The 2TB Samsung 850 Pro & EVO SSD Reviewby Kristian Vättö on July 23, 2015 10:00 AM EST
We've been looking at performance consistency since the Intel SSD DC S3700 review in late 2012 and it has become one of the cornerstones of our SSD reviews. Back in the days many SSD vendors were only focusing on high peak performance, which unfortunately came at the cost of sustained performance. In other words, the drives would push high IOPS in certain synthetic scenarios to provide nice marketing numbers, but as soon as you pushed the drive for more than a few minutes you could easily run into hiccups caused by poor performance consistency.
Once we started exploring IO consistency, nearly all SSD manufacturers made a move to improve consistency and for the 2015 suite, I haven't made any significant changes to the methodology we use to test IO consistency. The biggest change is the move from VDBench to Iometer 1.1.0 as the benchmarking software and I've also extended the test from 2000 seconds to a full hour to ensure that all drives hit steady-state during the test.
For better readability, I now provide bar graphs with the first one being an average IOPS of the last 400 seconds and the second graph displaying the IOPS divided by standard deviation during the same period. Average IOPS provides a quick look into overall performance, but it can easily hide bad consistency, so looking at standard deviation is necessary for a complete look into consistency.
I'm still providing the same scatter graphs too, of course. However, I decided to dump the logarithmic graphs and go linear-only since logarithmic graphs aren't as accurate and can be hard to interpret for those who aren't familiar with them. I provide two graphs: one that includes the whole duration of the test and another that focuses on the last 400 seconds of the test to get a better scope into steady-state performance. These results are for all drives 240GB and up.
In steady-state 4KB random write performance the EVO is actually slightly faster than the Pro, but given that the EVO employs more over-provisioning (12% vs 7%), it's not out of the ordinary. The 2TB EVO performs slightly lower than the 1TB model, so it seems that despite the upgraded DRAM controller the controller may not be ideal for more than 1TB of NAND (internal SRAM caches and the like are the same as in the MEX controller). Nevertheless, the difference isn't substantial and in any case the Pro and EVO both boast excellent steady-state performance.
Both 2TB drives also have great consistency, although the 2TB Pro can't challenge the 512GB Pro that clearly leads the consistency graph. Given the same amount of raw processing power, managing less NAND is obviously easier because the more NAND there is, the more garbage collection calculations the controller has to process, which results in increased variation in performance.
The behavior in steady-state is similar to other 850 Pro and EVO drives, which is hardly surprising given the underlying firmware similarities. One area to note, though, is the increased performance variation with additional over-provisioning (OP), whereas especially the 512GB Pro presents very consistent performance with 25% OP. In any case, performance with additional OP is class-leading in both Pro and EVO.