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
POST A COMMENT

90 Comments

View All Comments

  • BPB - Thursday, February 23, 2012 - link

    I just ordered the OCZ Synapse to use as a caching drive. The reviews all rave about speed increases. I am putting it in my main system. Would be nice if you could tell us where they fit in.

    Also, anybody got recommendations on an SSD to get an older system to feel faster for surfing and the like?
    Reply
  • Kristian Vättö - Thursday, February 23, 2012 - link

    It's hard for me to say anything about caching SSDs because we haven't reviewed any other than Intel 311 Series (yet). IIRC Synapse comes with its own caching software which is different from Intel's SRT.

    Fortunately, we have some more staff working on SSD stuff now. As you may have noticed, so far Anand has done all the SSD reviews. To reduce Anand's workload, I'll be doing some of the SSD reviews in the future, which should allow us to review more SSDs. In fact, I have Plextor M3 SSD on its way here :-)

    As for the SSD for an older system, is it a desktop or laptop? I think the best option would be a SSD+HD combo because that is cheap and still lets you have a decent amount of storage. You can try to find older SATA 3Gb/s SSDs (e.g. Intel X25-M G2 or Samsung 470 Series, they are very reliable). You can even hunt for a used drive, some people are already switching for faster SATA 6Gb/s SSDs so you may find a bargain.
    Reply
  • ckryan - Thursday, February 23, 2012 - link

    Synapse comes with NVELO's dataplex caching software, and there should be more consumer target caching solutions out soon. Reply
  • macuser2134 - Friday, February 24, 2012 - link

    An upcoming Plextor M3 review - this is exciting! It will certainly be interesting to find out how a Plextor drive compares to other manufacturers.

    As a side note the "Pro" version of the Plextor M3 just started selling on Newegg only 2 days ago. Models PX-128M3P, PX-256M3P etc.
    Reply
  • seanleeforever - Monday, February 27, 2012 - link

    there is something else the articles did not address. the life time of a certain SSD device is a combination of its cell reliability and how often it get re-written to.

    take a look at the second page, it may look like at 3x nm, the SLC can last 20 times more than MLC. However, from a device point of view, a 120 GB SLC can well last 40 or more time than a 120 GB MLC because as you write and delete file over and over.

    for example, in order to re-write the entire 120 GB of information, each cell of the SLC only get erase-write once while a 120GB MLC will most likely been erased-write twice (say to change 11 to 10 to and to 00 , a SLC will need to erase and write once on each of two cells where MLC will need to erase and write at least 3 times on a single cell), or try to imagine a super MLC cell that has all the voltage level needed for 120GB storage in one cell, then every-time something is changed that cell get re-written.

    this just get a lot worse in TLC design, as you reduce the number of cells to realize more storage space, you are reducing the error margin as well as increasing the cycles. the old saying still applies "there is no displacement for replacement". there is no free lunch.
    Reply
  • BPB - Friday, February 24, 2012 - link

    Thanks for the reply. I'll look into your suggestions, though given what I understand to be the limited life of SSDs I think I'll go new. Thanks again. Reply
  • Shadowmaster625 - Friday, February 24, 2012 - link

    Please keep an eye out for caching solutions such as NVELO's dataplex caching software. I am looking for one that works with XP. It doesnt make much sense to have to upgrade the OS if all you're looking for is a cheap upgrade to a 5 year old pc. $69 for a caching drive is one thing. $220 for a drive and an OS is quite another... Reply
  • JNo - Friday, February 24, 2012 - link

    Agree. I mentioned the NVELO caching software on these boards weeks (maybe months) ago after storagereview looked at them. It would be really good if you got some of their drives or even just software in to review. It is exciting that there is a competitor to Intel's caching in this space. Who knows? They may even be faster, cheaper and better.... Reply
  • xrror - Monday, February 27, 2012 - link

    I'd also like to see some analysis on NVELO "solutions." I was looking pretty seriously at buying Corsair's version of this with their Accelerator series drives as an "impulse purchase" but lack of availability of their 60Gb package and a good read of NVELO's software "licensing" put a quick halt on that.

    NVELO looks like a killer app assuming it works. Sadly I expect that it does live up to it's claims, but their DRM is pretty harsh. It's locked to your hardware, so if you say change your video card you must "re-activate."

    I know that for most people that's no worse than windows itself, but I change hardware a lot, and/or I'd like to be able to move the SSD cache to other machines in my house w/o worrying that I'll get DRM lockout.
    Reply
  • Roland00Address - Thursday, February 23, 2012 - link

    57 months ago (4.75 years) you could get a 16gb supertalent for 600 dollars
    41 months ago (3.41 years) you could get a 80gb intel (1st gen) ssd for 600 dollars.

    Small deviations make a big difference when you are calculating exponential growth (and decrease)
    Reply

Log in

Don't have an account? Sign up now