vides flexible cabling options, supporting up to 127 devices on a loop in which the nodes can be 65 feet apart using copper, and about 2500 feet apart using fiber.
SSA systems are typically configured in a loop topology. Data travels bidirectionally around the loop in bucket-brigade fashion. Packets of data hop from point to point as they travel around the loop, eliminating the need for systemwide bus arbitration and contributing to SSA's high potential bandwidth. Each loop supports more than one host, and each host can handle up to 80 MBps on the same loop simultaneously.
In theory, a single-loop SSA array supports two ports with two directions at 20 MBps, or a cumulative bandwidth of 80 MBps per adapter. But this bandwidth assumes an optimum combination of reads and writes that takes full advantage of SSA's bidirectional nature. A four-port controller connected to two array loops should support a peak bandwidth of 160 MBps.
The SSA Standard Committee's new specification increases the link speed to 40 MBps, which doubles the loop bandwidth for future SSA products.
Fibre Channel-Arbitrated Loop (FC-AL) is also emerging as an alternative high-speed storage interface. Fibre Channel, conceived for use as a fast, reliable backbone for linking channel-based systems, was adapted into FC-AL for use as a storage back end. FC-AL is a 100-MBps single-direction loop technology. Like SCSI, FC-AL is an arbitrated bus, and data transfer proceeds from only one node on the loop at a time.
SSA delivers flexible connections and a high device count, but much of its performance is still just a promise. IBM, for example, says that the throughput of its current SSA controller performing a combination of read/write operations is limited by its PCI interface chip. IBM says its adapter currently delivers between 43 and 70 MBps on a two-loop system (one adapter card with four ports).
"The bandwidth of 20-MB SSA is well balanced with the
chip set we've currently got, but we do recognize that we've got to go beyond that with 40-MB SSA," says Alistair Symon, project-control manager at IBM. "Future controllers will replace the PCI interface with an in-house design that removes the bottleneck."
A similar single-loop controller from Pathlight uses the same PCI interface chip, from PLX Technology. In tests that were performed at BYTE, a single StreamLine-PCI SSA adapter card ($995) delivered a throughput of approximately 40 MBps. In a random large-block read/write operation, the adapter supported 41 MBps in an array of 16 drives (theoretically, that number should be 80 MBps). In a sequential large-block read operation with one card, we observed approximately 39-MBps throughput.
Those numbers are slightly better than the fastest UltraSCSI performance that BYTE saw in a recent review (see "UltraSCSI Doubles Speed" August BYTE), but thanks to SSA's ability to host multiple adapters on a loop, SSA can deliver even better aggregate performan
ce. For example, with two computers on the same loop, each with a Pathlight adapter card, BYTE measured an aggregate throughput of 71 MBps.
Pathlight officials contend that the throughput is currently limited by a combination of the card's PCI interface chip, the current implementation of PCI, and the speed of system memory. Although IBM and Pathlight provide different explanations for SSA's current performance, both companies are promising even better throughput in their future products.
As with most cutting-edge technologies, it will take time before applications and OSes are updated to take full advantage of SSA's new capabilities. For example, SSA supports the connection of multiple computer systems to the same loop. But of the many OSes that SSA vendors currently support (e.g., Windows 95 and NT, NetWare, Mac OS, SCO Unix, and NextStep), only AIX currently supports shared access to the same data by those systems.
Despite its interface problems and currently incomplete OS support, at pres
ent SSA is poised to greatly extend the limit of what's possible for desktop storage subsystems.
Where to Find
Pathlight
Phone: (800) 334-4812
Fax: (607) 266-4010
Internet:
http://www.pathlight.com
SSA Products Deliver Better Storage
