Serial Storage Architecture (SSA) was a serial transport protocol used to attach
disk drives to
server computers.
History
SSA 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
Fibre Channel (FC) is a high-speed data transfer protocol providing in-order, lossless delivery of raw block data. Fibre Channel is primarily used to connect computer data storage to servers in storage area networks (SAN) in commercial data c ...
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 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.
Pathlight was involved in the development ...
and Vicom Systems produced products based on SSA. It was also adopted as an
American National Standards Institute
The American National Standards Institute (ANSI ) 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 organ ...
(ANSI) X3T10.1 standard. SSA devices are logically
SCSI
Small Computer System Interface (SCSI, ) is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI standards define commands, protocols, electrical, optical and logical interface ...
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 swap
Hot swapping is the replacement or addition of components to a computer system without stopping, shutting down, or rebooting the system; hot plugging describes the addition of components only. Components which have such functionality are said ...
pable 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
Raid, RAID or Raids may refer to:
Attack
* Raid (military), a sudden attack behind the enemy's lines without the intention of holding ground
* Corporate raid, a type of hostile takeover in business
* Panty raid, a prankish raid by male colleg ...
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.
SSA was deployed in server RAID environments, where it was 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.
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-D 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
The megabyte is a multiple of the unit byte for digital information. Its recommended unit symbol is MB. The unit prefix ''mega'' is a multiplier of (106) in the International System of Units (SI). Therefore, one megabyte is one million bytes o ...
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
In telecommunications, 8b/10b is a line code that maps 8-bit words to 10-bit symbols to achieve DC balance and bounded disparity, and at the same time provide enough state changes to allow reasonable clock recovery. This means that the dif ...
(10 bits per character). Higher protocol layers were based on the
SCSI
Small Computer System Interface (SCSI, ) is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI standards define commands, protocols, electrical, optical and logical interface ...
-3 standard.
Products
*
IBM 7133 Disk expansion enclosures
*
IBM 2105 Versatile Storage Server (VSS)
*
IBM 2105 Enterprise Storage Server (ESS)
*
IBM 7190 SBUS SSA Adapter
*
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.
Pathlight was involved in the development ...
Streamline PCI Host Bus Adapter, SSA Data Pump,
storage area network
A storage area network (SAN) or storage network is a computer network which provides access to consolidated, block-level data storage. SANs are primarily used to access data storage devices, such as disk arrays and tape libraries from se ...
gateway
See also
*
List of device bandwidths
References
{{Computer-bus
Serial buses
SCSI
IBM storage devices
American National Standards Institute standards
Computer storage buses