Authors: Michael Sebesta and Joseph Rebrovich
System Integration Test Lab
Hitachi Global Storage Technologies
Serial Attached SCSI (SAS) is a new serial interface for enterprise hard disk drives that will eventually replace aging parallel SCSI technology. The new serial interface provides much higher data transfer rates, improved scalability and superior reliability. SAS is also compatible with Serial ATA (SATA), which has become increasingly popular for use in nearline and entry-server applications. The SAS architecture allows IT managers to deploy either high-performance SAS or cost-effective SATA drives in the same storage enclosure, providing unprecedented design flexibility for server and storage subsystem deployments.
New storage interconnects can often present system integration challenges. SAS is no exception, although SAS’ unique development approach has, in many ways, helped to minimize the challenges.
One of the primary interoperability issues to arise is related to different interpretations of the new standard. With SAS, the standards committee did a thorough job of documenting the standard and gaining consensus to avoid misinterpretation. Because it was so clearly defined, the specification standard was agreed upon and promoted by all the industry suppliers, from the major hard drive companies to the storage integrators.
The SCSI Trade Association (STA) organized the SAS community early on and provided a forum for cooperation. Examples of this include the multiple SAS Plugfest events (held at the University of New Hampshire’s InterOperability Laboratory), which have provided a forum for companies developing SAS products to actually test their interoperability with other companies’ products before the start of normal production.
There are other attributes of the SAS technology itself that combine to ease integration challenges. Some of these include:
- Signaling Issues:
With parallel SCSI, IT managers had to be cautious about intermixing drives from various suppliers. The drives would each present a slightly different set of bus characteristics, in terms of signal properties and consistency. This could result in bus utilization or access issues. Due to the point-to-point and point-to-multipoint architecture of SAS, signaling issues are almost non-existent, even when using drives from multiple suppliers within the same enclosure. - Easily Scalable:
A significant limitation of the parallel SCSI interface is that it only supports up to 15 drives on a single bus. SAS improves drive addressability and connectivity with the use of fan-out and edge expanders, reducing addressing conflicts. Fan-out expanders can address up to 128 edge expanders and each edge expander can address up to 128 SAS devices for a total connectivity of 16,384 SAS devices. This allows large amounts of storage to be easily aggregated over multiple SAS links. - Maximum Flexibility:
IT managers are increasingly implementing tiered storage by combining SATA and SAS drives in a single enclosure to provide exactly the right disk drive/interface for a given application. Fibre Channel customers are also taking advantage of this tiered storage approach by using SAS/SATA drives within an enclosure and running Fibre Channel “outside of the box.” Fibre Channel customers can run longer cable lengths between their external storage systems and maximize their existing infrastructure investments. - SAS Analysis Tools:
Some of the most important SAS drive analysis and debugging happens long before customers begin working with their new drives. For example, test labs are using drive analysis tools that are newly available for SAS and far more robust than anything previously available for parallel SCSI. Labs use the new protocol tools to analyze drive and system behavior by manipulating frames of data to determine how the drives handle unexpected events. Injecting errors into drives with “jammers” allows test labs to project how the drive will manage error recovery and similar tasks over its lifetime. IT managers and other end-user customers benefit from drives that are more reliable in the field with greater resistance to integration issues. - Dual Ports / Wide Links:
Traditional parallel SCSI is limited by a single 320 MBps bus architecture. The dual ported SAS drives provide simultaneous command handling or data path redundancy. For system integration purposes, this allows the system to connect to either port for end user data reliability and maximum performance. In addition, wide links between SAS initiators and expanders currently provide up to 12 Gbps.
Together, these actions have helped to accelerate delivery of SAS products to the marketplace and will continue to smooth the industry transition from parallel SCSI to SAS.
