The Third Wave of Scalability

Authors: Franco Castaldini and Kent Bransford
Seagate Technology

The arrival of Serial Attached SCSI (SAS) marks a new era in storage scalability, wherein both the type and quantity of drives can easily be optimized. SAS compatibility with Serial ATA (SATA) enables seamless deployment of desktop-class SATA drives and enterprise-class SAS drives in the same SAS domain, giving IT managers unprecedented flexibility to specify the most appropriate drive for both online (transactional, high availability) and near-line (archival, low availability) duties. Moreover, employing a common SAS infrastructure for both types of drives minimizes hardware redundancy, further enhancing efficiency.

The Evolution of Scalability
Emerging in the early days of enterprise storage, the first wave of scalability was simply defined by the ability to increase the number of drives within a given environment–a purely quantitative criterion. Parallel SCSI addressed the need for such scalability by ensuring it could accommodate up to 15 drives on a single, shared data bus. But IT professionals soon learned parallel SCSI’s scalability imposes a significant cost–degraded performance. The more drives vying for access to the shared bus, the lower effective drive throughput drops.

A more sophisticated approach to scalability was clearly needed, one that incorporated the quantitative criteria of both drive count and drive performance. The second wave of scalability was made manifest with the advent of Fibre Channel, its fabric architecture ensuring throughput is maintained regardless of how many drives are added. But such scalability came at a price: Fibre Channel is a high-end solution, better suited to large enterprises that can justify the substantial infrastructure investment required.

Today’s ever-increasing need for efficiency has spawned the third wave of scalability, which emphasizes not only a storage solution’s quantitative criteria (drive count, performance) but also its qualitative characteristics–the fundamental nature of the devices being deployed. Serial Attached SCSI spearheads this new concept of scalability by matching Fibre Channel’s immunity to performance degradation as drive numbers escalate, and then adding compatibility with SATA to enable deployment of a heterogeneous mix of drives that cost-effectively meet a broad range of storage applications. These strengths, coupled with the proven SCSI command set of its parallel predecessor, give SAS a blend of performance, compatibility and reliability that redefines enterprise scalability.

SAS: Superset of SATA
Leveraging their common serial, point-to-point architecture, SAS encompasses all of SATA’s virtues and then surpasses them with a comprehensive range of enterprise-class capabilities far beyond those of its desktop-centric sibling. Specifically designed as a superset of SATA, SAS is able to synergistically interoperate with SATA, significantly enhancing the value of both technologies.

SAS, of course, is optimized for online, high-availability applications in the most demanding enterprise environments. To that end, it incorporates an impressive array of strengths (rock-solid reliability, rich and mature SCSI command set, advanced command queuing, sophisticated verification/error correction, full-duplex, dual-port operation) to deliver the throughput and dependability mission-critical environments demand. But the very strengths that make SAS an ideal performance solution render it a relatively over-engineered (and costly) choice for near-line, bulk storage chores. Conversely, SATA is ideally suited to such duties, where maximum capacity per dollar supersedes such factors as reliability and high availability.

By offering compatibility between these two complementary storage solutions, SAS enables IT managers to deploy both performance (SAS) and capacity (SATA) drives in a common SAS enclosure. This not only maximizes cost-effectiveness by eliminating the need for redundant SAS and SATA infrastructures (and the management headaches they entail), it also boosts SATA scalability far beyond the limits imposed by SATA-based infrastructures. SAS expanders are high-speed switches that enable a single SAS domain to contain over 16,000 SAS and/or SATA drives; by contrast, SATA Port Multipliers are hub-like devices that can handle far fewer (SATA-only) drives.

Greater storage efficiency continues to be an urgent priority for every enterprise, and SAS is uniquely positioned to facilitate this key goal. SAS was proactively engineered as a superset of SATA, and this innovative architecture pays multiple dividends. Not only does SAS (via its compatibility with SATA) enable IT managers to select the most cost-effective drive for any application, it also vastly improves SATA’s scalability. Furthermore, SAS eliminates the redundancies and inefficiencies of purchasing and maintaining separate infrastructures for high-performance (SAS) and high-capacity (SATA) storage environments. As such, SAS is clearly driving the third wave of scalability in enterprise storage.

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