Understanding SSD Performance Claims

Increases in storage capacity and the amount of sophisticated data processing being performed in modern systems, require a corresponding increase in data transfer speeds. Choosing a device based on exaggerated figures may lead to a system that does not meet its requirements.

It is generally understood that the real-world performance obtained from processors and storage devices is very different from that claimed in datasheets and benchmarks. Here we will look at what this means for flash-based storage such as Solid State Drives (SSD).

In the case of flash memory, this is not just the usual difference between unrealistic peak figures and application performance – even the raw performance numbers may not stand up to scrutiny in practice.

The two main measurements used to characterize an SSD are sequential access, where a large block of consecutive data is transferred to or from the storage device, and random access where smaller amounts of data are read or written to non-contiguous addresses. Sequential accesses are typically measured in megabytes per second (MB/s) while random accesses are usually given as input/output operations per second (IOPS).

The main factors that result in a difference between datasheet numbers and actual performance are:

• The differences between performance “out of the box” and after the device has been in use for some time (the steady state).
• Environmental effects, such as temperature sensitivity.
• Changes to the characteristics of flash memory over its lifetime.

Out of the box performance

We have carefully measured the real-world performance of several flash controllers, our own and some from competitors. To measure the difference between out-of-the-box and steady-state performance we use the CrystalDiskMark benchmark tool, then run a series of random writes for 72 hours before measuring the CrystalDiskMark performance again.

The difference in performance is shocking. In some devices that we have benchmarked, steady state speeds can be as little as 1% of the out-of-the-box performance.

What is more surprising is how quickly the performance falls off. In nearly all the drives tested, the performance decreased dramatically after about one and a half minutes of use.

There are several factors that cause the performance to drop over time. One is that the flash controller has to execute various background tasks such as garbage collection, wear leveling, dynamic data refresh and calibration. The controller can “hide” the effects of these for a short time by prioritizing and scheduling the IO operations. However, for longer transfers these tasks inevitably have some impact on performance. Over the longer term, the time required for some of these operations increases as well.

Other effects include caches becoming filled and losing their effectiveness after long sequences of either sequential or random accesses.

Finally, during use the SSD will get hotter. To protect itself from overheating, “thermal throttling” is employed. This limits the performance in order to reduce the power consumption and thus lower the temperature of the device. The effects of this will be dependent on the ambient temperature, the thermal design of the environment and the workload. This makes the effects on real-world performance hard to predict.