Conclusion

As the first SSD with QLC NAND to hit our testbed, the Intel SSD 660p provides much-awaited hard facts to settle the rumors and worries surrounding QLC NAND. With only a short time to review the drive we haven't had time to do much about measuring the write endurance, but our 1TB sample has been subjected to 8TB of writes and counting (out of a rated 200TB endurance) without reporting any errors and the SMART status indicates about 1% of the endurance has been used, so things are looking fine thus far.

On the performance side of things, we have confirmed that QLC NAND is slower than TLC, but the difference is not as drastic as many early predictions about QLC NAND suggested. If we didn't already know what NAND the 660p uses under the hood, Intel could pass it off as being an unusually slow TLC SSD. Even the worst-case performance isn't any worse than what we've seen with some older, smaller TLC SSDs with NAND that is much slower than the current 64-layer stuff.

The performance of the SLC cache on the Intel SSD 660p is excellent, rivaling the high-end 8-channel controllers from Silicon Motion. When the 660p isn't very full and the SLC cache is still quite large, it provides significant boosts to write performance. Read performance is usually very competitive with other low-end NVMe SSDs and well out of reach of SATA SSDs. The only exception seems to be that the 660p is not very good at suspending write operations in favor of completing a quicker read operation, so during mixed workloads or when the drive is still working on background processing to flush the SLC cache the read latency can be significantly elevated.

Even though our synthetic tests are designed to give drives a reasonable amount of idle time to flush their SLC write caches, the 660p keeps most of the data as SLC until the capacity of QLC becomes necessary. This means that when the SLC cache does eventually fill up, there's a large backlog of work to be done migrating data in to QLC blocks. We haven't yet quantified how quickly the 660p can fold the data from the SLC cache into QLC during idle times, but it clearly isn't enough to keep pace with our current test configurations. It also appears that most or all of the tests that were run after filling the drive up to 100% did not give the 660p enough idle time after the fill operation to complete its background cleanup work, so even some of the read performance measurements for the full-drive test runs suffer the consequences of filling up the SLC write cache.

In the real world, it is very rare for a consumer drive to need to accept tens or hundreds of GB of writes without interruption. Even the installation of a very large video game can mostly fit within the SLC cache of the 1TB 660p when the drive is not too full, and the steady-state write performance is pretty close to the highest rate data can be streamed into a computer over gigabit Ethernet. When copying huge amounts of data off of another SSD or sufficiently fast hard drive(s) it is possible to approach the worst-case performance our benchmarks have revealed, but those kind of jobs already last long enough that the user will take a coffee break while waiting.

Given the above caveats and the rarity with which they matter, the 660p's performance seems great for the majority of consumers who have light storage workloads. The 660p usually offers substantially better performance than SATA drives for very little extra cost and with only a small sacrifice in power efficiency. The 660p proves that QLC NAND is a viable option for general-purpose storage, and most users don't need to know or care that the drive is using QLC NAND instead of TLC NAND. The 660p still carries a bit of a price premium over what we would expect a SATA QLC SSD to cost, so it isn't the cheapest consumer SSD on the market, but it has effectively closed the price gap between mainstream SATA and entry-level NVMe drives.

Power users may not be satisfied with the limitations of the Intel SSD 660p, but for more typical users it offers a nice step up from the performance of SATA SSDs with a minimal price premium, making it an easy recommendation.

Power Management
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  • Spunjji - Wednesday, August 8, 2018 - link

    It was pretty obvious that the 840 was affected because it used the same NAND as the 840 Evo, just without the caching mode. It was also pretty obvious that Samsung didn't care because it was "old" so they never properly fixed it. Reply
  • OwCH - Wednesday, August 8, 2018 - link

    Ryan, I love that you will. It is not easy for the user to find real world data on these things and it is, at least to me, information that I want before making the decision to buy a drive.

    Looking forward to it!

    Thanks!
    Reply
  • Solid State Brain - Tuesday, August 7, 2018 - link

    The stated write endurance should already factor data retention, if it follows JEDEC specifications (JESD219A). For consumer drives, it should be be when the retention time for freshly stored data drops below 1 year after the SSD is powered off, at 30°C. Reply
  • BurntMyBacon - Wednesday, August 8, 2018 - link

    The Samsung 840 EVO would like to have a word with you. Reply
  • eastcoast_pete - Wednesday, August 8, 2018 - link

    Yes, it should factor data retention, and it should follow JEDEC specs. The problem is the "should". That doesn't mean it or they do. I found that "Trust but verify" is as important in IT as it is in life. Even the biggest names screw up, at least occasionally. Reply
  • IntenvidiAMD - Tuesday, August 7, 2018 - link

    Are there any reviewers that do test that? Reply
  • DanNeely - Tuesday, August 7, 2018 - link

    Over 18 months between 2013 and 2015 Tech Report tortured a set of early generation SSDs to death via continuous writing until they failed. I'm not aware of anyone else doing the same more recently. Power off retention testing is probably beyond anyone without major OEM sponsorship because each time you power a drive on to see if it's still good you've given its firmware a chance to start running a refresh cycle if needed. As a result to look beyond really short time spans, you'd need an entire stack of each model of drive tested.

    https://techreport.com/review/27909/the-ssd-endura...
    Reply
  • Oxford Guy - Tuesday, August 7, 2018 - link

    Torture tests don't test voltage fading from disuse, though. Reply
  • StrangerGuy - Tuesday, August 7, 2018 - link

    And audiophiles always claim no tests are ever enough to disprove their supernatural hearing claims, so... Reply
  • Oxford Guy - Tuesday, August 7, 2018 - link

    SSD defects have been found in a variety of models, such as the 840 and the OCZ Vertex 2. Reply

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