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Serial Storage Architecture

Definition: Serial storage architecture (SSA) is a standard for high speed data transfer between large arrays of disks and servers in mainframe computing environments.


Serial Storage Architecture

SSA Development

SSA was developed at IBM in the late 1980s and early 1990s. Its invention is attributed to an IBM engineer, Ian Judd, who based it on the small computer system interface (SCSI). IBM released the concept in 1990 and products using SSA started to emerge in 1994 and 1995. It was usually deployed in large-scale mainframes and networks, but could also be used in smaller systems.

There were a number of reasons for developing a new standard to link disks to servers, mostly around security and scalability. At the time the high performance parallel interface (HPPI) was being used for disk-to-server communications in larger computers. Although HPPI was capable of being run over fibre optic cable, this was expensive at the time so it was more normal to use a fifty strand twisted pair cable. This made it bulky and limited the number of devices that could be attached as well as the distance between them and the server.

Data Security and SSA

Data security, in the form of fault tolerance, was a prime driver for the development of SSA. The design was bi-directional, meaning that data could be sent in both directions at the same time (also known as full-duplex communication). It was intended to be implemented in a loop so that data could flow to its destination in either direction. The design also meant that a break in any one of the cable cores would not stop data flowing.

The implications of the protocol design meant that SSA also allowed hot-swapping. In the event of a disk drive failing it could be replaced without any of the other components being shut down. This also allowed a disk array to be expanded in a modular fashion, with initially 126 devices capable of being connected in an SSA environment. Resilience was also improved, with SSA capable of linking devices up to twenty-five metres apart.

Quick and Efficient Data Access

A pure speed increase was one of the many benefits of SSA. With two ports on each SSA device and each port capable of running a 40MB/s exchange in each direction, the aggregated bandwidth of Serial Storage ArchitectureSSA was 160MB/s. This was at a time when Ethernet was running at 10MB/s and the relatively new SCSI "fast-and-wide" offered 20MB/s.

SSA also spread the data among the devices in a fair fashion to counteract some inefficiencies of the SCSI interface. With SSA each device was allocated a fraction of the bandwidth. In large scale SCSI environments some devices would demand more time on the bus than others, disrupting the delivery of time-sensitive data such as video or audio.

Hard Disk Architecture Format Wars

Unfortunately for IBM they were developing SSA at the same time as Fibre Channel, a different but similar standard. Products using SSA were released by IBM and a number of other hardware vendors, including Viacom and Pathlight Technologies, but faced off against Fibre Channel in a battle similar to the VHS versus Betamax video format wars.

Although invented by IBM, and considered a proprietary format by the computer industry, IBM did submit SSA to ANSI as a proposal for an open standard and it was subsequently classified as ANSI X3T10.1. At roughly the same time Fibre Channel had been developed but with ANSI standard approval from the beginning.

Fibre Channel was not as fast as SSA but could travel further and had simpler interconnect requirements. Ultimately manufacturers and customers were unwilling for IBM to be in control of such a crucial link in their systems. Fibre Channel won out in the end and today even IBM is implementing Fibre Channel based hard disk arrays rather than using SSA.