Results: Performance Consistency
Increasingly, we pay close attention to the performance consistency of enterprise-class SSDs. This is what separates a good drive from a great one when all of the corner case testing seems equal. Over the past year, we measured this in terms of large-block transfers in our Enterprise Video Streaming section. Armed with that data and our exclusive analysis, the peaks, valleys, and frequency of each became clear. If you look at the information for long enough, you start to see fingerprints for each drive.
We started with large-block transfers because, in enterprise video applications, if you don't buffer or write data fast enough, you can lose it completely. Random 4 KB transfers are slightly more academic, but they also emulate database transfers pretty well. With this sort of workload, you might not actually lose anything, but your system will certainly slow down.
In the following tests, we subjected our enterprise SSDs to 25 hours of continuous random 4 KB writes across each drive. We recorded the IOPS every second, giving us 90,000 data points. We then zoomed in to the last 60 minutes to more coherently visualize the results.
As the graph clearly shows, Intel and Seagate take completely different approaches to latency. At one end, you have the SSD DC S3700, which delivers rock-solid performance with very little variation. On the other end, you have the S3500 that has higher latency and variance. When you zoom in on the data, though, the those "fingerprints" I mentioned earlier look almost identical. Even the histograms look similar. This comes as no surprise, since both Intel SSDs use the same controller.
Then there is Seagate's 600 Pro. Even though its average latency and IOPS are closer to the SSD DC S3700, the variance is so large that it approaches the maximum observed from Intel's SSD DC S3500.
A look at the histogram makes it easy to notice the two-level distribution of latency. Two peaks around 2.7 ms correlate to ~12,000 IOPS, while the peaks around 3.25 ms represent slightly less than 10,000 IOPS.
Intel makes it a point to specify the performance consistency of its enterprise SSDs. With the 480 GB SSD DC S3500, IOPS in the 99.9th slowest one-second interval should be within 75% of the overall average. In our testing, we recorded results closer to 80%.