By Barbara Craig, Sr. Product Marketing Manager,
Seagate

Storage professionals are faced with unprecedented demand for low-cost, high-capacity Tier 2 (also known as nearline) storage while satisfying internal policies specifying system uptime, reliability and availability of critical data. IT professionals cannot sacrifice internal quality of service requirements in exchange for lower storage acquisition costs. As a result, there's a growing interest in improving storage delivery and productivity to the higher levels of enterprise-class reliability and data integrity.

Critical data typically includes such diverse applications as database, file and print, rich media content like audio and video, reference and compliance data, email and messaging data, web and support for disk-to-disk or virtual tape backup and restore.

Leveraging the attractive economics of desktop drives, drive manufacturers continue to enhance the core technology with enterprise-grade features on more capable SAS products delivering lower acquisition costs (price/GB) and greater drive reliability, data integrity and power efficiencies.

Why SAS for Nearline?
While it may be tempting to focus on the significant performance advantages that Tier 2 nearline SAS drives offer over SATA drives, the more essential benefit to most IT operations is the superior data integrity and reliability that define 6Gb/s SAS storage solutions.

Superior Performance at Drive and System Level
The industry-standard Storage Performance Council (SPC) Benchmark tests simulate real-world workloads, demonstrating hard-drive performance with large-scale, sequential and random movements of data in a system environment. SPC Benchmark 2 sequential test results confirm that some SAS drives perform 40 to 78 percent better than competing SATA drives. (Figure 1) Similarly, SPC Benchmark 1 random tests reported that some SAS drives deliver 6 to 54 percent higher throughput than competing SATA drives.

Figure 1: SPC-2 Sequential Performance Test Results

Optimizing IOPS/Watts
With the growing emphasis on reducing energy consumption and lowering operating costs in the data center, hard drive power use has taken on greater significance. Tier 2 nearline SAS drives have only fractionally greater power consumption than SATA hard drives, yet offer clearly superior throughput, resulting in higher IOPS/watt performance. SAS drives clearly surpass SATA drives with regards to power/performance drive efficiencies. (Figure 2)

Figure 2: Operating Power and Performance Efficiencies

Greater Data Integrity at System Level
According to An Analysis of Data Corruption in the Storage Stack, a large-scale study of data corruption funded by the National Science Foundation, physical interconnect malfunctions cause up to 68 percent of storage subsystem failures. Lacking inherent dual-port failover capability, SATA systems often report such interconnect failures as a "drive not found" error resulting in a misdiagnosis as a drive failure instead of the silent data corruption that actually occurred while the data was in flight. SAS reduces data corruption with end-to-end IT Nexus checking, a key component of data integrity that ensures data traveling to or from the drive is never misdirected. SATA hard drives cannot achieve this level of enterprise-class data integrity because they lack native addressability (extra-cost interposers provide that capability). SAS also decreases storage system failure rates by reducing the number of physical interconnects and by adding dual-port redundancy. Users will also have better access to more advanced error reporting, which provides valuable information to debug the issue more effectively.

Simplicity and Continuity in Data Center Deployment
Reluctant to deploy new storage solutions that entail disruption in the data center? Tier 2 nearline SAS hard drives provide a welcome exception, seamlessly integrating into the same SAS infrastructures that currently support Tier 1 mission-critical data storage. Now, true-tiered storage across the enterprise is a reality. And, SAS command queuing enables one drive to accommodate up to 16 hosts, compared to the single host that a standard SATA drive can accept via NCQ¹. This also helps avoid the single-point-of-failure of a SATA interposer card and by reducing parts count, reliability goes up. Furthermore, SAS drives ensure operational continuity and investment protection thanks to their compatibility with existing SCSI software and middleware.

Summary
Enterprise-class Tier 2 drives are purpose-built and optimized to handle the rigors of 24x7 operation in the data center alongside their mission critical siblings. With the addition of the SAS interface, these drives now provide the most robust data integrity and performance delivering the maximum capacity, superior performance and rugged reliability that the vast array of nearline applications require.

¹ http://en.wikipedia.org/wiki/Native_command_queuing

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