A Brief Introduction to SSDs and Flash Memory

In almost every SSD review we have published, Anand has mentioned how an SSD is the biggest performance upgrade you can make today. Why would anyone use regular hard drives then? There is one big reason: price. SSD prices are still up in the clouds when compared to hard drive prices (especially before the Thailand floods) so for many, SSDs have not been a realistic option.

Forking over $700 for a 512GB SSD sounds crazy because a 500GB hard drive can be had for less than $50. Smaller capacities like 64GB and 128GB can already be bought for around $100 and $200 respectively, but unless you have the ability to have an SSD plus hard drive combo, such a small SSD doesn't usually cut it. If you have a desktop, the SSD + HDD combo should not be a problem but many laptops only have space for one 2.5" drive (unless you are willing to mod it afterwards by replacing the optical drive). SSD prices have been dropping for years now, but if the current rate continues it will take years before a $399 Walmart PC includes a reasonable size SSD. So what can be done?

Most of the time, SSD production costs are cut by shrinking the NAND die. Shrinking the die is the same as with CPUs: you move to a smaller manufacturing process, e.g. from 34nm to 25nm. In flash memory, this means you can increase the density per die and usually the physical die size is also smaller, meaning more dies from a single wafer. A die shrink is an effective way to lower costs but moving from one process to another takes time and the initial ramp of the new flash isn't necessarily cheaper. Once the new process has matured and supply has met demand, prices start to fall.

Since die shrinks are a relatively slow way to lower SSD prices and only contribute to steady reduction of prices, anyone looking to push higher capacity SSDs into the mainstream today will need something more. Right now, that "something more" is called Triple Level Cell flash, commonly abbreviated as TLC.

Rather than shrinking the die to improve density/capacity, TLC (like MLC) increases the number of bits per cell. In our SSD Anthology article, Anand described how SLC and MLC flash work, and TLC works the same way but takes things a step further. Normally, you apply a voltage to a cell and keep increasing it until you reach a point where the result is far enough from the "off" state that you now consider the cell as being "on". This is how SLC works, storing one bit per cell. For MLC, you store two bits per cell, which means instead of two voltage states (0 and 1) you have four states (00, 01, 10, 11). TLC takes that a step further and stores three bits per cell, or eight voltage states (000, 001, 010, 011, 100, 101, 110, and 111). We will take a deeper look into voltage states and how they work in the next page.

Even though SLC, MLC and TLC operate the same way, there is one crucial difference. Lets take a look at what happens to a NAND array depending on the amount of data per cell. The image above is a NAND array with ~16 billion transistors (one transistor is required per cell), i.e. 16 gigabits (Gb). This array can be turned into either SLC, MLC, or TLC. The actual array and transistors are equivalent in all three flash types; there is no physical difference. In the case of SLC flash, only one bit of data will be stored in one cell, hence your final product has a 16Gb capacity. When you up the bits per cell to two (MLC), you get 32Gb because now you have two bits per cell and there are still 16 billion cells. Likewise, three bits per cell (TLC) yields 48Gb.

However, TLC is a horse of slightly different color in this case. Capacities usually go in powers of two (2, 4, 8, 16 and so on) and 48 is not a power of two. To get a number that is a power of two, the original NAND array is chopped down. In our example, the array must be 10.67Gb in order to be 32Gb with three bits per cell, but since that is the same capacity as an MLC die, what is the benefit? You don't get more storage per die, but the actual die is smaller because the original 16Gb array has been reduced to a 10.7Gb array. That means more dies per wafer and hence lower cost.

Comparison of NAND Wholesale Prices
Cell Type SLC MLC TLC
Price per GB $3.00 $0.90

$0.60

Prices provided by OCZ

The theoretical price advantage of TLC isn't as great as SLC versus MLC, but it's still significant. In percentage, that is over a 30% reduction. The main reason is that MLC provides twice the capacity when compared to SLC (2bits per cell versus 1bit per cell), whereas TLC provides only 50% more than MLC (3bits per cell versus 2bits per cell). In fact, the price difference between MLC and TLC is directly proportional. TLC die is 33% smaller than a similar MLC die and in the prices provided by OCZ, TLC is also 33% cheaper than MLC. In theory, SLC should follow this equation as well and be priced at $1.80/GB, but there's limited 2Xnm SLC out in the wild, making SLC significantly more expensive than MLC and TLC at this point.

The reality of the matter is a little less clear. TLC NAND today isn't all that much cheaper than MLC NAND, which has contributed to its relative absence in the consumer SSD space. There's also a lack of controller support and market interest, which contribute to the higher prices of course. 

Weaknesses of TLC: One Step Worse than MLC
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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 ;-)
    Reply
  • 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.
    Reply
  • 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.
    Reply
  • 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.
    Reply
  • 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 .
    Reply
  • 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. Reply
  • jdjbuffalo - Thursday, February 23, 2012 - link

    As opposed to all the people who get new 2TB hard drives that fail in the first day? Reply
  • 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.
    Reply
  • pc_void - Thursday, February 23, 2012 - link

    Except for the exceptions. Reply
  • 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).
    Reply

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