06 May 2014

SanDisk’s collosal 4TB SSD: Does this mean SSDs will soon provide more storage than hard drives?

NAND flash memory die

Ever since SSDs first hit the consumer market, performance enthusiasts have had to juggle between high speed storage and disk space. Even today, buying a terabyte of SSD capacity is nearly an order of magnitude more expensive than a 1TB hard drive. Conventional wisdom has predicted that this will remain the case indefinitely, which is why it was surprising to see SanDisk announce an upcoming 4TB SSD. Granted, this is an enterprise drive, intended as a replacement for SAS HDDs but many consumer technologies start off in the enterprise before hitting dramatically better price points in the mainstream. Could this signal a sea change in SSD capacities? Will SSDs eventually provide more storage than HDDs?

SanDisk isn’t giving out all the details on this new drive, but we can infer some of the drive’s capabilities based on published data. First, this drive isn’t particularly fast. SanDisk is claiming a 400MB/s for read and write speeds, with 75K IOPS of read and 15K IOPS (Input/Output Operations Per Second) of write performance. The company’s 800GB Lightning Ultra Gen II SSDs, in contrast, top out at 1,000/600 MB per sec read-write performance and 190K and 100K of read-write IOPS respectively. Clearly, the company is pushing its controller technology to the limit to address enough NAND to make a 4TB drive possible and it’s taking a hit for doing so. Given that all SSDs are typically overprovisioned, the Optimus Max is probably carrying 4.2 - 4.4TB of NAND flash in total.

SanDisk is using this achievement to pound its chest over the imminent arrival of huge SSDs that will dwarf HDD capacities.

SanDisk HDD vs SSD
SanDisk’s perfectly wrong chart comparison

This is misleading for a host of reasons. The chart is flatly wrong you can buy a SATA 2.5-inch HDD with a 1.5TB capacity now, while Seagate has 10K SAS 2.5-inch drives with 1.2TB of total capacity.

But finally, there’s price. Seagate’s SAS 10K 2.5-inch drive is $650 for 1.2TB. Seagate’s 800GB SAS SSD, in contrast, retails for $6600. We can assume that SanDisk might get aggressive on pricing, but unless the company utterly blows its cost curve this 4TB SSD could easily run more than $10,000 multiple orders of magnitude more expensive than consumer HDDs in terms of cost-per-GB, and more than 10x as expensive as a SAS HDD.

Huge, cheap SSDs arriving no time soon

Unfortunately, all current evidence suggests that huge cheap SSDs that can keep pace with hard drives in terms of arreal density aren’t going to happen any time soon. Today, the vast majority of NAND flash is built in a planar (two-dimensional) structure. The same problems that have kept next-generation processors from scaling to ever-smaller sizes are keeping NAND flash stuck as well. NAND cell sizes aren’t falling the way they used to, which means the amount of NAND manufacturers can physically pack into the same area isn’t shrinking very much.

Cost reductions
We’re at 20nm now. The “reduction” isn’t scheduled to fall much further.

The alternative to shrinking physical cell sizes is to hold more charge levels (and thus more bits) per cell. Again, we’ve seen some moves to TLC NAND in the consumer space, with Samsung’s triple-level cell memory, but such memory is slower and can sustain fewer write cycles before failing. Better TLC NAND would be a huge boost for SSD densities, but no one is even talking about QLC (quad-level cell) NAND there’s just no way to store voltage at that fine-grained level while retaining enough write cycles to deploy the tech effectively.

If we can’t make NAND cells smaller and we can’t store more data per cell, can 3D NAND NAND built on its vertical edge save the day? Once upon a time, it looked like the answer was “Yes” Samsung was enthusiastic about its own plans for this market segment. Unfortunately, real-world data coming back suggests that the benefits of 3D NAND may have been overstated at least, relative to the early-ramp difficulties. Samsung was forced to fall back to a 40nm production node for its 3D NAND, and has announced that it intends to hold steady at that node for the next five technology generations. It’s also had to use very large NAND cell sizes to overcome certain manufacturing problems related to material deposition and it’s not clear if the company’s manufacturing strategy with V-NAND can scale down to smaller nodes effectively.

Samsung cell size

Right now, Samsung’s 40nm 3D NAND cell size is 5x the size of a conventional 40nm NAND cell and 30x the size of a conventional cutting-edge 16nm NAND cell. That means that current 3D NAND, even at 24 layers thick, isn’t competitive with cutting edge planar.

That scaling is critical to any long-term cost equalization between NAND and HDDs because stacking multiple dies on top of each other is precisely how 3D NAND was supposed to continue to improve its own cost structure. Analysts and industry experts have begun quietly pushing out dramatic cost reductions for SSDs, as in the recent graph we showed where HDDs retain a significant cost advantage in the foreseeable future.

NAND vs HDD expected cost trends
Towards NAND as “big enough”

I suspect that the majority of end consumers won’t be much bothered by the slowing rate of progress. NAND prices and capacities will still improve over the next few years, just not necessarily at the meteoric rates of the last half decade. There’s enough headroom left in existing technology that we should see TLC-based 512GB drives pushing below the $250 mark in the next few years. I suspect that this represents a “good enough” point for the overwhelming majority of users even if you regularly install games that require 30-50GB of storage, 512GB of storage gives you some elbow room.

Current market trends suggest that we’ll see a shift towards high end PCI Express based NAND for the enthusiast market and larger, slower NAND pools for the consumer and enterprise spaces. Increasing densities will give companies like SanDisk room to bring down the price on commercial drives like this one, but with 4TB hard drives selling for ~$200 online, it’ll be a very long time before we see cost parity between SSDs and HDDs.

Courtesy Extremetech

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