c11e7f1f5438
        Total devices 4 FS bytes used 3.88TiB
        devid    1 size 3.62TiB used 3.62TiB path /dev/sdi2
        devid    2 size 2.73TiB used 2.72TiB path /dev/sdh
        devid    3 size 5.43TiB used 1.42TiB path /dev/sdg2
        devid    4 size 20.00GiB used 14.00GiB path /dev/sda1

btrfs fi usage /local

Overall:
    Device size:                  11.81TiB
    Device allocated:              7.77TiB
    Device unallocated:            4.04TiB
    Device missing:                  0.00B
    Used:                          7.76TiB
    Free (estimated):              2.02TiB      (min: 2.02TiB)
    Data ratio:                       2.00
    Metadata ratio:                   2.00
    Global reserve:              512.00MiB      (used: 0.00B)

Data,RAID1: Size:3.87TiB, Used:3.87TiB
   /dev/sda1      14.00GiB
   /dev/sdg2       1.41TiB
   /dev/sdh        2.72TiB
   /dev/sdi2       3.61TiB

Metadata,RAID1: Size:11.00GiB, Used:9.79GiB
   /dev/sdg2       5.00GiB
   /dev/sdh        7.00GiB
   /dev/sdi2      10.00GiB

System,RAID1: Size:32.00MiB, Used:572.00KiB
   /dev/sdg2      32.00MiB
   /dev/sdi2      32.00MiB

Unallocated:
   /dev/sda1       6.00GiB
   /dev/sdg2       4.02TiB
   /dev/sdh        5.52GiB
   /dev/sdi2       7.36GiB

----------------------
btrfs fi df /local
Data, RAID1: total=3.87TiB, used=3.87TiB
System, RAID1: total=32.00MiB, used=572.00KiB
Metadata, RAID1: total=11.00GiB, used=9.79GiB
GlobalReserve, single: total=512.00MiB, used=0.00B

I would have presumed that a balance would take blocks found on both the
3TB and 4TB, and move one of them over to the 6TB until all had 1.3TB of
unallocated space.  But this does not happen.  Any clues on how to make
it happen?