The IBM 7311-D20 is a rack-mounted, high-density expansion drawer that attaches to the pSeries 630, 650 Model 6C4 to provide remote I/O. There are seven hot-swap PCI-X I/O slots in the 7311-D20 and 12 optional hot-swap disk drive bays.The 7311-D20 occupies 4U (7 inches) of space in a rack and mounts in a 19-inch standard rack drawer. The Model D20 is 24 inches deep. The fans, power supplies, and PCI adapters are top-accessible, while the disk drives are front-accessible for easy service and maintenance.