Confusing output of btrfs fi df

[Stefan Malte Schumacher <s.schumacher@netcologne.de>]

Hello

Yesterday I created a btrfs-filesystem on two disk, using raid1 for
data and metadata. I then mounted it and rsynced several TB of data
onto it.

mkfs.btrfs -m raid1 -d raid1 /dev/sdf /dev/sdg

The command btrfs fi df /mnt/btrfs result in the following output:

Data, RAID1: total=2.64TiB, used=2.22TiB
Data, single: total=8.00MiB, used=0.00
System, RAID1: total=8.00MiB, used=380.00KiB
System, single: total=4.00MiB, used=0.00
Metadata, RAID1: total=4.00GiB, used=2.94GiB
Metadata, single: total=8.00MiB, used=0.00

I am a bit confused because of the "single"-entries. They are not
shown in the UseCases-example on the btrfs-website and I wonder if I
did something wrong.

I also would like to know if its possible to label a multi-disk
filesystem after creation. 

For your information, I am using Btrfs v3.12+20131125 and kernel
3.11.10-7 64bit. My distribution is an openSUSE 13.1.

Yours
Stefan