Expanding BTRFS Pool

To create the BTRFS pool:

sudo mkfs.btrfs /dev/nvme --label mofp0
sudo mkdir -p /mnt/mofp0
sudo mount LABEL=mofp0 /mnt/mofp0

Then modify /etc/fstab to mount the pool on boot:

LABEL=mofp0 /mnt/mofp0 btrfs defaults 0 0

To add a new device to the existing BTRFS pool:

sudo btrfs filesystem show

Label: 'drive0_wd_2tb'  uuid: 4451c98c-4577-43ab-b4c2-59832905d010
        Total devices 2 FS bytes used 2.37TiB
        devid    1 size 1.82TiB used 1.21TiB path /dev/nvme2n1
        devid    2 size 1.82TiB used 1.21TiB path /dev/nvme1n1

sudo btrfs device add /dev/nvme /mnt/mofp0
sudo btrfs balance start /mnt/mofp0 --bg # run in the background

WARNING:

        Full balance without filters requested. This operation is very
        intense and takes potentially very long. It is recommended to
        use the balance filters to narrow down the scope of balance.
        Use 'btrfs balance start --full-balance' option to skip this
        warning. The operation will start in 10 seconds.
        Use Ctrl-C to stop it.
10 9 8 7 6 5 4 3 2 1
Starting balance without any filters.

sudo btrfs device stats /mnt/mofp0