Red Hat Knowledgebase

Short Answer

Rebuild your ramdisk after creating the array with mdadm. See the Knowledgebase article regarding rebuilding the initial ramdisk image.

Long Answer

Assuming the machine already has/had at least one software RAID array in use prior to you making this new one, you should not have to do anything. If there were no arrays in use prior to this, then you should rebuild your initrd image (using mkinitrd) so that it will know to load the RAID modules. mkinitrd basically checks to see if there are any active RAID devices at the time it is run. If there are, then it loads the right RAID modules and starts the RAID arrays in the initrd image. Therefore, when running mkinitrd after creating the new software RAID array, make sure that any arrays you want started in the initrd are up and running and mkinitrd will do the rest.