I've install Ubuntu 11.10 with btrfs as the / file system (was a bit of a mess to do, I'll explain if people are interested) so I can expand the primary file system into the second drive on the system(*).

After installing the system, I ran btrfs device add /dev/sdb1 / and it added the new device and expanded the file system on to it, and all was good. But according to the (wrong) manual I was reading, I also had to run btrfs filesystem balance and this apparently converted my filesystem to "raid1" mode so everything is stored redundantly twice - once on each drive, and I can only use 50% of my total capacity:

$ btrfs filesystem df /

Data, RAID0: total=78.00GB, used=41.57GB

System, RAID1: total=8.00MB, used=16.00KB

System: total=4.00MB, used=0.00

Metadata, RAID1: total=3.75GB, used=355.06MB

Its a nice feature, but I was kind of wanting to use "raid0" (stripping). I've tried to remove the new device so I can re-add it, but when I try that I get an error and syslog has this:

btrfs: unable to go below two devices on raid1

So my question is: how can I convert my filesystem back to raid0 so I can use the total space of both disks?

(*) Like can be done with LVM, but with btrfs you can host multiple "partitions" on the same "file system" and space is allocated dynamically where you need it - unlike in LVM.