SAS: reinventing flexible storage in the enterprise.

The advent of Serial Attached SCSI (SAS) answers the growing demand for more flexible solutions that deliver both higher performance and greater cost effectiveness. Tightening IT budgets certainly play a key role in this movement, but it is also driven by a fundamental change in the nature of enterprise data management. A new concept, near-line or secondary storage, has become increasingly popular because it recognizes that data has a lifecycle, during which its need for accessibility and security will vary. Rather than a simple online/offline dichotomy, enterprise storage now requires a continuum along which data can easily and economically migrate as its value and relevance change.

From its inception, Serial Attached SCSI was designed to be the cornerstone of this continuum, retaining its parallel predecessor's renowned reliability while utilizing an advanced serial architecture to set new standards for performance and scalability. While these attributes enable it to efficiently function in a broad variety of applications, it is Serial Attached SCSI's compatibility with Serial ATA (SATA) that most significantly contributes to its unprecedented flexibility. The authors of the Serial Attached SCSI standard well understood SATA's emerging role in the enterprise as a near-line storage solution for non-critical data and backups/restores, and they recognized the synergies (both fiscal and physical) that would result if SAS and SATA drives could be consolidated in a single storage subsystem.

Background: SCSI and the Road to Serialization

SCSI has been the workhorse of enterprise storage for over two decades. Originally designed to surpass the slow transfer rates of contemporary serial technology by utilizing multiple signal paths, parallel SCSI combined exceptional speed with rock-solid reliability. For many years, it steadily evolved in performance and, with the additional flexibility afforded by various RAID configurations, was able to keep pace with growing data storage requirements. But in recent years, the enterprise storage landscape has changed dramatically with the rapid growth of the Internet spurring explosive demand for online/transactional storage solutions that delivered data with high availability and reliability. The traditional Direct Attached Storage (DAS) model has been augmented by Storage Area Network (SAN) and Network Attached Storage (NAS) environments.

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During this period, SCSI received further speed boosts, but its parallel bus architecture had become increasingly problematic--demanding ever more precise timing to properly synchronize the arrival of multiple signals. In addition, its daisy-chain configuration offers only limited scalability (15 drives per bus), and the presence of multiple devices competing for bandwidth slows throughput. Furthermore, this shared-signal bus is susceptible to data degradation from both signal interaction and external (RF) interference.

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While parallel SCSI wrestled with its inherent limitations, Fibre Channel (FC) arrived with a sophisticated serial interface to achieve stunning throughput and scalability, soon superseding parallel SCSI as the premium storage solution for mission-critical data in large enterprises. For mid-size enterprises, Fibre Channel's infrastructure cost was daunting, limiting the applications in which it was appropriate or practical. Thus, many mid-size firms soldiered on with parallel SCSI for mission-critical use, while investigating ways to move their vast quantities of non-critical, infrequently accessed data from online SCSI subsystems onto less costly near-line alternatives.

The debut of affordable, voluminous Serial ATA drives handily addressed this near-line storage need. SATA provides the requisite high capacity at low cost and offers vastly faster access (at appropriate capacities) than tape. However, SATA lacks the scalability, performance and reliability crucial for high-availability, mission-critical enterprise duty. The burgeoning enterprise storage continuum had strong serial solutions at the high end (FC) and low end (SATA), but the critical midrange was conspicuously lagging behind. While parallel SCSI was reaching the practical limits of its developmental potential, serial interfaces had gained a new lease on life, thanks to recent breakthroughs in VLSI technology and high-speed serial transceivers. The solution was obvious.

Quite simply, serial architecture liberates SCSI from the constraints of its parallel past. Serial Attached SCSI retains the proven strengths of its predecessor (rock-solid reliability, a rich and mature command set) while achieving blazing throughput (3.0 Gb/s) and remarkable scalability (over 16,000 devices through the use of expanders). But perhaps SAS's most compelling feature is its compatibility with SATA, ensuring unprecedented freedom to specify and consolidate the most appropriate, cost-effective storage solutions for a broad range of applications. With the addition of SAS, the enterprise storage continuum is now complete, offering a comprehensive range of solutions that address storage needs at every level.

SAS: Flexibility is the Key

The flexibility offered by SAS's new serial architecture is extensive and far-reaching, benefiting key criteria such as speed and scalability. Furthermore, by marrying serial technology with established SCSI command sets, SAS ensures the ability to leverage investment in existing SCSI software and middleware. Most compellingly, SAS's compatibility with SATA introduces a degree of storage flexibility and freedom of choice previously unknown to IT professionals.

Performance

With a transfer rate of 3.0 Gb/s and a clear roadmap to 12.0 Gb/s, Serial Attached SCSI accelerates storage to remarkable speeds across SAS's, point-to-point architecture. Full-duplex operation enables signal transfers to take place in both directions simultaneously, doubling effective throughput. By contrast, SATA is restricted to half-duplex operation, in which data transfer in one direction must be completed before transfer in the opposite direction can occur.

Point-to-point cabling ensures maximum bandwidth by providing a dedicated signal path for each device--no more shared bandwidth slowdowns and bottlenecks. In addition, this means the end of parallel SCSI's daisy-chain address conflicts and termination headaches. SAS performance is further enhanced by adopting the most advanced command queuing, ensuring superior throughput under high-demand enterprise traffic conditions such as e-commerce applications, which entail frequent, concurrent access by multiple initiators or hosts.

Scalability

As enterprise storage requirements relentlessly expand, the ability to seamlessly increase capacity and throughput is paramount. Such scalability was never among parallel SCSI's strengths, and a key goal of Serial Attached SCSI was to vastly improve the ease with which drives could be added. In concert with SAS's point-to-point architecture, switches (known as expanders), enable quick aggregation of many drives, allowing a single SAS domain to contain up to 16,384 devices while preserving performance.

Furthermore, the compact cabling and connectors employed by SAS (and SATA) free up valuable space inside enclosures, improving airflow and cooling while making drive installation and removal simpler. Hot plug capability enables drive swapping without system shutdown.

Investment Protection

The appeal of any new technology can quickly fade if it requires discarding existing, expensive infrastructure. Serial Attached SCSI was specifically designed to leverage current investment in storage management and enterprise application software by integrating the same SCSI command sets that support today's enterprise storage environment. Migration to SAS is straightforward, without the need for system-level workarounds as required by ATA.

And SAS drives will be available in both 3.5-inch and 2.5-inch form factors, the former ensuring compatibility with existing enclosure form factors while the latter's compact size opens exciting new possibilities in high-density computing environments.

Compatibility

Compatibility with SATA is key to the enhanced flexibility SAS brings to enterprise storage. It's no secret that pressure to contain costs and increase productivity continues to escalate, and savvy IT managers are constantly looking for ways to consolidate and rationalize their enterprise storage mix.

Extensively tested by industry leaders such as Seagate and LSI Logic, Serial Attached SCSI cables/connectors, backplanes and host bus adapters (HBAs) are fully compatible with SATA drives. Furthermore, SAS employs three different protocols to transport information over its serial interface, one of which (Serial ATA Tunneling Protocol, or STP), enables SAS controllers to identify and communicate with SATA devices.

The result? SAS provides the path through which deployment of SATA in the enterprise offers the greatest strategic benefit--the freedom to select the most appropriate disc drive for a given task. SAS disc drives are clearly the best choice for mission-critical enterprise use where transactional/online performance and reliability are crucial, while SATA disc drives are cost-effective for lighter-duty use such as near-line and backup/restore storage. SAS and SATA drives can be consolidated on a common back-plane and housed in a single enclosure, enabling one SAS-based subsystem to economically handle the full gamut of enterprise storage duties.

SAS and SATA: Compatible, Not Interchangeable

To fully benefit from the remarkable flexibility SAS/SATA compatibility offers, it's important to remember the fundamental differences between these complementary serial-based storage solutions. SAS was designed and engineered for high availability, enterprise-class data storage where random-read performance, reliability and data integrity are absolutely paramount. SATA is a desktop-class solution that offers high capacity at low cost, and impressive through-put on sequential reads (e.g., backup and restore operations).

Recent demonstrations jointly conducted by Seagate and LSI Logic at Storage Networking World in April and WinHEC in May clearly illustrated the superior IO/sec performance of SAS. In tests of random reads and writes of 100/MB files, SAS measured five times faster than SATA (SAS returned an average benchmark score of 250 IO/sec, while SATA averaged 50). In sequential reads and writes, the two interfaces scored virtually the same.

Pressured to reduce costs and flushed with enthusiasm for SATA's apparent price/performance advantage, some IT managers have begun deploying SATA drives in enterprise roles for which they are wholly unsuited. Keep in mind that SAS and SATA drives are vastly different. SAS drives are engineered for rugged enterprise duty and every component (drive motor, spindle, actuator, magnetic recording heads, control and servo processors, firmware, etc.) is specifically designed and manufactured for that rigorous use.

Conclusion

As IT budgets continue to undergo intense scrutiny, the pressure to rationalize and consolidate enterprise storage systems has never been greater. Clearly, the more flexibility an IT manager has to choose the most targeted and efficient solutions, the more likely optimal performance and cost-effectiveness will be achieved. Serial Attached SCSI provides that flexibility, ensuring unprecedented freedom to create high-performance storage solutions that can be easily adapted, reconfigured and scaled as needs inevitably change.

    Device       Duty Cycle   MTBF (Typical)   Seek Time (Typical)

SAS (15K rpm)    24 hrs/day   1,200,000 hours  3.6 ms
                 7 days/week

SATA (7200 rpm)  8 hrs/day    600,000 hours    9.5 ms
                 5 days/week

     Solution             Application                  Notes

Fiber Channel       Online, high           Suitable for
                    availability, random   mission-critical data in
                    reads                  large enterprises [e.g.,
                                           Storage Area Network
                                           (SAN)], support up to 16
                                           million addresses and
                                           maximum cabling distance of
                                           ten kilometers, relatively
                                           costly

Serial Attached     Online, high           Suitable for
SCSI (SAS)          availability, random   mission-critical data in
                    reads                  mid-size enterprises, high
                                           performance coupled with
                                           exceptional scalability on
                                           local level, more
                                           affordable than FC,
                                           compatible with SATA

Serial ATA (SATA)   Near-line, low         High capacity at low cost,
                    availability,          not suitable for
                    sequential reads       mission-critical data;
                                           Suitable for high-capacity,
                                           low $/GB non-mission
                                           critical data storage,
                                           enables tiered storage in
                                           SAS systems

Tape Libraries      Offline                High capacity, ability to
                                           move data offsite for
                                           archival and security

Franco Castaldini is product marketing manager at Seagate Technology (Scotts Valley, CA), with additional contribution by Kent Bransford.

http://specials.seagate.com/sas

www.scsita.org