Author: Mike Micheletti, Product Manager
With Serial Attached SCSI (SAS) products now entering the market, system integrators and storage vendors are accelerating their transition to serial-based storage infrastructure. By taking the lead on delivering SAS-based solutions, these vendors can build high performance sub-systems while developing valuable expertise in this next generation direct-attach storage technology.
As storage integrators ramp their SAS design and validation efforts, they may benefit from using specialized test equipment to accelerate product qualification. Below are three protocol-level test tools that are playing a key role in validating SAS components as ready for enterprise-worthy applications.
Even when a system is working properly, using a protocol analyzer to observe what’s really happening on the link allows validation engineers to increase system performance and to identify bottlenecks or latent design flaws. Protocol analyzers are generally available as dedicated “PC-hosted” instruments, and are designed to unobtrusively tap the link between devices and record data into an on-board memory buffer. Software residing on an attached PC enables users to view the “raw” data in the form of frames and primitives.
Today’s analyzers have evolved far beyond the simple data recorders common in early computer labs. By decoding SAS link layer events into high level Serial Management Protocol (SMP) transactions and SCSI operations, protocol analyzers provide a logical view of traffic on the bus that will save an enormous amount of debug time. Figure 1 below shows how Protocol Analyzers can decode the payload of a frame, including SCSI-specific parameters.
Figure 1: Protocol Analyzers decode raw bytes into high-level language instructions
to provide a logical view of events on the bus
Some analyzers generate traffic metrics, including performance statistics and compliance reports, to help users evaluate protocol behavior. For SAS testing, analyzers are most often equipped with four independent recording channels to better accommodate “wide port” analysis.
There are hardware- and software-based bus exercisers, which vary significantly in their purpose and functionality. Hardware-based exercisers, or “testers,” provide precise control of the data stream (down to the bit-level), and allow engineers to test the performance of a device when encountering unexpected or illegal traffic conditions. To establish and maintain the communication link with a device under test, the exerciser must be capable of emulating a SAS end-device. Exercisers commonly involve programming — typically using a text-based Application Program Interface (API) for authoring test scripts. In addition to error injection, they can also be used for compliance testing and are often utilized side-by-side with an analyzer to capture and view exerciser sessions.
Software-based traffic generators typically sit on top of commercially available host controllers, and allow users to send individual SCSI operations with extensive control of command-level parameters like block size and queue depth. They provide an easy method of stimulating specific traffic patterns, but do not provide error injection below the command level.
Traffic Impairment Systems
Traffic impairment systems are designed to sit in the data path on “live” systems and programmatically alter or corrupt traffic. Unlike traffic generators, impairment systems do not require scripts or programming. They typically monitor an established communication link between two devices — in real-time — and then inject errors either randomly or programmatically. Typically associated with system-level testing, impairment devices offer several advantages. Specifically, they are easy to use and designed to operate using actual workloads to help uncover latent issues that may only surface under “load” testing.
Even though SAS building blocks are just entering production, system integrators will benefit from a stable and mature selection of readily available verification tools. OEMs considering SAS for their next storage platform can amortize their investment in test equipment over the long expected life of SAS technology. Depending on the size and scope of the development project, the return on investment for these tools can take the form of higher performance and reduced time-to-market for SAS-based storage systems.