Wikipedia
Mdadm
mdadm is a Linux utility used to manage and monitor software RAID devices.
The name is derived from the md (multiple device) device nodes it administers or manages, and it replaced a previous utility mdctl. The original name was "Mirror Disk", but was changed as more functions were added.
mdadm is free software maintained and copyrighted to Neil Brown of SUSE, and licensed under the terms of version 2 or later of the GNU General Public License.