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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  • 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... :-)
    Reply
  • 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. Reply
  • 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.
    Reply
  • AnnihilatorX - Friday, February 24, 2012 - link

    It has been claimed that algorithms to minimize write amplification will follow Moore's Law


    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)
    Reply
  • 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... Reply
  • 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
    Reply
  • 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.)
    Reply
  • Hulk - Friday, February 24, 2012 - link

    Timely, informative, well written, and just the right amount of technical detail.
    Really nice job.
    Reply
  • 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.
    Reply
  • 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
    Reply

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