Difference between revisions of "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 [[Digital Equipment Corporation|DEC]] designed [[disk]] [[drive]] to [[device controller|controller]] interface | + | The '''Standard Disk Interconnect''' (usually referred to by the acronym, '''SDI'''; sometimes called the '''Standard Disk Interface''') is a [[Digital Equipment Corporation|DEC]] designed [[disk]] [[drive]] to [[device controller|controller]] interface, intended to support large [[mass storage]] devices. It is a key element of the [[Digital Storage Architecture]] (DSA), used with [[PDP-11]]s and with [[VAX]] single and multiple [[Central Processing Unit|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 [[server]]s. | |
| − | The | + | 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 [[protocol]]s are adapted to sequential storage needs. |
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| + | ==Technical details== | ||
| + | |||
| + | The SDI 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. | ||
| + | |||
| + | Electrically, the SDB consists of four signal carriers, using small-diameter [[coaxial cable]]s; two for moving user data, commands, and responses between drive and controller, and two for continuous real time transmission of drive and controller status information. The SDB signals are [[transformer]] coupled (economically feasible with such a small number of links), so the drive and controller do not require a common [[direct current|DC]] [[ground]], simplifying installation. Transformer coupling also eliminates unpredictable and unrepeatable "[[ground loop]]" problems, and provides for installation flexibility, since SDB drives can be physically added to or removed from a subsystem without disrupting its operation. These four cables are emplaced in a single [[connector]] for actual installation of SDB units. | ||
| + | |||
| + | 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 path]]s, and indicates a forced error in case a hard media error causes sector replacement. | ||
| + | * A Bad Block Replacement Algorithm which dynamically substitutes good [[disk block]]s 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: | ||
* SDI Disk controllers: | * SDI Disk controllers: | ||
| + | ** [[UDA50]] ([[UNIBUS]]) | ||
** [[KDA50]] ([[QBUS]]) | ** [[KDA50]] ([[QBUS]]) | ||
** [[KDB50]] ([[VAX Bus Interconnect|VAXBI]]) | ** [[KDB50]] ([[VAX Bus Interconnect|VAXBI]]) | ||
** [[KDM70]] ([[Extended Memory Interconnect|XMI]]) | ** [[KDM70]] ([[Extended Memory Interconnect|XMI]]) | ||
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* SDI [[Hierarchical Storage Controller]]s (HSCs): | * SDI [[Hierarchical Storage Controller]]s (HSCs): | ||
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** [[ESE 25]] | ** [[ESE 25]] | ||
** [[ESE 50]] | ** [[ESE 50]] | ||
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==Further reading== | ==Further reading== | ||
* ''Digital Storage Technology Handbook'', 1989 (EC-H0374-45/89) - Chapter 4, The Digital Storage Interconnects | * ''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== | ==External links== | ||
Revision as of 16:03, 24 February 2023
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.
Contents
Technical details
The SDI 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.
Electrically, the SDB consists of four signal carriers, using small-diameter coaxial cables; two for moving user data, commands, and responses between drive and controller, and two for continuous real time transmission of drive and controller status information. The SDB signals are transformer coupled (economically feasible with such a small number of links), so the drive and controller do not require a common DC ground, simplifying installation. Transformer coupling also eliminates unpredictable and unrepeatable "ground loop" problems, and provides for installation flexibility, since SDB drives can be physically added to or removed from a subsystem without disrupting its operation. These four cables are emplaced in a single connector for actual installation of SDB units.
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:
- SDI Solid State Disk Drives
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
- SDI Disk Controllers - brochure
