Performance Consistency

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 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.

Steady-State 4KB Random Write Performance

For a mainstream drive, the 850 EVO mSATA/M.2 does relatively well in IO consistency except for the highest capacity 1TB model. Strangely enough the 2.5" 1TB 850 EVO does just fine, so this issue seems to be limited to the mSATA version. 

Steady-State 4KB Random Write Consistency

Looking at the standard deviation reveals why: the IO consistency of the 850 EVO mSATA 1TB, even with overprovisioning, is horrible compared to the rest of the 850 EVO lineup. 

Samsung 850 EVO mSATA 250GB
25% Over-Provisioning

The issue with the 1TB mSATA is actually worse than I expected because it's literally stopping for seconds in a frequent manner. The pauses can even be over 50 seconds, so this isn't just some normal garbage collection that's happening in the background. I find this to be very alarming because it may have dramatic impact to user experience and it's simply something that no modern SSD should do anymore. I did let Samsung know about my findings before publishing this review, but I wasn't really able to get any comment from them regarding this issue and whether Samsung has noticed something similar in its internal tests. Adding over-provisioning seems to help as the pauses become much more infrequent, but for now I would still advise against buying the 1TB mSATA version until there's a fix for the IO consistency.

As for the other capacities, the 850 EVO has excellent consistency and steady-state performance for a mainstream drive. The capacity has some effect on performance, but even the 250GB model has roughly twice the performance of 240GB Ultra II thanks to faster V-NAND.

Samsung 850 EVO mSATA 250GB
25% Over-Provisioning
Introduction, The Drives & The Test AnandTech Storage Bench - The Destroyer
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  • Peichen - Tuesday, March 31, 2015 - link

    Shouldn't mSATA/M.2 intereface drives be a lot faster than SATAIII drives due to the much faster interface? I was kinda expecting 1GB/sec. speed consider there are already drive tested at 1.4 and 2.7GB/sec.
  • MrCommunistGen - Tuesday, March 31, 2015 - link

    mSATA is SATA in a different formfactor. M.2 can be either SATA or PCI-E. As stated in the article, this drive comes (only) in the SATA form.
  • foxtrot1_1 - Tuesday, March 31, 2015 - link

    The interface is still SATA, even if the connector is M.2. I assume PCIe M.2 drives will be coming later.
  • Murloc - Tuesday, March 31, 2015 - link

    in a very short time they introduced a bunch of connectors and interfaces and it's all gotten quite confusing.
  • foxtrot1_1 - Tuesday, March 31, 2015 - link

    Don't worry, it's not like there's also three different mainstream USB standards with two different plugs. Oh wait.

    Well, at least we have one agreed-upon display connection, that makes shopping for monitors and graphics cards easier. Oh wait.
  • lazarpandar - Tuesday, March 31, 2015 - link

    That's the great thing about standards, you've got so many to choose from!
  • yslee - Tuesday, March 31, 2015 - link

    xkcd #927 puts it very nicely. :P
  • Artuk - Wednesday, April 1, 2015 - link

  • blanarahul - Tuesday, March 31, 2015 - link

    You need yo put /sarcasm tag so people don't get confused.
  • Callitrax - Tuesday, March 31, 2015 - link

    One thing you should probably do in M.2 SSD reviews is include how the drives are keyed, preferably in one of the tables. This is important since the M.2 interface is actually 4 semi compatible "standards" (see As a result not all M.2 SSDs will fit in all M.2 slots. This one appears to be both B and M keyed so I think it should be pretty universal, but as an example the Samsung XP941 is only M keyed and thus will not work in the HP Stream Mini's B keyed SSD slot. (Did whoever came up with M.2 make a crappy standard that will cause lots of customer support calls and RMA's when consumers M.2 drives don't work with their M.2 equipped computers? Yes they did.)

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