Performance Consistency - Average IOPS

Performance consistency tells us a lot about the architecture of these SSDs and how they handle internal defragmentation. The reason we do not have consistent IO latency with SSDs is because inevitably all controllers have to do some amount of defragmentation or garbage collection in order to continue operating at high speeds. When and how an SSD decides to run its defrag or cleanup routines directly impacts the user experience as inconsistent performance results in application slowdowns.

To test IO consistency, we fill a secure erased SSD with sequential data to ensure that all user accessible LBAs have data associated with them. Next we kick off a 4KB random write workload across all LBAs at a queue depth of 32 using incompressible data. The test is run for just over half an hour and we record instantaneous IOPS every second.

We are also testing drives with added over-provisioning by limiting the LBA range. This gives us a look into the drive’s behavior with varying levels of empty space, which is frankly a more realistic approach for client workloads.

Each of the three graphs has its own purpose. The first one is of the whole duration of the test in log scale. The second and third one zoom into the beginning of steady-state operation (t=1400s) but on different scales: the second one uses log scale for easy comparison whereas the third one uses linear scale for better visualization of differences between drives. Click the dropdown selections below each graph to switch the source data.

For more detailed description of the test and why performance consistency matters, read our original Intel SSD DC S3700 article.

Samsung 845DC PRO 400GB

The 845DC PRO is just amazing. While it only has 28% over-provisioning, the 845DC PRO is able to provide 50K IOPS while for instance Intel's SSD DC S3700 is only capable of 35K IOPS, even though both have the same over-provisioning. A part of that can come from the controller and firmware design, but it is obvious that the lower program and erase latencies of V-NAND are behind the performance. It looks like Samsung's heavy investment in 3D NAND technology is really paying off.

The 845DC EVO does very well too despite the slower TLC NAND as it is still able to achieve steady-state performance of ~10K IOPS. The impact of NAND management from higher capacity is evident since the 240GB 845DC EVO performs better than the 960GB version, although I need to run a longer test for the full review to see if the difference evens out.

The PM853T, on the other hand, is a weird case because it is considerably slower than the 845DC EVO. I asked Samsung about this and they told me that there are some differences in garbage collection and wear-leveling algorithms, which causes the performance to be bumpy at first but it should even out after about 3,000 seconds. I will confirm this in the full review.

Samsung 845DC PRO 400GB

Samsung 845DC PRO 400GB

Samsung PM853T Performance Consistency - Worst-Case IOPS
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  • hojnikb - Wednesday, September 3, 2014 - link

    Looks like, they ain't doing Turbowrite on TLC models :) Reply
  • Kristian Vättö - Wednesday, September 3, 2014 - link

    TurboWrite wouldn't make any sense on an enterprise drive because all that matters is sustained performance. Client drives are a different case because IO activity tends to happen in bursts, so having a fast buffer is beneficial (and it makes the drive look better in benchmarks). Reply
  • hojnikb - Wednesday, September 3, 2014 - link

    Yeah i figured as much. Aside from being more suited for short bursts (as you'd said) its really great for marketing department, since they can slap magical 500MB/s+ speeds, that every uninformed buyer is after. 500MB/s sure sells a lot better than 250MB/s (or even less, if we're dealing with lower capacities) :) Reply
  • Spirall - Wednesday, September 3, 2014 - link

    Thanks for the article. Would suggest to make a $/manufacturer declared write endurance comparison (PB writen until manufacturer write warranty end). If there's a chance (due to the difficult on testing time) also make a $/tested write life (PB writen until tested dead). Reply
  • Kristian Vättö - Wednesday, September 3, 2014 - link

    $/endurance is certainly something I will be looking at in the future. This was more of a preview than a full review and in the full review I will have more graphs and comparisons to support the conclusions :)

    As for actual endurance testing, that is something I will not do. It took me two weeks of continuos writing to get the rated endurance to drop by 1% in the 845DC PRO, so it would take way too long to wait for the drive to die.
    Reply
  • hojnikb - Wednesday, September 3, 2014 - link

    And there is a good chance, that thing would still go strong after rated endurance would drop to 0% (unless its hardcapped to die after that). Reply
  • Essence_of_War - Wednesday, September 3, 2014 - link

    Another excellent article, Kristian.

    A question/comment, though, do you think it would make more sense to report the stdev measurements as an "error bar" attached to each data point rather than an additional graph with a 2nd y-axis? I think that might be more compact without having to worry about having multiple y-scales to read. Then it might even be possible to plot an average + error bars data set for multiple different SSDs on the same axis w/o having to worry about which curve is the avg, which curve is the stdev, etc.
    Reply
  • hulu - Wednesday, September 3, 2014 - link

    Another way to present the standard deviation data would be to draw it in two average +/- stdev graphs above and below the average graph. This would better allow visualizing the actual values the average minus stdev has. Reply
  • Kristian Vättö - Thursday, September 4, 2014 - link

    Thanks for the suggestion. I will certainly look into alternative ways to present the standard deviation. I need to redo the graphs anyway for our new suite, so while I'm at it I can see if there is a better way to present the data. Reply
  • Essence_of_War - Thursday, September 4, 2014 - link

    Best luck then, I'm sure you'll figure out a good way to do it. Reply

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