Serial Attached SCSI (SAS) is the next-generation replacement of SCSI Ultra320. SAS is a serialized protocol. The serial interface offers several benefits over the existing SCSI parallel bus; allowing for high availability, complex storage topologies, and a lower cost structure.
High Availability: As with SATA, Serial Attached SCSI is a high-speed, point-to-point technology initially designed to operate at speeds up to 6Gb/sec. System designers have found that point-to-point serial connections are inherently more reliable than shared bandwidth parallel connections. As a result, point-to-point serial connections have become the preferred method for implementing high-availability systems. SAS based disk drives are dual ported, taking advantage of this feature. This is a key requirement of high-availability SANs that call for redundant paths to all devices in the system.
Complex Storage Environments: SAS storage environments usually consist of a mixture of disk drives, host bus adapters and expanders. SAS allows for addressing of over 16,000 devices. SAS Expanders are routing devices that forward service requests between the source and destination devices. Expanders can be attached to other expanders, disk drives, and other hosts. An important characteristic of Expanders is their ability to aggregate bandwidth by combining multiple physical links into a "Wide" port. These wide ports can function as intelligent switches and can independently send commands across one physical path with data being returned over a different physical path. This increases the complexity of SAS test and debug because it requires users to monitor multiple links concurrently to record all DWORDs associated with a single SCSI transaction. The usage of expanders also allows for usage of a storage concept called Zoning. Zoning allows administrators to restrict access to devices by other devices. Applications of this include separating storage systems within a company by department.
Lower Cost Structure: Serial Attached SCSI also contains support for SATA drives in its storage environments. This allows support for both enterprise-class performance offered by SAS and low cost storage with SATA drives in the same system. Since the cost of the SAS infrastructure is expected to be comparable to SATA, it is likely that SAS will be the preferred interface. Since both SAS and SATA drives can be used in this environment, it will simplify and standardize the storage infrastructure, allowing for the ability to choose which drive will best fit individual storage needs.
SAS Test and Verification: LeCroy's protocol analysis tools are designed from the ground up to address these unique characteristics of SAS including wide-links and Serial ATA interoperability. Sierra’s analysis and design suites are developed specifically for use with the SAS and SATA protocols, and provide extensive protocol decoding, expert error analysis, and complete user support when decoding and viewing the recorded traffic. This extensive protocol support, combined with the different traffic views, advanced triggering, data filtering, traffic generation, and error injection capability, allows engineers to rapidly become familiar with SAS- and SATA-specific issues, and quickly understand new issues the first time they encounter them. Every engineer becomes a protocol expert with the support of Sierra’s detailed expert analysis.
