The Intel Optane SSD DC P4800X (375GB) Review: Testing 3D XPoint Performance
by Billy Tallis on April 20, 2017 12:00 PM EST3D XPoint Refresher
Intel's 3D XPoint memory technology is fundamentally very different from NAND flash. Intel has not clarified any more low-level details since their initial joint announcement with Micron of this technology, so our analysis from 2015 is still largely relevant. The industry consensus is that 3D XPoint is something along the lines of a phase change memory or conductive bridging resistive RAM, but we won't know for sure until third parties put 3D XPoint memory under an electron microscope.
Even without knowing the precise details, the high-level structure of 3D XPoint confers some significant advantages and disadvantages relative to NAND flash or DRAM. 3D XPoint can be read or written at the bit or word level, which greatly simplifies random access and wear leveling as compared to the multi-kB pages that NAND flash uses for read or program operations and the multi-MB blocks used for erase operations. Where DRAM requires a transistor for each memory cell, 3D XPoint isolates cells from each other by stacking them each in series with a diode-like selector. This frees up 3D XPoint to use a multi-layer structure, though not one that is as easy to manufacture as 3D NAND flash. This initial iteration of 3D XPoint uses just two layers and provides a per-die capacity of 128Gb, a step or two behind NAND flash but far ahead of the density of DRAM. 3D XPoint is currently storing just one bit per memory cell while today's NAND flash is mostly storing two or three bits per cell. Intel has indicated that the technology they are using, with sufficient R&D, can support more bits per cell to help raise density.
The general idea of a resistive memory cell paired with a selector and built at the intersections of word and bit lines is not unique to 3D XPoint memory. The term "crosspoint" has been used to describe several memory technologies with similar high-level architectures but different implementation details. As one Intel employee has explained, it is relatively easy to discover a material that exhibits hysteresis and thus has the potential to be used as a memory cell. The hard part is desiging a memory cell and selector that are fast, durable, and manufacturable at scale. The greatest value in Intel's 3D XPoint technology is not the high-level design but the specific materials and manufacturing methods that make it a practical invention. It has been noted by some analysts that the turning point for technologies such as 3D XPoint may very well be in the development in the selector itself, which is believed to be a Schottky diode or an ovonic selector.
In addition to the advantages that any resistive memory built on a crosspoint array can expect, Intel's 3D XPoint memory is supposed to offer substantially higher write endurance than NAND flash, and much lower read and write times. Intel has only quantified the low-level performance of 3D XPoint memory with rough order of magnitude comparisons against DRAM and NAND flash in general, so this test of the Optane SSD DC P4800X is the first chance to get some precise data. Unfortunately, we're only indirectly observing the capabilities of 3D XPoint, because the Optane SSD is still a PCIe SSD with a controller translating the block-oriented NVMe protocol and providing wear leveling.
The only other Optane product Intel has announced so far is another PCIe SSD, but on an entirely different scale: the Optane Memory product for consumers uses just one or two 3D XPoint chips and is intended to serve as a 32GB cache device accelerating access to a mechanical hard drive or slower SATA SSD. Next year Intel will start talking about putting 3D XPoint on DIMMs, and by then if not sooner we should have more low-level information about 3D XPoint technology.
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Billy Tallis - Friday, April 21, 2017 - link
I said the NVMe driver wasn't manually switched into polling mode; I left it with the default behavior which on 4.8 seems to be not polling unless the application requests. I'm certainly not seeing the 100% CPU usage that would be likely if it was polling.If I'd had more time, I would have experimented with the latest kernel versions and the various tricks to get even lower latency.
tuxRoller - Friday, April 21, 2017 - link
I wasn't claiming that you disabled polling only that polling was disabled since it should be on be default for this device.Assuming you were looking at the sysfs interface, was the key that was set to 0 called io_poll or io_poll_delay? The later set to 0 enables hybrid polling, so the cpu wouldn't be pegged.
Either way, you wouldn't need a new kernel, just to enable a feature the kernel has had since 4.4 for these low latency devices.
Also, did you disable the pagecache (direct=1) in your fio commands? If you didn't, that would explain why aio was faster since it uses dio.
Btw, it's not my intent to unnecessarily criticize you because i realize the tests were performed under constrained circumstances. I just would've appreciated some comment in the article about a critical feature for this hardware was not enabled in the kernel.
yankeeDDL - Friday, April 21, 2017 - link
Optane was supposed to be 1000x faster, have 1000X endurance and be 10x denser than NAND (http://hothardware.com/ContentImages/NewsItem/4020...I realize this is the first product, but saying that it fell short of expectation is an understatement.
It has lower endurance, lower density and it is measurably faster, but certainly nowhere close 1000X.
Oh, did I mention it is 5-10X more expensive?
I am quite disappointed, to be honest. It will get better, but @not ready@ is something that comes to ind reading the article.
Billy Tallis - Friday, April 21, 2017 - link
3D XPoint memory was supposed to be 1000x faster than NAND, 1000x more durable than NAND, and 10x denser than DRAM. Those claims were about the 3D XPoint memory itself, not the Optane SSD built around that memory.ddriver - Friday, April 21, 2017 - link
It is probably as good as they said... if you compare it to the shittiest SD card from 10 years ago. Still technically NAND ;)yankeeDDL - Monday, April 24, 2017 - link
I disagree. I can agree that the speed may be limited by the drive, but even so, it falls short by a large factor. The durability and the density, however, are pretty much platform independent and they are not there by a very, very long shot. Intel itself demonstrated that it is only 2.4-3X faster (https://en.wikipedia.org/wiki/3D_XPoint).It clearly has a future, especially as the NAND is approaching the end of its scalability. Engineering wise is interesting, but today, it makes really little sense, while it should have been a slam dunk. I mean, who would have thought twice before buying a 500GB drive that maxes out the SATA for $20-30? But this one ... not so much.
zodiacfml - Friday, April 21, 2017 - link
It will perform better in DIMM.factual - Friday, April 21, 2017 - link
I don't see xpoint replacing dram due to both latency and endurance not being up to par , but It's going to disrupt the ssd market and as the technology matures and prices come down, I can see xpoint revolutionizing the storage market as ssd did years ago.Competition is clearly worried since seems like paid trolls are trying to spread falsehoods and bs here and elsewhere on the web.
ddriver - Saturday, April 22, 2017 - link
I just bet it will be highly disturbing to the SSD market LOL. With its inflated price, limited capacity and pretty much unnecessary advantages I can just see people lining up to buy that and leaving SSDs on the shelves.factual - Saturday, April 22, 2017 - link
You are either extremely ignorant or a paid troll !!! anyone who understands technology knows that new tech is always expensive. When SSDs came to the market, they were much more expensive and had a lot less capacity than HDDs but they closed the gap and disrupted the market. The same is bound to happen for Xpoint which performs better than NAND by orders of magnitude.