That's a good example of a historical accident rather than anything planned. Iris is the older news machine, serving the primary server for local and internal newsgroups and as a backup news server for the remaining Usenet groups. The MegaRAIDs feature three independent ultra-wide SCSI channels, an I960 processor, and up to 128Meg of memory on the controller.
Local drives hold basic system software and OS files for each system.
Nyx1, Nyx2, and Nyx3 are all login machines and in addition to the internal drives listed above, mount partitions off of four nine gig drives located on Nyx0.