Trends in storage: Low-Cost High-Capacity Arrays

In an earlier post I talked about the trend in cost-effective, high-performance arrays being focused on the use of 10K/rpm 2.5" SAS hard drives due to their cost per GB, low power consumption, high performance, and total capacity per rack unit as compared to 15K/rpm 3.5" SAS drives. Today I'm writing about high capacity solutions where total cost per GB and the highest possible capacity per rack unit are the main criteria for a server solution design.

While 10K/rpm 2.5" SAS hard drives are the reigning king for high performance, when it comes to solutions where performance is not critical, but capacity is, they are not ideal. The largest shipping 2.5" SAS drive today has a capacity of 600GB. In a 2U chassis with twenty-four drive slots the total raw capacity provided by these high end drives is 14.4TB. In contrast, the same basic chassis with twelve 7.2K/rpm 3.5" SATA or NL-SAS with current peak capacity of 2TB each can house a total raw capacity of 24TB in the same rack space. This makes 3.5" SATA or NL-SAS hard drives more ideal for high capacity when performance isn't critical.

The industry has been offering both 2.5" and 3.5" SATA and NL-SAS drives for years, but unlike the high-performance SAS hard drive market, the 2.5" SATA and NL-SAS drives do not provide greater value than the 3.5" SATA and NL-SAS hard drives. 

Since the platters used for enterprise class 2.5" SATA drives aren't also used for high volume desktop or mobile solutions, the economy of scale created by the leveraged 3.5" SATA drives simply isn't achieved. As such, 3.5" SATA platters are considerably more affordable per GB than their equivalent 2.5" brethren.

One interesting twist in this discussion is the introduction of Near Line SAS (NL-SAS) several years ago by Dell and some other providers. While the physical spinning platters and drive mechanics of a 7.2K/rpm SATA drive offer the highest capacity per dollar and rack space, utilizing an enterprise class SAS controller on those platters results in many benefits which justify the very minimal cost adder for the controllers over SATA.

When you compare SAS to SATA, the SAS controllers provide a more robust signaling, bi-directional dual-port communication, tagged command queuing, advance SAS error-recovery and error-reporting and other high end enterprise capabilities not available with SATA. As such, I generally recommend NL-SAS over SATA when it comes to high capacity storage arrays for server applications.

When my customers are asking for the absolute highest capacity at the lowest possible cost, I recommend 3.5" NL-SAS arrays. There are instances where the equivalent SATA solution is preferred, but that is rare. If the highest performance solution at reasonable prices is called for, then 10K/rpm 2.5" SAS drive arrays are the way to go.

While these guidelines are great for today and probably ideal for the next year or so, we are nearing the point where flash-based solid state drive technology will become acceptable, or even more ideal, when the absolute highest performance is required.

Do you agree? How are you managing your drive technologies for high-performance environments?

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About the Author: Franklin Flint