IBM 3850

The IBM 3850 Mass Storage Subsystem was a mass storage subsystem from IBM. It was a forerunner of today's tape libraries, enabling it to manage data on the virtual disk files.

Design and operation

The IBM 3850 Mass Storage Subsystem consisted of a kind of high-bay warehouse, the IBM 3851 Mass Storage Facility, in which cylindrical magnetic tape cartridges (called cartridges ) were stored in hexagonal honeycomb. These cartridges had a diameter of 2 inches ( 5.08 cm) and a height of 4 inches (10.16 cm). The length of the magnetic tape was 770 inches ( 19.55 m). By means of helical recording technology could be 50 MB stored on each cartridge. Two cassettes formed from a virtual disk type IBM 3330-1.

The honeycombs were arranged along the two walls of the IBM 3851. A robot transporting the cassettes from the honeycomb to the read and write stations. A Ein-/Ausgabestation was the introduction of new and ejection of old, bad cartridge.

The built-in control unit managed an inventory of the cartridges, the MSS Volume Inventory, which is based on two plates, the table packs, was stored redundantly. The table packs were not directly accessible by the operating system of the connected mainframe. On the Table also packs the microprogram control unit was recorded, which was coded as an overlay program and its parts were loaded on demand in the core memory.

In addition to the MSS included a number of plates, the so-called staging Drives. These were initially of type IBM 3330 (1 or 11), and later also of type IBM 3350, which was, however, operated in 3330 compatibility mode. The staging Drives were managed by an IBM 3830 Storage Control (Model 3 ).

The IBM 3850 had a built-in error correction procedures. Once the number of correctable errors a cassette exceeded a threshold value, the contents were copied onto a blank tape, tracked inventory and eject the old cartridge.

Models

The IBM 3850 were available in different models, partly a reconstruction was possible. Basically, there were A- and B- models, the latter containing an additional control unit. The smallest model, the A1 could save on 706 cassettes 35.3 GB, the A4 brought it to 236 GB.

IBM 3850 and MVS

In the MVS operating system is so-called virtual units had to be defined. Once files from the MSS were requested by an application, the data from the corresponding virtual disks in blocks have been written on 4 cylinders on the staging Drives and could be read from there as normal disk files. This process was called staging. After the Close the data has been written back to the cartridges (so-called least Aging, ) but remained as long as possible get to the staging Drives.

The administration of the MSS was special IDCAMS commands. There were chargeable additional programs such as MSVIZAP.

History

In the late 1960s the IBM laboratory began in Boulder (Colorado) under the code name Comanche with the development of a mass storage subsystem. On October 9, 1974, was officially announced as the IBM 3850. A real boom time of the IBM 3850 in the early 1980s. Since the IBM 3380 came with the delivery of disks of type IBM in default, some companies were forced to have a MSS with even then obsolete technology designed to capture data growth. Support for the IBM 3850 ended on August 6, 1986.