Using RAID with CoreOS (btrfs; RAID0, RAID1)

reference.md

btrfs adding devices and rebalancing: https://btrfs.wiki.kernel.org/index.php/Using_Btrfs_with_Multiple_Devices#Adding_new_devices

btrfs rebalance command: http://askubuntu.com/a/412307 essence of that: don't use -mconvert=raid0 since that would mean "use raid0 for metadata also" which is not advisable

running fdisk in non-interactive mode: http://xmodulo.com/how-to-run-fdisk-in-non-interactive-batch-mode.html

setup-raid-coreos.sh

#!/bin/sh

# replace with whatever your additional device is called
# /dev/sda should be where your CoreOS install lives right now
newdevice="/dev/sdb"
# 0 or 1
raidtype="0"

# make sure to download the single-partition.layout file as well

# create single primary partition on $newdevice (you could also run fdisk interactively, choose p command and use defaults it suggests)
sudo sfdisk $newdevice < single-partition.layout

# mount current root; I suppose /dev/sda9 would work as well?
sudo mount /dev/disk/by-label/ROOT /mnt

# add btrfs device
sudo btrfs device add /dev/sdb1 /mnt

# convert to raid of given type
sudo btrfs balance start -dconvert=raid$raidtype /mnt

# should take a bit

# verify things have worked
sudo btrfs fi df /mnt
# should show something like...
#Data, RAID0: total=6.00GiB, used=303.25MiB
#System, DUP: total=8.00MiB, used=16.00KiB
#System, single: total=4.00MiB, used=0.00
#Metadata, DUP: total=108.06MiB, used=18.78MiB
#Metadata, single: total=8.00MiB, used=0.00
#unknown, single: total=16.00MiB, used=0.00

sudo umount /mnt

single-partition.layout

# partition table of /dev/sdb
unit: sectors

/dev/sdb1 : start=     2048, size=937701040, Id=83
/dev/sdb2 : start=        0, size=        0, Id= 0
/dev/sdb3 : start=        0, size=        0, Id= 0
/dev/sdb4 : start=        0, size=        0, Id= 0

Here's another piece of documentation I hadn't seen: https://coreos.com/docs/cluster-management/debugging/btrfs-troubleshooting/#adding-a-new-physical-disk
Maybe it's even easier and the "create partition" step isn't even required. That hadn't worked for me when trying to add the disk to btrfs.