To convert a non-raid file system to a raid, add a device and run a balance filter that changes the chunk allocation profile.

Converting a btrfs file syst em

To convert an existing single device system, /dev/sd b1 in this case, into a two device, raid1 system in order to protect against a single disk failure, use the following commands:

# mount /dev/sdb1 /mnt
# btrfs device add /dev/sdc1 /mnt
# btrfs balance start -dconvert=raid1 -mconvert=raid1 /mnt

Important
If the metadata is not converted from the single-device default, it remains as DUP. This does not guarantee that copies of the block are on separate devices. If data is not converted it does not have any redundant copies at all.