SAS: The Next Evolution of SCSI

The Small Computer System Interface (SCSI) has enjoyed over twenty years as the defacto standard in the storage industry. In recent years SCSI development has appeared to reach its final iteration as it will likely have difficulty in responding to the new challenges of tomorrow’s mission critical application environments — including smaller form factors, scalability, storage optimization and performance. With global commerce and 24/7 information available on-demand now in the forefront, businesses are now turning to a new standard to meet their current and future requirements.

SAS is the newest serial interconnect and successor to iSCSI. SAS has leveraged SCSI technology and improved upon it to offer an improved host interface, as well as drive technology, to meet these increasingly rigorous performance and scalability requirements. With a solid roadmap and industry acceptance behind it, SAS technology is proving to be the next evolution of SCSI. As a large proponent of leading technologies,Sun has been a leader in SAS with a range of SAS platforms available. The most recent of these is with the introduction of the Sun StorageTek 2500 series of arrays based on SAS technology.

SCSI Replacement Requirements

While SAS overcomes many of parallel SCSI’s interface limitations through serial-based technologies, it also retains and leverages the 20 years of proven SCSI development. SAS preserves the SCSI legacy and provides investment protection by retaining SCSI block protocol and SCSI command set. By incorporating this time-tested command set, the Sun StorageTek 2500 arrays with SAS technology protects a users existing investment in SCSI software, middleware and drives. This provides a seamless transition from legacy SCSI interfaces. To have a greater understanding of SAS improvements over SCSI, it is advantageous to look at SAS technology from both an interface and drive perspective.

Interface Improvements

Increased Performance
Leveraging SAS technology, the Sun StorageTek 2500 series of arrays support faster data transfer rates then parallel SCSI. SAS drives also provide full-duplexing allowing simultaneous in-bound and out-bound communication which effectively doubles performance. Full-duplexing inherently provides better performance then parallel connections in which communication occurs in one direction at a time often becoming a bottleneck in some environments. And with 3Gbps SAS wide interfaces consisting of 4channels, aggregate throughput can theoretically achieve up to 24Gbps in full-duplex mode (four 3Gbps channels x2 with full duplex), far exceeding what could be reached by Ultra320 SCSI.

SAS host interface with 4 channels

ScalabilitySAS also breaks free of the 15 drive barrier per channel of parallel SCSI technology and scales with the needs of its endusers. With its high addressability and connectivity, the SAS interface dynamically increases connectivity and can address over 16,000 devices per domain – well beyond SCSI‘s drive maximum with the user of SAS expanders. These expanders enable the necessary fan-out to create highly scalable configurations to support a growing organization and their data. The Sun StorageTek 2500 arrays leverage the SAS scalability and can accommodate up to a total of 36 drives per array ensuring adequate growth support when needed.Smaller Form Factor The smaller form factor of SAS includes several attributes. The serial design of SAS provides more flexible and thinner cabling and smaller connectors. This is versus the large SCSI cables which are typically not suitable for dense computing environments. This allows for improved routing and airflow as well as reduced susceptibility to overheating. And the cross-talk that SCSI at times is prone to is reduced with more predictable and reliable SAS signaling mechanisms.

Drive Technology

Improved Reliability
SAS technology provides enhanced reliability and availability with point-to-point and dual-ported architecture. With point-to-point topology, direct communication with each drive is achieved and each drive is isolated from the other. This allows for problems to be located and remedied more quickly vs. parallel SCSI’s loop topology. With a looped architecture, communication must travel through eachnode to get to the troubled drive. Where multiple drives may be operating on a given loop, it can be extremely difficult to identify which drive is having a problem and the entire system’s performance could be degraded.

Dual-ported technology also provides additional assurance of reliability. The dualported interface provides redundant paths which increases availability to each drive by having an alternate link to maintain connectivity in the case that one link fails.

SAS and SATA Intermix Support
Previously with SCSI arrays, organizations could not take advantage of a tiered storage solution as SCSI drives could not support intermixing with any other type of drive technology. As well, previously available tiered storage solutions, such as with FC and SATA intermix, was leveraged primarily be the enterprise. Due to the cost and complexity of these types of arrays which supported FC and SATA intermix, tiered storage was not a viable option for smaller organizations.

With the ability of SAS drive technology to support intermix with SATA drives, a wide range of businesses can now optimize their applications with the appropriate drive technology. Where SAS drives are notable for high availability and typically utilized for mainstream transaction-based applications, SATA technology is generally deployed for near-line data and can drastically reduce the cost of a storage investment when cost/GB is the most important factor. By optimizing the right workload demand with the appropriate drive technology, the Sun StorageTek 2500 arrays can support the transition to tiered storage without the high cost typically associated with this strategy.

Summary

The market transition from parallel to serial interface technologies is irrefutable. With improved performance, reliability and availability, SAS is a leading serial interface technology and Sun is at the forefront of bringing it to its customers. SAS allows organizations to retain their current legacy investment in SCSI while improving their overall storage performance and scalability with Sun systems such as with the Sun Storagetek 2500 arrays. The Sun StorageTek 2500 arrays leverage SAS technology and is positioned as a new low-cost SAS/SATA disk subsystem providing consolidated RAID to FC and SAS host interfaces. In environments were entry-level storage devices are yet another commodity, Sun in combination with SAS technology, trumps the competition with its industry leading value, simple server-oriented management, performance and flexibility.