Btrfs RAID

With Btrfs you no longer need to use mdadm to create mirrored volumes or to create RAIDs. This is already included in btrfs, and very easy to use. There are even advanced features bult in:

Add devices to the volume This will integrate a device into the mounted volume
root # btrfs device add [/dev/sdz7] / COPY TO CLIPBOARD

Remove devices from the volume. This will not delete any data, but remove the device from the volume. Bevorehand all data will be copied to the remaining devices of the volume.
root # btrfs device delete [/dev/sdz8] / COPY TO CLIPBOARD

Use devices with different sizes in one volume
Switch the volume between RAID levels
Convert data to different RAID levels
Do this while the volume is mounted and being used

Raid Levels

There’s some similarity with traditional RAID levels, but this could be confusing to users familiar with the traditional meaning. Due to the similarity, the RAID terminology is widely used in the documentation (of btrfs). See RAID@wikipedia,RAID profiles@btrfs.readthedocs