IBM 3850
| Company type | Product |
|---|---|
| Industry | Computer storage |
| Founded | October 9, 1974 |
| Defunct | August 5, 1986 |
| Headquarters | Boulder, Colorado , United States |
| Products | Nearline tape storage system |
The IBM 3850 Mass Storage System (MSS) represented an online tape library solution engineered to accommodate vast amounts of data that were accessed infrequently. It stands as one of the pioneering implementations of nearline storage technology.
History
During the late 1960s, IBM's research facility in Boulder, Colorado, created an economical mass storage solution utilizing magnetic tape cartridges. This system featured automation through a robotic accessor mechanism, which inserted tapes into a reader/writer unit capable of processing multiple tapes concurrently. Originally designed to function as directly attached memory, the technology evolved to serve as a repository for rarely used data offloaded from hard disk drives as computing speeds advanced. Internally referred to as Comanche, IBM formally introduced the product as the IBM 3850 on October 9, 1974. The system remained operational for more than ten years until its retirement on August 5, 1986.
Description
The architecture consisted of cylindrical plastic cartridges measuring two inches in width and length, each housing tape capable of storing 50 MB; every virtual disk utilized two such cartridges. These cartridges were organized in a hexagonal matrix within the IBM 3851 Mass Storage Facility, where newly introduced cartridges were automatically allocated to empty slots. Data retrieval was facilitated through virtual IBM 3330 disk drives, with physical caching implemented via a combination of 3330 and 3350 staging drives. The IBM 3830 Storage Control managed the transfer processes—known as staging (from tape to disk) and destaging (from disk back to tape)—while also overseeing independent disk storage operations.
Two motor-driven accessor arms transported cartridges along a flat cable wrapped around a drum to reading stations. The typical delay for staging data from cartridge to disk was approximately 15 seconds, including about two seconds for cartridge positioning and eight to ten seconds for reading roughly 200 feet of tape.
The recording technique employed was distinctive: tape was extracted from the cartridge and helically wrapped once around a cylindrical mandrel. A rotating head then sequentially read or wrote diagonal tracks as the tape advanced incrementally. This method is recognized as an early form of digital helical scan recording, predating later analog helical scan devices such as those developed by Exabyte.
When releasing disk capacity became necessary, designated cylinders were destaged back to tape with minimal formatting alterations. Each tape contained images of 202 cylinders, each with 19 tracks—half of the total 404 cylinders found in a 3330 disk pack—with fixed cylinder markers along the tape edge.
Models
The 3851 family included multiple variants, starting with the smallest A1 model holding 706 cartridges and providing 35.3GB of storage, up to the largest A4 model accommodating 4,720 cartridges and offering 236GB within a unit measuring 20 feet in length. Models designated "B" featured an additional controller enabling online backup capabilities and offline storage functions. On March 6, 1980, a second series was introduced that doubled the maximum capacity to 472GB. Production and support for the entire series concluded on August 5, 1986.