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Samsung SSD 850 EVO 32-Layer 3D V-NAND-Based SSD Tested
MojoKid writes Samsung just took the wraps off a new family of mainstream solid state drives, targeting the market segment previously occupied by its popular SSD 840 EVO series. The new Samsung SSD 850 EVO series is the follow-up to the company's current flagship SSD 850 PRO, but the new EVO is Samsung's first to pack 32layer 3D VNAND 3-bit MLC flash memory. The move to 32layer 3D VNAND 3-bit MLC flash brings pricing down to the .50 to .60 per GiB range, but doesn't adversely affect endurance because the cell structure doesn't suffer from the same inherent limitations of planar NAND, since the cells are stacked vertically with the 3D VNAND. The new 850 EVO drive performs well with large sequential transfers and also offered very low access times. The compressibility of the data being transferred across the Samsung SSD 850 EVO had no impact on performance and small file transfers at high queue depth were fast. Small file transfers with low queues depths, which is what you'd expect to see with most client workloads, were also very good. The Samsung SSD 850 EVO drives also put up excellent numbers in trace-based tests like PCMark 7.
They're not stacking silicon wafers on top of each other. Rather, they're putting more layers of oxide, semiconductor, etc onto each wafer in order to produce the 3d stacking. Yes, it's more complex. But it's a pretty mature technology.
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With increased density from 32 layers (despite larger feature size) why don't they have a 2tb (or 1920gb) model yet?
Anandtech wrote:
Initially I was told that the 850 EVO would come in 2TB capacity as well, but later on Samsung opted against it due to the limited demand.
Most likely because there's no savings whatsoever, if a 1TB drive is $500 then 2TB is probably like $980. They scale almost perfectly, all you need is an extra SATA port and you'd get a lot better performance with two in RAID0.
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TFA says:
The move to 32-layer 3D VNAND 3-bit MLC flash brings pricing down to the .50 to .60 per GiB range, but doesn't adversely affect endurance because the cell structure doesn't suffer from the same inherent limitations of planar NAND, since the cells are stacked vertically with the 3D VNAND.
which didn't make sense to me. Luckily Anandtech has a non-gibberish explanation:
Rather than increasing density by shrinking cell size, Samsung's V-NAND takes a few steps back in process technology and instead stacks multiple layers of NAND cells on top of one another. ...In the floating gate MOSFET, electrons are stored on the gate itself - a conductor. Defects in the transistor (e.g. from repeated writes) can cause a short between the gate and channel, depleting any stored charge in the gate. If the gate is no longer able to reliably store a charge, then the cell is bad and can no longer be written to. Ultimately this is what happens when you wear out an SSD.
With V-NAND, Samsung abandons the floating gate MOSFET and instead turns to its own Charge Trap Flash (CTF) design. An individual cell looks quite similar, but charge is stored on an insulating layer instead of a conductor. This seemingly small change comes with a bunch of benefits, including higher endurance and a reduction in overall cell size. That's just part of the story though.
V-NAND takes this CTF architecture, and reorganizes it into a non-planar design. The insulator surrounds the channel, and the control gate surrounds it. The 3D/non-planar design increases the physical area that can hold a charge, which in turn improves performance and endurance.
The final piece of the V-NAND puzzle is to stack multiple layers of these 3D CTF NAND cells. Since Samsung is building density vertically, there's not as much pressure to shrink transistor sizes. With relaxed planar space constraints, Samsung turned to an older manufacturing process (30nm class, so somewhere between 30 and 39nm) as the basis of V-NAND.
By going with an older process, Samsung inherently benefits from higher endurance and interference between cells is less of an issue. Combine those benefits with the inherent endurance advantages of CTF and you end up with a very reliable solution. Whereas present day 19/20nm 2-bit-per-cell MLC NAND is good for around 3000 program/erase cycles, Samsung's 30nm-class V-NAND could withstand over 10x that (35K p/e cycles).