Seagate 15K.2 Inflection Point - The New Era of Serial Attached SCSI - Page 2

SCSI Inflection Point: Standardizing on Serial Attached SCSI (SAS) The SCSI Inflection Point Flash forward to the present, where the technological landscape has been radically transformed. Dramatic advances in processor speed, RAM size and RAM speed have combined to accelerate system performance to levels unthinkable just a few years ago. Yet one aspect of system capability has conspicuously lagged behind: parallel SCSI drive performance. Of course, remarkable advances in areal density have yielded exponential growth in drive capacity, but parallel SCSI transfer rates have achieved only modest gains. And those gains have come grudgingly, as the inherent limitations of the parallel, shared bus architecture have made each speed enhancement increasingly problematic and costly. Yet while parallel SCSI is clearly approaching its practical performance limits, serial interfaces have gained a new lease on life due to recent breakthroughs in Very Large Scale Integration (VLSI) technology and high-speed serial transceivers. SCSI technology has reached a fundamental inflection point, where the constraints of parallel interfaces have discouraged further development, while the burgeoning potential of serial interfaces has already delivered remarkable performance and scalability benefits. To better understand the significance of this inflection point, as well as the specific challenges presented by parallel SCSI and the corresponding advantages of Serial Attached SCSI (SAS), a summary table and detailed analyses are provided below. Parallel SCSI: The Art of Compromise The very elements that underpinned parallel SCSI's initial appeal (multiple data paths for greater throughput, a shared bus to enable easy connection of multiple SCSI devices) gradually became stubborn obstacles on the road to improved performance and scalability. Over the years, juggling design parameters (for example, increase the clock rate but shorten cable length) to circumvent SCSI's inherent limitations has wrought hard-won advances, but in recent times the shortcomings of its parallel, shared-bus architecture have become increasingly apparent. The SCSI Inflection Point: Transitioning From Parallel SCSI to Serial Attached SCSI (SAS) Architecture Performance Scalability Compatibility Max. Cable Length Cable Form Factor Hot Pluggability Device Identification Parallel SCSI Parallel, all devices connected to shared bus 320 MB/s (Ultra320 SCSI); performance degrades as devices added to shared bus 15 drives Incompatible with all other drive interfaces 12 meters total (must sum lengths of all cables used on bus) Multitude of conductors adds bulk, cost No Manually set, user must ensure no ID number conflicts on bus SAS Serial, point-to-point, discrete signal paths 3 Gb/s, roadmap to 12 Gb/s; performance maintained as more drives added Over 16,000 drives Compatible with Serial ATA (SATA) 8 meters per discrete connection; total domain cabling thousands of feet Compact connectors and cabling save space, cost Yes Worldwide unique ID set at time of manufacture; no user action required Termination Manually set, user must ensure proper installation and functionality of terminators Discrete signal paths enable devices to include termination by default; no user action required 2