IBM 3850 - Description


The MSS (as it was known) consisted of a library of cylindrical plastic cartridges, two inches wide and 4 inches (100 mm) long, each holding a spool of tape 770 inches (20 m) 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.

Cartridges were moved into and out of read stations by two motorized accessor arms, electrically connected via flat cable on a drum. Stage time for data from cartridge to disk was typically 15 seconds, including about two seconds to move the cartridge into a read station, and eight to ten seconds to read the 200-foot tape.

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 (which was based on analog video helical recording systems 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 of 19 tracks each, half of the 404 cylinders in a 3330 disk pack. Cylinder locations on the tape were fixed and identified by markers along the edge.

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