Seagate ST3146855LC Optimizing Storage with SAS: Beyond the 10K Compromise (72 - Page 3

Online Storage: 15K SAS Is King Online storage application requirements can be neatly summed up with an age-old aphorism: Time is money. Transactional data is the very lifeblood of many enterprises, and online storage is all about keeping that data flowing. In the online world, diminished throughput and bouts with downtime are far more than inconvenient; they often translate into substantial lost revenues. The advanced architecture of SAS (point-to-point connectivity, full duplex operation, dual ports) eliminates the significant performance bottleneck imposed by parallel SCSI's shared-bus topology. It makes little sense to deploy a SAS infrastructure and then effectively reintroduce a performance bottleneck by using slower SAS drives. While both 10K SAS and 15K SAS drives offer exemplary reliability in transactional environments, only 15K SAS can boast the highest available throughput for online applications. Simply put, 3.5-inch 15K SAS disc drives are purpose-built to address the performance deficiencies of 10K SAS drives, and in so doing enable greater cost-effectiveness in storage. Requiring fewer drives to provide equal or better performance, 15K drives deliver lower TCO by ensuring: • Less supporting infrastructure • More enclosure/rack space for nearline storage • Lower maintenance and storage-management costs • Greater reliability due to fewer potential points of failure • Higher IOPS/U 15K drives achieve their superior performance by providing quicker access to data. Of course, 15K drives spin their internal discs faster which reduces latency. But they also utilize smaller diameter discs (approximately 2.6 inches vs. 3.0 inches for many 10K drives) to shorten the distance an actuator must move, decreasing seek time. Costly 10K Kludges Some IT departments may attempt to meet their online application needs with 10K SAS drives, employing unorthodox techniques to wrest improved performance from them. For example, they might deploy lower-capacity drives to ensure each drive accesses less data, thus lowering drive access times. In effect, these departments would be purchasing more actuators to simultaneously access their data. Or they may resort to storing data only on the outer diameter of the disc, a practice referred to as short-stroking (or destroking). This reduces the distance the actuator must move to access the data, thus improving disc seek time at the cost of severely limiting effective capacity. Both of these techniques vastly increase storage costs due to the high number of drives needed to meet a given capacity requirement, and this also drives up associated costs such as infrastructure, required space (data center ft3 and rack U), power consumption and cooling, and storage maintenance. Nearline Storage: Nearline SATA vs. 10K Nearline applications can be divided into two categories: data protection and reference data. Both entail data that doesn't justify the cost of high-availability, mission-critical storage, but must still be readily accessible at any time. While nearline data activity is far less frequent than online activity, both involve random reads/writes that force drive heads to rapidly and repeatedly traverse a drive's discs. To deliver the nearline-class reliability standard of 1.0 million hours MTBF, nearline-ready SATA drives are specifically designed to withstand the rigors of random reads/writes and 24x7, always-on operation. In contrast, the typical 600,000 hours MTBF rating of desktop-class SATA drives is obtained in the mild environment of sequential reads/writes and 8x5 power-on hours, and thus has no relevance when considering the use of such drives in nearline applications. Nearline-ready SATA drives also incorporate Workload Management to dynamically protect them from excessive peak workloads. To further safeguard reliability, these drives perform "offline scans" during drive idle time to periodically test the media surface for defects. Designed for online applications, 10K SAS drives offer even greater performance and reliability than nearline-ready SATA drives. However, when used in less demanding nearline applications these performance and reliability advantages are largely academic. More importantly, 10K SAS drives are far more expensive in terms of capacity, with cost/GB roughly four times higher than nearlineready SATA drives. 3