1971: Track-following servo quadruples HDD density

IBM 3330 incorporated important hardware and software advances

Beginning in the late 1960s, engineers began to leave the security of IBM to seek greater financial reward in the entrepreneurial climate of Silicon Valley. “The Dirty Dozen” departed to found the first IBM disk drive spinout, Information Storage Systems (ISS) in 1967, and a large group followed Alan Shugart to Memorex in 1969. These departures exacerbated the task of meeting aggressive goals set in 1965 for the “Merlin” project - to build a system with four times the capacity and half the access time of the IBM2314. A team led by Jack Clemens, with key contributions from Les Adams, Dick Charlton, and Dick Wilmer, finally delivered the IBM 3330 subsystem, comprising a 3330 Disk Storage Unit and a 3830 Storage Control unit in 1971. The controller employed IBM’s first floppy disk drive to load microcode.

The 3330 system incorporated technical advances that contributed to a quadrupling of practical storage density. Removable 100 MB disk packs with 30 milliseconds access time were enabled by coupling a track-following servo system and a rectangular ceramic slider that reduced the head to disk spacing to 50 microinches together with the voice-coil actuator of the IBM 2310. Shugart later described this feature as one of the four most significant technical developments in the history of mass storage. The closed-loop servo continuously corrected the position of the drive heads from a reference track recorded on one of the surfaces of the total of 20 available in the disk pack. The 3330 also introduced the use of error correction, which made the drives more reliable and reduced costs because small imperfections in the disk surface could be tolerated. Track-following servos have continued to be an essential technology used in all modern hard disk drives.

  • Oswald R. “Servo positioning system for magnetic disk memory” U.S. Patent # 3725764 (Filed: Apr 1, 1971 Issued Apr 3, 1973)
  • Allan, Iain D. and Per-Erik Walberg. "Method and apparatus for weighting the priority of access to variable length data blocks in a multiple disk drive data storage system" U. S. Patent #4,200,928 (Filed: Jan 23, 1978 Issued: April 29, 1980)
  • Sordello, Frank J. “Magnetic track following servo system” U.S. Patent # 3404392 (Filed: Apr 14, 1967 Issued Oct 1, 1968)
  • IBM 3300 Marketing brochure (Retrieved on 1.15.15 from: https://www-03.ibm.com/ibm/history/exhibits/storage/storage_3330b.html)

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