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The IBM 3850 Mass Storage System was an online tape library used to hold large amounts of infrequently accessed data.


Starting in the late-1960s IBM's lab in Boulder, Coloradomarker began development of a low-cost mass storage system based on magnetic tape cartridges. The tapes would be accessed automatically by a robot (known as an accessor) and fed into a reader/writer unit that could work on several tapes at the same time. Originally the system was going to be used as a directly attached memory device, but as the speed of computers grew in relation to the storage, the product was re-purposed as an automated system that would offload little-used data from hard disk systems. Known internally as Comanche while under development, IBM management found a number of niche uses for the concept, and announced it officially as the IBM 3850 on October 9th, 1974.


Data cartridge
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. 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 was accessed via one or two IBM 3330 disk 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.

The recording method was unusual for its time. The tape was wound around a cylindrical mandrel in a helix and stopped. The drive head rotated once (on a rotating drum) to record a diagonal track. Then tape was wound a small step, so the head could iterate over next diagonal track. Depending on technical definitions this might be even considered a first example of a digital helical scan recording, long before Exabyte's helical drive (although analog video helical recording systems were developed earlier).

When free disk space was required a group of cylinders were selected to be destaged to tape, these were transferred with minimal or no change of format. Each tape could store 202 cylinder images each of 19 tracks which corresponded to half of a 3330 disk pack. Cylinder locations on the tape were fixed and identified by markers along the edge.


Several models of the 3851 were available. The smallest A1 holding 706 cartridges storing 35.3GB, while the largest A4 held 4,720 cartridges storing 236GB in a long unit. All of the units were also available in the "B models" which added a second controller for on-line backups, as well as offline storage. A second series of 3581s was released on March 6th, 1980, but the entire series was discontinued on August 5, 1986.

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