The MSS (as it was known) consisted of a library of cylindrical plastic cartridges, two inches wide and long, each holding a spool of tape long storing 50MB; each virtual disk required a pair of cartridges. These cartridges were held in a hexagonal array of bins in the IBM 3851 Mass Storage Facility. New cartridges were rolled into the facility and were automatically stored in a vacant bin. The data were accessed via virtual IBM 3330 disk drives, and physically cached on a combination of 3330 and 3350 "staging drives", the data being transferred automatically between cartridge and disk drive in processes called "staging" and "destaging". These were all connected together with the IBM 3830 Storage Control (also used for disk storage alone), the entire system making up a 3850 unit.