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
Price per GB $3.00 $0.90


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 - Thursday, February 23, 2012 - link

    That's definitely a very interesting idea, I haven't actually thought about it. Maybe we will see something like that in the future. It should be feasible since we have products like Momentus XT.
  • marraco - Thursday, February 23, 2012 - link

    That's what I was planning to write, so I agree, but it should be taken further: A weared out TLC cell should not be taken away by the controller/firmware. Instead it should be "degraded" to a MLC, and once it degrades, it should be used a s SLC.
  • hechacker1 - Friday, February 24, 2012 - link

    While that's an interesting concept, you would have to over provision it even more to account for storage loss as it is unable to store as many bits. It would be easier to define at creation that some of the NAND would be MLC, and some TLC.

    With really large SSD's, I think the life of TLC will be pretty good simply because we'll have so much storage to work with.

    Imagine if you had a 1TB SSD, with a low 750 cycles. You could still potentially get around 750TB (minus amplification) of writes onto it.
  • xrror - Monday, February 27, 2012 - link

    Hah, I had this same thought also.

    What would be great fun (but a nightmare for OEMs to validate) is if you as a user could arbitrarily choose what "modes" to run the flash in.

    It'd also be ironic that as an SSD "wore out" it would drastically lose capacity but yet become faster while doing it ;p
  • ViRGE - Friday, February 24, 2012 - link

    It seems like this would play hell with wear leveling though. Even though you're trying to segregate (largely) static data into TLC NAND, you're still going to periodically write to TLC NAND and as such need to do wear leveling to keep from burning out a smaller number of the TLC NAND cells too soon. It seems like the need to wear level would largely negate the segregation of data.
  • Mr. GlotzTV - Thursday, February 23, 2012 - link

    Since SLC, MLC & TLC are physically the same why not make the firmware dynamic?
    A new (empty) starts storing information as SLC, when more storage is needed saves it as MLC or TLC. To ensure good performance and a long life of the drive it should store frequently modified & temporary files as SLC, other things like movies and music (files where speed is not important and aren't modified a lot) should be stored as TLC.
    other thoughts:
    when a TLC cell is too worn change it to MLC and later to SLC!

    I know this would require a new very complex (and probably buggy) firmware. But are there any concepts or something?
  • jjj - Thursday, February 23, 2012 - link

  • Mr. GlotzTV - Thursday, February 23, 2012 - link

    as far as I understood it is about removing DRAM and using some kind of pseudo SLC cache instead. Not exactly what I was thinking about but good to know anyway.
    THX for the link.
  • Kristian Vättö - Thursday, February 23, 2012 - link

    Interesting idea, though I'm not sure if it's possible. While SLC, MLC and TLC are physically the same (i.e. they consist of the same transistors), I'm not sure what kind of process needs to be used to turn a NAND array into MLC or TLC instead of SLC. I would guess that it's more than what a simple SSD controller can do.

    I can try to dig up more info on NAND manufacturing and hopefully it will shed some light to this. Either way, it does sound very complicated and the possibility of data loss is huge if the NAND type is changed during use (you can't really go from TLC to SLC without having a huge cache).
  • ckryan - Thursday, February 23, 2012 - link

    There is MLC-1, which is MLC which stores only 1 bit like SLC. It's almost as good as SLC, but I assume is much cheaper -- MLC is much cheaper than SLC (even if you're discarding half the capacity). I believe FusionIO uses this in some applications.

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