Beginning in the late-1960s, IBM engineers in Boulder, Colorado, began development of a low-cost mass storage system based on magnetic tape in cartridges. By 1970, the proposed device was code named "Comanche" and described as an online tape library to provide computer-controlled access to stored information. Numerous marketing studies and design changes were made during the early 1970s, and finally Comanche was announced as the IBM 3850 Mass Storage System (MSS) in October 1974.
The components of the 3850 were new data cartridges, one or two units of the IBM 3851 Mass Storage Facility, the new IBM 3830 Storage Control Model 3, and the widely-used 3330-series disk subsystem.
The data cartridges were circular cylinders, two inches in diameter and four inches long, each holding a spool holding 770 inches of tape. Cartridges were stored in a two-dimensional array of bins, which were hexagonal, rather than square, to save space.
During its development, Comanche evolved in concept and design from a tape library to a means for storing infrequently-used information that otherwise would reside on disk storage. Information under control of the MSS was stored in Direct Access Storage Device (DASD) format images on the low-cost cartridge tape. The MSS transferred data from a cartridge to the 3330 DASD drive when requested, in a process called "staging." Once staged, the data were treated the same as any other data resident on a 3330 disk file. Conversely, when additional 3330 space was needed for incoming data, a cylinder of 3330 data was automatically selected for "destaging" back to its data cartridge. Each cartridge was capable of storing 202 cylinders in the 3330 format.
There were eight models of the MSS, differing in storage capacity as well as in staging capability. The smallest model (Models A1 and B1) accommodated 706 tape cartridges or 35 billion bytes, and the largest held 472 billion bytes (Models A4 and B4).