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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Kristian Vättö - Friday, February 24, 2012 - link
There is HET-MLC (or usually known as eMLC) which is MLC NAND aimed for enterprises. It stores two bits per cell like normal MLC but its P/E cycle rate is much higher (IIRC something like 50,000). Unfortunately I don't know how it really differs from regular MLC but it's a lot more expensive than regular MLC.SLC, MLC and TLC simply refer to the amount of bits per cell, there is no 1-bit-per-cell MLC as MLC alone means multi-level-cell, and one isn't multi ;-)
ckryan - Friday, February 24, 2012 - link
I wasn't aware that this was going on until I read the UCSD paper "the Bleak Future of NAND Flash Memory. Somehow, you can use MLC to store just one bit, and it gets similar, but not identical, performance to SLC.http://cseweb.ucsd.edu/users/swanson/papers/FAST20...
This was the study that was said to cast doubt on the ability to scale NAND effectively past 2024.
They tested this particular MLC-1 setup. Even if you discard half the capacity of MLC, it's still cheaper than SLC bit for bit.
HET-MLC and eMLC really are just highly binned MLC NAND. Toshiba gives it's eMLC 10kPE cycles. But enterprise drives only have to retain data for 3 months at MWI=0, so some of this extra endurance comes from that.
Kristian Vättö - Friday, February 24, 2012 - link
Ooh, interesting, thanks for the link! I was sure that you had mixed up eMLC and MLC because MLC-1 doesn't make much sense, at least not without further explanation.Does the study say how much cheaper MLC-1 is when compared with SLC (I don't have time to read it thoroughly now, but will do tomorrow)? Like I said in the article, MLC is the highest volume product and gets the new process nodes first, so that might be the reason why MLC-1 is a bit cheaper than SLC. Shouldn't be much, though.
This Guy - Friday, February 24, 2012 - link
There is a better tech in the wings anyway. Memristors:http://www.electronicsweekly.com/Articles/06/10/20...
Memristors will create a giant performance boost. Their low latency and high density will allow for the replacement of HDDs and RAM. And this could be a second generation product out in three years.
If the worst memory latency was ~20ns instead of ~50µs (SSD) or ~20ms(HDD), cache misses would stop being a problem. Old, simple CPU architectures could be reproduced (with I/O upgrades) and bundled with memristor storage and compete with current designs.
In 10 years we could see CPUs replaced with multi-chip modules containing large memristor banks, ALU's with a far larger variety of operations (including GPGPU operations) and the system's I/O. No cache. No registers. No stalls.
jjj - Saturday, February 25, 2012 - link
Those are dreams for now.Anyway Sandisk/Toshiba sell a lot of TLC already, in certain products.They even had 4 bits per cell but that's not being produced anymore.As for the future they got 2 things, BiCS and 3D ReRAM. We'll see soon enough if any of those make it to market .
rpmurray - Thursday, February 23, 2012 - link
While cheaper is sure tempting I'm not making the move until I stop seeing so many users giving one-star ratings on Newegg when their nice new SSD bricks itself anywhere between one day and three months.jdjbuffalo - Thursday, February 23, 2012 - link
As opposed to all the people who get new 2TB hard drives that fail in the first day?pc_void - Thursday, February 23, 2012 - link
So true. It is said that if it lasts for 3 months then it will probably last for years - talking about hard disk drives or anything for that matter.In my opinion, people brick ssd drives because they are not dummy proof.
pc_void - Thursday, February 23, 2012 - link
Except for the exceptions.Folterknecht - Thursday, February 23, 2012 - link
The same goes for many other components - hdds, gpu, mobo ...And often enough there is either a crappy PSU or RAM involved doesnt work as it should. I dont really trust those "user reviews" on sites like Newegg, to many people writing there that have no idea what they are talking about.
Of course firmware issues exist, not denying that, but thats no reason pass the best possible upgrade for your pc (in most cases).