Standard Disk Interconnect

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The Standard Disk Interconnect (usually referred to by the acronym, SDI; sometimes called the Standard Disk Interface) is a DEC designed disk drive to controller interface, intended to support large mass storage devices. It is a key element of the Digital Storage Architecture (DSA), used with PDP-11s and with VAX single and multiple CPU systems.

A new standard, it was introduced (in 1981) because no existing standard had the capabilities that DEC deemed necessary in the mass storage arena. SDI claims to offer high performance, availability, and configuration flexibility, and does so across a large range of configurations (ranging from small individual systems, to groups of large servers.

A variant, the Standard Tape Interconnect (STI), is used with magnetic tape systems, and other sequential storage applications. It shares its physical connection technology (called the Standard Drive Bus, SDB) with SDI; the higher layer protocols are adapted to sequential storage needs.

Technical details

The SDB drive to controller interconnect is a serial link which connects each drive directly (i.e. individually), or 'radially' (a term for such a device connection approach which DEC also used in its early machines), to its controller.

Logically, at a higher layer, SDI consists of a protocol by which a controller can recognize a drive's characteristics, direct its mechanical operation, store data on or retrieve data from it, and recognize and recover from error conditions.

SDI Features

To support DSA data integrity goals and provide compatibility and interchange, SDI controllers include advanced data integrity features:

  • An advanced error-correction code (ECC).
  • An Error Detection Code (EDC) which verifies ECC computation logic and the ECC correction process, checks controller data paths, and indicates a forced error in case a hard media error causes sector replacement.
  • A Bad Block Replacement Algorithm which dynamically substitutes good disk blocks for potentially defective ones. The goal of the DSA is to anticipate developing defects and retire blocks containing them before user data loss occurs.
  • Automatic Revectoring for transparent access to replaced blocks.
  • Quadruplicate Headers for verification of read/write head position prior to data transfer, even if media defects exist in the header area.
  • Firewalling against single failure points to detect and isolate single component failures so that no erroneous data is passed to the user.

SDI Family of DSA Products

The family of products which implement the SDI includes:

Further reading

  • Digital Storage Technology Handbook, 1989 (EC-H0374-45/89) - Chapter 4, The Digital Storage Interconnects
  • Digital Large System Mass Storage Handbook, 1986 - Chapter 7, The Storage Interconnect

External links