The Intel SSD 660p SSD Review: QLC NAND Arrives For Consumer SSDs
by Billy Tallis on August 7, 2018 11:00 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.
As with the Heavy test, the SLC cache of the Intel SSD 660p is extremely beneficial and brings the average data rate of the 660p up into high-end NVMe territory. When the drive is full and the SLC cache has been reduced to its minimum size, performance suffers and drops below the Crucial MX500 mainstream SATA drive but not all the way down to the level of the Toshiba RC100 DRAMless NVMe SSD.
The best-case latency scores from a freshly-erased 660p are acceptable for a high-end NVMe SSD and excellent for an entry-level drive. In the worst case of a full drive, the average latency is far higher but still low enough that the drive won't actually feel much slower. The 99th percentile latency climbs very high by SSD standards, but is still barely up into the average latency range of hard drives.
On the Light test, the average read latency of the 660p stays comfortably below that of SATA drives even for the worst-case full drive test run, and only the average write latency shows a serious problem from filling up the whole drive and not giving it enough time to empty the now-reduced SLC cache.
The full-drive 99th percentile write latency of the 660p on the Light test is almost as bad as the 600p or the Toshiba RC100. Otherwise, the 660p doesn't have any worrying QoS problems on this test and users won't notice serious pauses from the drive.
The energy usage of the 660p on the Light test is below most other NVMe drives when the test is run on an empty drive, and even with the extra background work and longer test duration of the full-drive test run the 660p is only a little less efficient than the average for this bunch of NVMe SSDs.
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woggs - Tuesday, August 7, 2018 - link
2D nand was abandoned because it squeezed the storage element down to a size where it became infeasible to scale further and still store data reliably. The move to 3D nand took back the needed size of the memory element to store more charge. Cost reduction from scaling is no longer reliant directly on the reduction of the storage element. This is a key enabler for TLC and QLC.woggs - Tuesday, August 7, 2018 - link
Stated another way... Scaling 2D flash cells proportionally reduced the stored charge available to divide up into multiple levels, making any number of bits per cell proportionally more difficult. The the question of cost reduction was which is faster and cheaper: scale the cell to smaller size or deliver more bits/cell? 2 bits per cell was achievable fast enough to justify it's use for cost reduction in parallel with process scaling, which was taking 18 to 24 months a pop. TLC was achievable on 2D nodes (not the final ones) but not before the next process node would be available. 3D has completely changed the scaling game and makes more bits per cell feasible, with less degradation in the ability to deliver as the process scales. The early 3D nodes "weren't very good" because they were the first 3D nodes going through the new learning curve.PeachNCream - Tuesday, August 7, 2018 - link
Interesting performance measurements. Variable size pseudo-SLC really helps to cover up the QLC performance penalties which look pretty scary when the drive is mostly full. The .1 DWPD rating is bad, but typical consumers aren't likely to thrash a drive with that many writes on a daily basis though Anandtech's weighty benchmarks ate up 1% of the total rated endurance in what is a comparable blink of an eye in the overall life of a storage device.In the end, I don't think there's a value proposition in owning such the 660p in specific if you're compelled to leave a substantial chunk of the drive in an empty state so the performance doesn't rapidly decline. In effect, the buyer is purchasing more capacity than required to retain performance so why not just purchase a TLC or MLC drive and suffer less performance loss and therefore gain more usable space?
Oxford Guy - Tuesday, August 7, 2018 - link
The 840's TLC degraded performance because of falling voltages, not because of anyone "thrashing" the drive.However, it is also true that the performance of the 120 GB drive was appalling in steady state.
mapesdhs - Wednesday, August 8, 2018 - link
Again, 840 EVO; few sites covered the standard 840, there's not much data. I think it does suffer from the same issue, but most media coverage was about the EVO version.Spunjji - Wednesday, August 8, 2018 - link
It does suffer from the same problem. It wasn't fixed. Not sure why Oxford *keeps* bringing it up in response to unrelated comments, though.Oxford Guy - Friday, August 10, 2018 - link
The point is that there is more to SSD reliability than endurance ratings.Oxford Guy - Friday, August 10, 2018 - link
"few sites covered the standard 840"The 840 got a lot of hype and sales.
FunBunny2 - Tuesday, August 7, 2018 - link
with regard to power-off retention: is a stat estimation from existing USB sticks (on whatever node) and such, meaningful? whether or not, what might be the prediction?milkywayer - Tuesday, August 7, 2018 - link
My question is, should I truest this drive with valuable info if endurance can be an issue?If the PC is frequently powered On, will it refresh the cells?