From LQWiki

mdadm is a tool to manage software RAID on linux.

To create a RAID-0 array at /dev/md0 consisting of /dev/sda1 and /dev/sdb1:

 # mdadm --create --verbose /dev/md0 --level=0 --raid-devices=2 /dev/sda1 /dev/sdb1
 mdadm: chunk size defaults to 64K
 mdadm: array /dev/md0 started.