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SSDs In RAID: A Performance Scaling Analysis

I/O Performance In Abundance

Sequential read rates of more than 200 MB/s no doubt speak for SSDs, but whether the drive peaks at 230 or even 260 MB/s is of less importance when it comes to professional use. What is more important, especially in servers, is the number of I/O operations such a system can handle per second. Because the access times of SSDs are in the microsecond range, they outclass traditional hard drives in this respect.

HDD versus SSD: where magnetic drives can only handle a few hundred I/Os per second, current SSDs are delivering five-digit values. Source: Samsung

Using the right flash drive can increase the I/O performance at a factor of up to three digits, according to Korean manufacturer Samsung. The image above shows the I/O performance of Samsung’s current generation of SSDs compared to a 15 000 RPM SAS hard drive, demonstrating a whooping 106-fold performance increase. In our own testing, we found this performance advantage to be similar on many flash drives. In our comparison tests of the best SSDs we particularily noticed major differences in I/O performance between the various participants. But, apart from some unsuitable candidates, even the lower- to middle-class SSDs outperform current enterprise hard drives by a factor of at least 10, on average.

Power Saving With SSDs

Another important reason for the use of SSDs in enterprise environments is the lower power consumption of the devices. While good flash drives never exceed their maximum load claims of 2 W, and do not use even 10% of that at idle, enterprise hard drives frequently use 10 W or more. These are all certainly very low numbers, but important nevertheless.

Server hard drive installations in data centers are real power hogs, costing accordingly. The Fraunhofer Institute and market analysis group IDC estimate that air conditioning makes up about 50% of the total power consumption in data centers. Obviously, that's significant. With SSDs however, low power consumption and high I/O performance go hand-in-hand. The I/O per watt ratio is both reliable and often quoted, ensuring that energy efficiency stays relative to performance, which also speaks for the use of flash drives.

Expandability Of SSD RAIDs

For enterprises, it may well be worth the cost to replace a hard drive-equipped server with a flash-based system. Take the significantly higher I/O values, lower energy consumption, and heat generation into consideration and the calculation may still favor SSDs, despite the individual flash drives being more expensive to replace and storage capacities almost always smaller. And of course, you have to take into account how many SSDs would be needed to enhance the performance of an existing multi-hard drive system.

However, the most interesting question, by far, is about the expansion capability of an SSD server. We have a test system up and running to examine the scalability of SSD RAID arrays.