3D memory goes mainstream
Without giving away too much, too soon, you may have noticed that both of our showcase SSDs this issue hail from the same outfit, namely Samsung. Without doubt, the Korean giant dominates the market for solid-state drives. That applies in terms of sheer number and presence, plus in terms of technical innovation. And it’s the latter that qualifies the 850 Evo for special attention, though that doesn’t mean it’s doing something absolutely new or that it’s necessarily a winner. Still, it’s all a bit galling when you consider Samsung is also dominant in the smartphone market. And HDTVs. And just about any other area of consumer electronics you care to mention. Resistance, it seems, is futile.
Not that it’s terribly surprising to find big players dominating a technology like solidstate drives. At their core, SSDs depend upon memory chips created courtesy of some of the most advanced technology in the world. For advanced, read expensive. It now costs many billions of dollars to build a factory for memory chips and there are very few companies who can stump up that kind of cash. Consolidation down to a few key players is inevitable.
However, as dominant as Samsung is, it’s not actually unchallenged in the SSD market. While only a handful of companies make memory chips, there are a fair few who knock out controller chipsets and even more who put controllers together with memory chips to create drives. That means Samsung’s innovations can be largely seen as adding to healthy competition rather than tools of competitive oppression. For now, at least.
Whatever, the big news with the Samsung 850 Evo is its use of 3D memory chips. It’s not actually the first time we’ve seen 3D memory in an SSD, known as V-NAND or vertical NAND in Samsung speak. That distinction goes to its Samsung sibling, the 850 Pro. Yeah, it was always going to be another Samsung drive. But the 850 Evo brings 3D memory to a more mainstream part of the market and signals what looks like being a shift in memory tech that will eventually take over every SSD.
Welcome to the layer cake
The future, then, is 3D memory. But what does that mean? In simple terms, we’re talking about memory chips that go up in layers as well as across in terms of the pattern of the circuit. Say what? Haven’t integrated circuits always been built up in layers? That’s nothing new, is it?
Actually, yes. Previously, the layers in a chip were all about providing connectivity between the thousands and then millions and latterly billions of transistors. That’s such a complex job, it had to be achieved using multiple layers. But the network of the critical component – those transistors – was essentially laid out on a 2D plane. Chips had a single layer of transistors.
With 3D memory, the big innovation is multiple layers of transistors. As we’re talking memory chips, that effectively means multiple layers of memory cells. Immediately, the impact of that is obvious. Two layers of cells gives you twice the memory capacity for any given area of chip, albeit at the cost of increased thickness. But it’s easy to see how memory capacity suddenly skyrockets when you start to build up multiple layers of cells. As it happens, Samsung has also upgraded its V-NAND from two bits per cell to three bits per cell for the 850 Evo, thereby upping capacity even further.
That’s important for all kinds of reasons. For starters, the chip industry is having increasing problems keeping Moore’s Law on track. Moore’s Law says the chip industry will squeeze twice the number of transistors into a given area of chip every two years. In other words, it’s a doubling of transistor density that results in the exponential growth of transistor counts and therefore processing power and memory capacity over time.
Eventually, we’ll arrive at transistors made of one or two atoms and the fun will stop. We’re not far off that today, so a new approach is needed if we’re to keep increasing memory capacity. The other big upside is also related to the manufacturing processes used to create chips. As transistors and in turn memory cells shrink ever smaller, they become less robust. In other words, they can sustain fewer read and write cycles before they fail.
The same applies to memory cells with the capacity to store multiple bits of data. As you go from single-bit to two-bit and threebit cells, robustness tends to fall off. All of which means that the endurance of flash memory has been getting worse and worse as capacities have increased.
Introduce 3D memory, however, and you can wind back the clock a bit on that problem. That’s exactly what Samsung has done with its own 3D NAND memory. Because it’s composed of 32 layers of memory cells, Samsung can achieve excellent memory density without the latest manufacturing tech. The 850 Evo’s memory chips are 40nm items but still achieve 128Gbit per chip. That’s several generations behind its cutting-edge 19nm 2D chips.
Oh, and as if that wasn’t enough, the vertical connections between the layers of memory cells, known in tech speak as through-silicon vias or TSVs, also help boost bandwidth inside the chip and therefore overall memory performance. So that’s more storage, increased endurance and better performance. Yup, 3D NAND looks like being all things to all PCs.
Not far enough
In capacities up to and including this 500GB model, the 850 Evo also gets Samsung’s latest MGX SATA SSD controller. So there’s an awful lot of newness all round. But does all this theoretical hotness actually make the new 850 Evo any better in the real world? Unfortunately, there’s an immediate and very obvious problem. This is a conventional SATA drive suffering the AHCI protocol and a modest 6Gbps bandwidth ceiling. It’s not a fancy new M.2 drive with oodles of PCI Express-powered bandwidth and SSDoptimised NVMe gubbins. Oh dear.
It’s no surprise, then, to find our test results look largely pedestrian. So, that’s 500-odd MB/s for peak sequential performance and healthy but ultimately unremarkable 4K random numbers. The same goes for our file transfer and compression tests. The 850 Evo doesn’t do anything terribly special. Nor does it set the world alight for performance consistency. We should also point out that Samsung’s RAPID mode feature is pretty useless in the real world. On paper, it uses your system memory as a super-fast drive cache. In practice, it only delivers really tangible results in synthetic tests.
All of which means the upsides of the 850 Evo’s fancy pants 3D NAND memory end up feeling a little theoretical. That’s not to say the Evo is in any way a bad SSD. Taken in context – as a 500GB SATA drive at this price point – it’s definitely still highly competitive. It’s just not the game changer we’d been hoping for.