How to convert a btrfs file system in raid1 mode to raid0?

Solution 1:

btrfs balance start -dconvert=raid0 /

That's all you need to do. The system will busily move the existing data around to be raid0 (striped, no parity) and any further data will be written that way as well.

METAdata will still be written in duplicate. If you want to live EXTRA SUPER dangerously, feel free to tack on an -mconvert raid0 argument into the above command as well, then both data AND metadata on the array at / will be converted to raid0.

Solution 2:

You read the right manual; you do need to run balance after adding the drive in order to restripe your data across the new disk. It also happens to convert any DUP chunks to RAID1, and the default mkfs options use RAID1 or DUP for metadata depending on whether you have multiple disks or not. There is not currently a supported way to convert back, but there are some restriper patches floating around on the btrfs mailing list that eventually will allow this sort of thing.