Serial Storage Architecture
Encyclopedia
Serial Storage Architecture (SSA) is a serial transport protocol used to attach disk drives to server
s. It was invented by Ian Judd of IBM
in 1990. IBM produced a number of successful products based upon this standard before it was overtaken by the more widely adopted Fibre Channel
protocol.
SSA was promoted as an open standard by the SSA Industry Association, unlike its predecessor the first generation Serial Disk Subsystem. A number of vendors including IBM, Pathlight Technology
and Viacom
produced products based on SSA. It was also adopted as an ANSI
X3T10.1 standard. SSA devices are logically SCSI
devices and conform to all of the SCSI command
protocols.
SSA provides data protection for critical applications by helping to ensure that a single cable failure will not prevent access to data. All the components in a typical SSA subsystem are connected by bi-directional cabling. Data sent from the adaptor can travel in either direction around the loop to its destination. SSA detects interruptions in the loop and automatically reconfigures the system to help maintain connection while a link is restored.
Up to 192 hot swappable hard disk drives can be supported per system. Drives can be designated for use by an array in the event of hardware failure. Up to 32 separate RAID arrays can be supported per adaptor, and arrays can be mirrored across servers to provide cost-effective protection for critical applications. Furthermore, arrays can be sited up to 25 metres apart - connected by thin, low-cost copper cables - allowing subsystems to be located in secure, convenient locations, far from the server itself.
With its inherent resiliency and ease of use, SSA is deployed in server/RAID environments, where it is capable of providing for up to 80 Mbyte
/s of data throughput, with sustained data rates as high as 60 Mbytes/s in non-RAID mode and 35 Mbytes/s in RAID mode.
connectors. Impedances are 75 ohm single-ended, and 150 ohm differential. For longer-distance connections, it is possible to use fiber-optic cables up to 10 km (6 mi) in length. Signals are differential TTL
. The transmission capacity is 20 megabyte
s per second in each direction per channel, with up to two channels per cable. The transport layer protocol is non return to zero
, with 8B/10B encoding
(10 bits per character). Higher protocol layers were based on the SCSI
-3 standard.
Server (computing)
In the context of client-server architecture, a server is a computer program running to serve the requests of other programs, the "clients". Thus, the "server" performs some computational task on behalf of "clients"...
s. It was invented by Ian Judd of IBM
IBM
International Business Machines Corporation or IBM is an American multinational technology and consulting corporation headquartered in Armonk, New York, United States. IBM manufactures and sells computer hardware and software, and it offers infrastructure, hosting and consulting services in areas...
in 1990. IBM produced a number of successful products based upon this standard before it was overtaken by the more widely adopted Fibre Channel
Fibre Channel
Fibre Channel, or FC, is a gigabit-speed network technology primarily used for storage networking. Fibre Channel is standardized in the T11 Technical Committee of the InterNational Committee for Information Technology Standards , an American National Standards Institute –accredited standards...
protocol.
SSA was promoted as an open standard by the SSA Industry Association, unlike its predecessor the first generation Serial Disk Subsystem. A number of vendors including IBM, Pathlight Technology
Pathlight Technology
Pathlight Technology was a pioneer in the field of Storage Area Networks. Based in Ithaca, NY, Pathlight was formed in 1994 as a spin-out from Ironics, a manufacturer of VME computer, IO and memory boards....
and Viacom
Viacom
Viacom Inc. , short for "Video & Audio Communications", is an American media conglomerate with interests primarily in, but not limited to, cinema and cable television...
produced products based on SSA. It was also adopted as an ANSI
American National Standards Institute
The American National Standards Institute is a private non-profit organization that oversees the development of voluntary consensus standards for products, services, processes, systems, and personnel in the United States. The organization also coordinates U.S. standards with international...
X3T10.1 standard. SSA devices are logically SCSI
SCSI
Small Computer System Interface is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI standards define commands, protocols, and electrical and optical interfaces. SCSI is most commonly used for hard disks and tape drives, but it...
devices and conform to all of the SCSI command
SCSI command
In SCSI computer storage, a command is the basic unit of communication. The SCSI command architecture was originally defined for parallel SCSI buses but has been carried forward with minimal change for use with Fibre Channel, iSCSI and Serial Attached SCSI....
protocols.
SSA provides data protection for critical applications by helping to ensure that a single cable failure will not prevent access to data. All the components in a typical SSA subsystem are connected by bi-directional cabling. Data sent from the adaptor can travel in either direction around the loop to its destination. SSA detects interruptions in the loop and automatically reconfigures the system to help maintain connection while a link is restored.
Up to 192 hot swappable hard disk drives can be supported per system. Drives can be designated for use by an array in the event of hardware failure. Up to 32 separate RAID arrays can be supported per adaptor, and arrays can be mirrored across servers to provide cost-effective protection for critical applications. Furthermore, arrays can be sited up to 25 metres apart - connected by thin, low-cost copper cables - allowing subsystems to be located in secure, convenient locations, far from the server itself.
With its inherent resiliency and ease of use, SSA is deployed in server/RAID environments, where it is capable of providing for up to 80 Mbyte
Megabyte
The megabyte is a multiple of the unit byte for digital information storage or transmission with two different values depending on context: bytes generally for computer memory; and one million bytes generally for computer storage. The IEEE Standards Board has decided that "Mega will mean 1 000...
/s of data throughput, with sustained data rates as high as 60 Mbytes/s in non-RAID mode and 35 Mbytes/s in RAID mode.
Link characteristics
The copper cables used in SSA configurations are round bundles of two or four twisted pairs, up to 25 metres long and terminated with 9-pin micro-DD-subminiature
The D-subminiature or D-sub is a common type of electrical connector. They are named for their characteristic D-shaped metal shield. When they were introduced, D-subs were among the smaller connectors used on computer systems....
connectors. Impedances are 75 ohm single-ended, and 150 ohm differential. For longer-distance connections, it is possible to use fiber-optic cables up to 10 km (6 mi) in length. Signals are differential TTL
Differential TTL
Differential TTL is a type of binary electrical signaling based on the TTL standard. Normal TTL signals are single-ended, which means that each signal consists of a voltage on one wire, referenced to a system ground. The "low" voltage level is zero to 0.8 volts, and the "high" voltage level is 2...
. The transmission capacity is 20 megabyte
Megabyte
The megabyte is a multiple of the unit byte for digital information storage or transmission with two different values depending on context: bytes generally for computer memory; and one million bytes generally for computer storage. The IEEE Standards Board has decided that "Mega will mean 1 000...
s per second in each direction per channel, with up to two channels per cable. The transport layer protocol is non return to zero
Non-return-to-zero
In telecommunication, a non-return-to-zero line code is a binary code in which 1's are represented by one significant condition and 0's are represented by some other significant condition , with no other neutral or rest condition. The pulses have more energy than a RZ code...
, with 8B/10B encoding
8B/10B encoding
In telecommunications, 8b/10b is a line code that maps 8-bit symbols to 10-bit symbols to achieve DC-balance and bounded disparity, and yet provide enough state changes to allow reasonable clock recovery. This means that the difference between the count of 1s and 0s in a string of at least 20 bits...
(10 bits per character). Higher protocol layers were based on the SCSI
SCSI
Small Computer System Interface is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI standards define commands, protocols, and electrical and optical interfaces. SCSI is most commonly used for hard disks and tape drives, but it...
-3 standard.
Products which used SSA
- IBM 7133 Disk expansion enclosures
- IBM 2105 Versatile Storage Server (VSS)
- IBM 2105 Enterprise Storage ServerIBM Enterprise Storage ServerThe IBM Enterprise Storage Server or the Shark is an enterprise storage array from IBM.-History:Originally, in 1998 IBM released the IBM 2105 Versatile Storage Server...
(ESS) - IBM 7190 SBUS SSA Adapter
- Pathlight TechnologyPathlight TechnologyPathlight Technology was a pioneer in the field of Storage Area Networks. Based in Ithaca, NY, Pathlight was formed in 1994 as a spin-out from Ironics, a manufacturer of VME computer, IO and memory boards....
Streamline PCI Host Bus Adapter - Pathlight TechnologyPathlight TechnologyPathlight Technology was a pioneer in the field of Storage Area Networks. Based in Ithaca, NY, Pathlight was formed in 1994 as a spin-out from Ironics, a manufacturer of VME computer, IO and memory boards....
SSA Data Pump - Pathlight TechnologyPathlight TechnologyPathlight Technology was a pioneer in the field of Storage Area Networks. Based in Ithaca, NY, Pathlight was formed in 1994 as a spin-out from Ironics, a manufacturer of VME computer, IO and memory boards....
SAN Gateway