quoted

9.08GB + 992.48MB*2 == 11.02GB
10.85GB + 518MB*2 == 11.86GB
That's nearly a GB smaller.

That, too; I missed the "DUP". Not quite as pronounced as in my
calculations, then, but still a significant enough difference.

great. now were down to 7-8%

just FYI:

ive retested with max_inline=0 but with leafsize=64K this time:
# mkfs.btrfs -l 64K -L root /dev/sda2
...
real    4m45.878s
user    1m44.730s
sys     1m7.226s

thats 2% slower for this one test (no big deal really)

# btrfs filesystem show
Label: 'root'  uuid: dd2951da-2529-4320-a952-e692ea5bdbc3
        Total devices 1 FS bytes used 11.37GB
        devid    1 size 232.63GB used 13.04GB path /dev/sda2

Btrfs v0.20-rc1

# btrfs filesystem df /mnt/point/
Data: total=11.01GB, used=10.89GB
System, DUP: total=8.00MB, used=64.00KB
System: total=4.00MB, used=0.00
Metadata, DUP: total=1.00GB, used=487.94MB
Metadata: total=8.00MB, used=0.00

(1024*10.89 + 2*487.94) / 1024 = 11.84GiB

still around 7-8%

now lets see what everyday use with
max_inline=0 and leafsize=64K
feels like :)

Ahmet