The Silicon Motion SM2262EN SSD Controller Preview: Good And Bad Amplified
by Billy Tallis on August 1, 2018 11:45 AM ESTAnandTech Storage Bench - Light
Our Light storage test has relatively more sequential accesses and lower queue depths than The Destroyer or the Heavy test, and it's by far the shortest test overall. It's based largely on applications that aren't highly dependent on storage performance, so this is a test more of application launch times and file load times. This test can be seen as the sum of all the little delays in daily usage, but with the idle times trimmed to 25ms it takes less than half an hour to run. Details of the Light test can be found here. As with the ATSB Heavy test, this test is run with the drive both freshly erased and empty, and after filling the drive with sequential writes.
The difference between freshly-erased and full drive test run performance is greater for the Silicon Motion SM2262EN than for any other drive. The fresh out of the box performance on the Light test is faster than anything else we've tested, and the full-drive performance is below par for a mainstream SATA SSD. Where this drive transitions between these two modes and how steep that transition is will make or break the product.
Despite having a slightly higher average data rate, the SM2262EN has slightly worse average and 99th percentile latency scores than the Samsung 970 EVO. When the drive is full, the average latency is almost as high as the Crucial MX500 SATA drive, and the 99th percentile latency is worse.
Average read and write latencies from the SM2262EN are more or less tied for first place when the test is run on an empty drive. When the drive is full the read latency is a bigger problem than write latency: reads are slower than the MX500, while writes stay in low-end NVMe territory.
The 99th percentile read and write latency scores are good but not the best for the empty drive test run. When the SM2262EN is full, both scores are vastly worse, with the 99th percentile read latency ending up twice as slow as the Crucial MX500.
The SM2262EN's energy consumption during the Light test is slightly higher than average for the empty-drive test runs, and higher than any other M.2 drive when the test is run on a full drive.
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DigitalFreak - Wednesday, August 1, 2018 - link
One thing has always confused me about these benchmarks. Does performance get progressively worse as the drive fills up? For example, the ATSB - Light average latency for the drive is 48 mu empty and 330 mu full. Does that mean when the drive is 50% full the latency would be around 189 mu? Or does it run at 48 mu until the drive hits 100% full? Same for the average data rate.Billy Tallis - Wednesday, August 1, 2018 - link
I think there's usually a threshold at which performance drops pretty rapidly because the SLC cache or spare area is no longer large enough. Unfortunately, determining the shape of the curve and where the threshold is (if there is one) is extremely time-consuming, and the tools used for the ATSB tests don't make it easy to test multiple drives in parallel.I did run the Heavy and Light tests on this drive with it 80% full and the results were similar to the 100% full case. But manual overprovisioning like that doesn't necessarily have the same impact that re-tuning the firmware would. A typical variable-size SLC cache won't be significantly larger for an 80% full drive than for a 100% full drive.
And there's still the problem that the ATSB tests don't give the drive any long idle periods to flush the SLC cache. The SLC cache on a full drive might be large enough to handle the Heavy test reasonably well if it gets a realistic amount of idle time to flush the cache mid-test. But that would take the Heavy test from 1.5 hours to a full day.
DigitalFreak - Wednesday, August 1, 2018 - link
Understandable. With the huge performance difference between empty and full with this controller, I was just curious at what percentage used the drive performance tanked. Based on your test we already know that 80% full is just as bad as 100%. Hopefully it's not any lower than that.justaviking - Wednesday, August 1, 2018 - link
I had the exact same question. How full is full?If the performance hit did not occur until 95% full or more, then it would be easily avoidable and acceptable (to me). If it happens at 30% full, it's a deal breaker. Or a linear degredation would also unacceptable to me since the degredation is so extreme.
I STRONGLY ENCOURAGE taking the time to explore the "degradation curve" relative to "fullness" for this drive, since it is so dramatic. It could make a special article of the type AnandTech excels at.
29a - Wednesday, August 1, 2018 - link
I agree.jtd871 - Wednesday, August 1, 2018 - link
How long of a "long idle time" do you need? Are you talking about 1.5h run time for ATSB to 8h or 24h with sufficiently long "long idle times"?Billy Tallis - Wednesday, August 1, 2018 - link
Currently, the ATSB tests cut all idle times down to a maximum of 25ms. I suspect that idle times on the order of seconds would be sufficient, but I don't think we even still have the original traces with the full idle times. In the near future I'll do some SYSmark runs with a mostly-full drive; that's a similar intensity of storage workload to the ATSB light, but with a fairly realistic pacing including idle.I'll also try to compare the power data against the performance test duration for the synthetic tests. That should reveal how long the drive took to return to idle after the writing stopped, and give us a pretty good idea of how quickly the drive can empty the SLC cache and how high of a duty cycle it can sustain for writes at full speed.
Dark_wizzie - Wednesday, August 1, 2018 - link
A larger drive helps mitigate the issues because 1) Larger drives tend to have large SLC cache? Or 2) There is more normal free space for the drive?Billy Tallis - Wednesday, August 1, 2018 - link
Both, in a big way when it's 2TB, and especially when you have a variable-size SLC cache. A mostly-empty 2TB drive can have over 100GB of SLC cache, which is absolutely impossible to fill up with any real-world client workload.mattrparks - Wednesday, August 1, 2018 - link
I wonder if...I think you could get similar results (stellar performance characteristics at low drive usage) by using a larger DRAM read/write cache when the drive mapping table is not taking up as much RAM. With 2GB of DDR4, let's say arbitrarily that 1.5GB of that is used by FTL page mapping tables when the drive is full. What if you found a way in firmware to manage your memory such that when most of the drive FTL is unmapped, that you could use say only 0.5GB for the mapping table and have an extra 1GB available for caching? Many of the synthetic tests could be gamed by keeping that much drive cache. I don't remember your drive testing methodology fully, but perhaps a full power cycle of the drive after the data is written, before the read tests, would make sure that all the performance is indeed SLC speed and not just enormous amounts of caching.