If you have a RAID in your machine, the kernel installation will pull in the mdadm tool as a dependency. It is important to edit the /etc/mdadm.conf file prior to rebooting the machine so the RAID is properly recognised and set up before the kernel attempts to mount it in the tree. Failing to do so can result in an unusable or even unbootable system! For specific details, consult the mdadm man page man mdadm or the mdadm ebuild page.