ERROR: error during balancing '.' - No space left on device

[Marc MERLIN <marc@merlins.org>]

legolas:/mnt/btrfs_pool2# btrfs balance .
ERROR: error during balancing '.' - No space left on device
There may be more info in syslog - try dmesg | tail
[ 8454.159635] BTRFS info (device dm-1): relocating block group 288329039872 flags 1
[ 8590.167294] BTRFS info (device dm-1): relocating block group 232494465024 flags 1
[ 9200.801177] BTRFS info (device dm-1): relocating block group 85928706048 flags 1
[ 9533.830623] BTRFS info (device dm-1): 824 enospc errors during balance

But:
legolas:/mnt/btrfs_pool2# btrfs fi show `pwd`
Label: btrfs_pool2  uuid: 6afd4707-876c-46d6-9de2-21c4085b7bed
	Total devices 1 FS bytes used 646.41GiB
	devid    1 size 820.45GiB used 820.45GiB path /dev/mapper/disk2
Btrfs v3.12
legolas:/mnt/btrfs_pool2# 
Data, single: total=800.42GiB, used=636.91GiB
System, DUP: total=8.00MiB, used=92.00KiB
System, single: total=4.00MiB, used=0.00
Metadata, DUP: total=10.00GiB, used=9.50GiB
Metadata, single: total=8.00MiB, used=0.00

I can't see how I'm full, and now that I can't run balance to fix
things, this is making things worse.
Kernel is 3.14.

What am I missing?

Thanks,
Marc
-- 
"A mouse is a device used to point at the xterm you want to type in" - A.S.R.
Microsoft is to operating systems ....
                                      .... what McDonalds is to gourmet cooking
Home page: http://marc.merlins.org/                         | PGP 1024R/763BE901