Understanding TLC NAND
by Kristian Vättö on February 23, 2012 1:14 PM EST- Posted in
- Storage
- SSDs
- OCZ
- Indilinx Everest
- TLC
Our thoughts are a bit mixed. On the one hand, cheaper SSDs are exactly what consumers want. The performance is still there compared to hard drives, no matter what NAND is used. If you go to an Apple Store today and try out MacBook Air and Mac Pro, the MacBook Air will often feel faster, even though it's the slower Mac in terms of processing power. This is solely due to the presence of an SSD. An SSD can bring new life to a computer that is otherwise considered obsolete. That's why we think everyone would want an SSD, but it's understandable that the masses won't adopt SSDs until the price and capacities are reasonable. This is definitely where TLC shines—it provides us with noticeably cheaper SSDs, possibly cheap enough for the masses to adopt (e.g. well under $1 per GB).
On the other hand, we're concerned that the cut in prices is done at the expense of endurance. One advantage often heard about buying an SSD is that SSDs are a lot more reliable than hard drives. In terms of P/E cycles, that is probably true with current MLC NAND. However, there have been quite a few widespread firmware issues, such as SF-2281 BSOD and Intel 320 Series 8MB bugs. Those have been fixed, and we may finally be looking at SSDs which have good performance, adequate endurance, and are more or less trouble-free. However, TLC will require new controller logic, and new logic may result in additional firmware issues.
The earliest SSDs lacked performance, even though they were faster than most hard drives, especially in seek times. In just a few years, performance has increased exponentially, maybe even to a point where the average user won't notice the difference between the fastest SSD and a mediocre SSD.
Given the desire for performance, reliability, and cost, TLC NAND may take away one from the triplet: endurance. Notice we said "may", because P/E cycles aren't everything. It has been claimed that algorithms to minimize write amplification will follow Moore's Law, just like NAND does. In other words, every time there is a die shrink, wear leveling has been improved in order to keep endurance the same. On top of that, improvements in manufacturing technologies can keep the P/E count up as well. 20nm IMFT MLC is claimed to have 3000-5000 P/E cycles, just like 25nm IMFT MLC.
The good news is, MLC NAND will stay in production and hence MLC NAND based SSDs are not going anywhere. What TLC will provide is freedom of choice. If you use your computer for checking email and browsing the Internet, no doubt a TLC based SSD will be sufficient. For the majority of consumers, TLC SSDs should meet their demands.
In addition, the SSD market is evolving quickly; if you buy the best SSD today, it won't be the best for very long. Let's say that it lasts you for four years. In that time, the SSD market will change a lot—four years ago, we were looking at 16GB SSDs for nearly $600! By the time a typical SSD is ready for replacement, you will be looking at much faster SSD with more capacity, and likely for a lower price. In 4.5 years, we have gone from that 16GB offering with performance that often trailed behind contemporary HDDs to 120GB SSDs that are up to a couple orders of magnitude faster than HDDs on random access patterns (and still several times faster for sequential tranfers), all for a starting price of around $170. If that pattern holds for the next four years, we'll be looking at ~1TB SSDs in four years that offer transfer rates that would saturate multi-lane PCIe interfaces at even lower prices. While we expect the rate of progress to be quite a bit slower over the next four years, there's still plenty of room for improvements in SSD technology.
As far as TLC-based SSDs are concerned, all we can do now is to wait for the first product announcements to come. Once we get some review samples, we'll be sure to put them through our SSD test suite and see how they stack up to existing drives.
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themossie - Friday, February 24, 2012 - link
The information is straight from Micron, it's just an awkward way to explain the concept. If you want to keep the industry standard capacities in your explanation, perhaps show the math as capacity/(1, 2, 3) = transistors rather than transistors * (1, 2, 3) = capacity? If capacity is fixed, solving for number of transistors required seems more intuitive.Corsair, OCZ and Kingston all make 90 GB Sandforce 2281 SSDs. I don't know how many channels / what NAND die they use. Searching that information brought up this website first every time! Upon further consideration, I blame aNAND... :-)
Kristian Vättö - Saturday, February 25, 2012 - link
90GB SSDs have 96GB of NAND in them (remember that SandForce drives have ~7% over-provisioning). Most 2.5" drives have sockets for 16 NAND devices so that's simply twelve 8GB packages.Confusador - Friday, February 24, 2012 - link
I read the comments thread looking for this answer, so thank you. I still don't see the logic behind it, as others have pointed out that storage capacities haven't been power-of-2 for decades. It could conceivably be firmware related, but given that overprovisioning makes (e.g.) 60 and 120 GB fairly common that seems unlikely.Anyway, just some questions to keep in mind as you're in contact with the manufacturers. Thanks again for the great article, as the coverage here continues to be second to none.
AnnihilatorX - Friday, February 24, 2012 - link
That's not really possible due to information theory. You can only compress information to reduce write by so much (entropy theory). The improvement will be more like an exponential decay rather than an exponential growth (Moore's law)
Shadowmaster625 - Friday, February 24, 2012 - link
I estimate somewhere around $80 billion has been invested in the NAND flash market, cumulatively. Despite this enormous capital investment, I am surprised prices are still so high. You'd think with this type of mass economy of scale, it wouldnt cost so much to produce 1TB of flash. I wonder how much energy it takes to produce 1TB of flash...MrSpadge - Friday, February 24, 2012 - link
There's so much unused space in 2.5" SSDs, let alone 3.5" drives for desktops. People wouldn't need to worry about TLC endurance, if the NAND was put into sockets and could easily be replaced. Or upgraded later on for higher capacities. And by the time you'd be doing this NAND prices will have fallen again. There'd need to be a standard for this, though...MrS
mark53916 - Friday, February 24, 2012 - link
As late as 2010 SLC's typically had 10 year retention time when new, down to
about 1 year as cells got reprogrammed and the end of life was
indicated for the device. (The number of erase cycles was
also higher than now, but had be decreasing for a few years prior
also.)
I don't know about new cell retention time when new for SLC's
now, but MLCs either show no spec or the retention time spec for NEW
cells is about 18 months.
For the various reasons mentioned in the article and earlier comments,
the effect of MLCs is that speed has been reduced and data retention time
is reduced and the fraction of long error correction time has increased
dramatically.
MLCs are not suitable for long term backups and spinning drives were never
good for more than 5 years EXPECTED powered off life)
MLCs just get 2 times as much storage for the same price 18 months earlier.
In the meantime, due to supply issues (capacity being used for MLC instead
of SLC) Thus SLC typically cost 8 times as much per GB compared
to MLC, rather than less than 2 times as much.) This amounts
to about a 3 year delay in SLCs reaching a given price level.
(MLC also typically comes with implementation side effects
[interleaved data layout, in particular] that means that data in
unchanged pages as seen outside of the SSD is rewritten
because data was changed at the interleaved logical location,
not because the SSD software decided that the data was getting
"weak" and needed to be refreshed.)
Hulk - Friday, February 24, 2012 - link
Timely, informative, well written, and just the right amount of technical detail.Really nice job.
valnar - Friday, February 24, 2012 - link
I'm not sure who the target audience of TLC is. Is there really a group of people out there that is willing to sacrifice reliability and data integrity for price or capacity? I certainly wouldn't.It's bad enough that modern hard drives in the 2TB range have longevity problems. I don't want my SSD to be in the same boat, especially since that SSD tends to be the boot drive on most PC's.
foolsgambit11 - Friday, February 24, 2012 - link
I'm assuming TLC is a subclass of MLC, and not actually distinct as it's laid out in this article. Before TLC came along, all MLC belonged to (what I'll call) the DLC subclass, yeah?SLC = Single level cell
MLC = Multi level cell
a. DLC = Dual level cell
b. TLC = Triple level cell