My system uses an NVMe drive for primary storage, so I didn't have CONFIG_DM_RAID set. When I tried to create a raid-1 volume on sda2 and sdb2, mdadm fell over every time. After setting CONFIG_DM_RAID=y (in the SCSI section) and rebooting, the same command worked as expected. Please add a short section to the wiki to make this requirement clear: https://wiki.gentoo.org/wiki/Complete_Handbook/Software_RAID#Installing_the_Tools Perhaps most people will have this set already, but some evidently won't.
Bugs are not the place to suggest wiki changes. That goes in the Discussion page of the article. Wiki bugs are for the software itself
That said, most articles not in a namespace are editable by anyone who wishes to take on the task.
