The mdadm tool manages MD devices, aka Linux Software RAID
# emerge mdadm
However, in most cases the package is pulled in during the kernel installation as a dependency when a RAID is present.
Configuring mdadm is done via the
/etc/mdadm.conf file. By default, after installation this file is entirely commented out. One may remove the comment hash
# in front of the line that most suits the RAID in use and alter the line to point to the correct devices, or add a new line, based on the examples given.
If you're upgrading an existing system to Funtoo, backup the existing
/etc/mdadm.conf file to a secure location and copy it back prior to rebooting into the new install.
mdadm can be very tricky to administer for novices. Install Webmin and use its webinterface to create and use a new RAID.