How SSDs Work

The building block of NAND flash is the N-channel MOSFET:

Each "cell" is made up of one of these transistors. In a single-level cell (SLC) flash device, one of these transistors can hold 1-bit of data. You write data to the cell by electron tunneling; apply a high enough voltage to the gate, create a powerful enough electric field, and electrons will tunnel through the oxide and into the floating gate. Remove the voltage and the electrons will remain in the floating gate. Apply the voltage across the channel instead of the gate, reverse the bias and the electrons will go in the other direction. Simply put, that's how flash works - you've got two states, 0 and 1, and the state is preserved even if the cell has no power, making it ideal for a storage device.

Programming flash is an iterative process. The controller will apply voltage to the gate (or the channel), allow some electrons to tunnel and check the threshold voltage of the cell. When the threshold voltage has reached some predetermined value, it’s now programmed and your data is stored.

MLC vs. SLC

There are two forms of NAND flash used in SSDs today: Single-Level Cell (SLC) and Multi-Level Cell (MLC). The difference between the two is the amount of data stored per cell, with SLC it's 1-bit per cell and with MLC it's 2-bits per cell. The key here is that both SLC and MLC take up the same amount of die area, so MLC effectively doubles your capacity at the same price.

Intel actually uses the same transistors for its SLC and MLC flash, the difference is how you read/write the two. With SLC there are only two voltages to worry about, since there are two states (0 or 1). With MLC, there are four states (00, 01, 10, 11) and thus it takes longer to access since you don't want to accidentally write the wrong bit of data; you've got the same min and max voltage, you simply have more graduations in between the two now:


SLC (left) vs. MLC (right)

Below is a table of some basic stats on SLC vs. MLC performance:

  SLC NAND flash MLC NAND flash
Random Read 25 µs 50 µs
Erase 2ms per block 2ms per block
Programming 250 µs 900 µs

 

Erasing performance is the same between the two, read performance takes twice as long on MLC flash and write performance can take almost four times as long. If you've ever heard people complain about MLC write speed before, this partly why. Do keep in mind though, the numbers we're talking about here are ridiculously low - even 900 µs to write to MLC flash is much faster than writing to a mechanical hard disk.

The biggest advantage of SLC ends up not being performance, but lifespan. To understand how flash wears, we first need to look at how it's organized in a storage device.

Index The Flash Hierarchy & Data Loss
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  • Anand Lal Shimpi - Tuesday, September 09, 2008 - link

    I think the question was: how much more performance is left untapped by current controller designs? The JMicron issues are a limited case, what will truly be telling is what happens when we see Intel vs. Samsung with SLC drives...

    The dominating the charts line was in reference to the Crysis results. If you've ever run the Crysis GPU bench you'll know that it is extremely disk intensive (particularly the first run). As I mentioned in the article, it over emphasizes the importance of disk performance but that's not to say that the results aren't valid.

    I do see your point however, let me see what I can do about clarifying that statement.

    -A
    Reply
  • yyrkoon - Tuesday, September 09, 2008 - link

    Ok, I guess I missed the JMicron 'thing', but to be perfectly honest I dislike *anything* JMicron and try to avoid them whenever possible. I guess I am just so interested in these Intel drives, I just tuned everyting else out. However, I did read what you mentioned about 'trouble-shooting' the JMicron MLC issue.

    Never ran Crysis, and do not plan on running it anytime soon if ever, but I am somewhat of a hardcore gamer.

    Keep up the good work, and PLEASE do keep us informed on at least these Intel SSD drives :)
    Reply
  • BD2003 - Monday, September 08, 2008 - link

    If the achilles heel of the JMicron MLC is the random write speed, why couldnt a ram buffer be used to cache writes? Sure this would cause a serious problem if the power went out, but thats an issue some would be willing to live with.

    I'm fairly sure vista has an option for this in the device manager in the properties tab of a drive - "enable advanced disk performance". I wonder if that would have any effect on the results?
    Reply
  • DigitalFreak - Monday, September 08, 2008 - link

    Yet more proof that JMicron products are shit. Reply
  • ggordonliddy - Monday, September 08, 2008 - link

    For the love of all humanity: If you are going to write for a living, please learn basic comma usage!

    It is NOT okay to just stick a comma in the middle of a sentence anytime you want. And it gives readers a headache.

    Here is just one of numerous examples of improper comma usage I've seen so far (and I've only gotten to the 3rd page!):

    "Intel certifies its drives in accordance with the JEDEC specs from 0 - 70C, at optimal temperatures your data will last even longer [...]"

    The comma before "at optimal" should be replaced with a semicolon or a period (I prefer the semicolon).

    Did you actually pass your English classes? I'm guessing that you probably did and you are just a product of our miserable public school system that refuses to hold students to any real level of accountability.


    (And BTW, your quoting system is broken. When I enter text in the Quote Text dialog and click OK, nothing new appears in the Comment compose field.)
    Reply
  • 7Enigma - Friday, September 19, 2008 - link

    Honestly man, you need to seriously relax. My personal rule of thumb for grammar is does the mistake make the understanding of the sentence difficult to comprehend.

    Writing something like, "Intel certifies its drives in accordance with the JEDEC specs from 0 - 70C, at optimal temperatures your data will last even longer [...]", while not grammatically correct is completely readable.

    If it was something like, ""Intel certifies drives to accordance with the JEDEC specs from 0 - 70C, at optimal data your temperatures will last even longer [...]", now you have a legitimate beef.

    The former can easily be forgiven, the latter makes my head hurt when I read it. Trust me, whatever you do, do not go to Dailytech.com and read the articles. Those even I get annoyed at frequently and I'm very forgiving.
    Reply
  • Anand Lal Shimpi - Tuesday, September 09, 2008 - link

    You're quite right, thanks for the heads up :) Some of the article was directly from my notes while I was working on the tests, so that's one source of unpolished bits. I know I'm far from perfect, so I do appreciate your (and anyone else's) assistance.

    Thanks :)

    Anand
    Reply
  • pkp - Tuesday, September 09, 2008 - link

    Thanks for posting, Anand. I see you're already aware of the problem, but I wanted to throw my two cents in.

    What is the usual editing process? I think a once over by a second set of eyes would have caught the bulk of the grammatical errors.

    Of course, the ultimate issue isn't commas. It's readability. However, the problem was bad enough that I'm making this comment without having even gotten through the first page of this article.
    Reply
  • JarredWalton - Tuesday, September 09, 2008 - link

    I'm often the content editor for posted articles, but often we skip that stage due to late nights and schedules. Doing a final thorough edit can require a couple hours (edit and then HTLM-ize), and when someone finishes an article at 5AM or whatever and it's an NDA type piece, delaying it any further is usually not desired by the readers or us.

    I do read all posted articles, and often I take the time to go through and fix any noteworthy errors. A few misplaced commas don't really detract from a 5000 word article, however, and depending on what else is going on I may or may not edit the text. If anyone takes the time to point out specific errors, i.e. "on page 3 you write "...." they always get corrected - at least if I see it. General complaints are much more difficult to address though, i.e. "You used passive voice and therefore you must DIE!" LOL.

    I know personally that when you write a long article with lots of testing, certain thoughts tend to appear in multiple places and the final result isn't always as coherent as I would like. Trying to "fix" problems relating to flow and readability is difficult at best, and requires more time than we generally spend. If anyone wants to make specific suggestions, though, we're open for input as always.

    Perhaps it's useful to compare the process to print publications. Magazines usually have several editors on staff whose job is solely to edit other authors' work; I can say that we don't have anyone at AnandTech in that position these days. (I edit some of the articles, but not all, and even then I make mistakes.) That's probably why we have more typos than magazines, but then we provide far more thorough coverage as well. Last I saw, most magazine hardware reviews end up being one page and ~1000 words, with a couple charts.

    At the end of the day, I get most of my detailed information from the internet. Magazines might be more grammatically correct, and they make for great toilet reading, but I don't generally depend on them as a source of credible information. I'd say it's safe to say we won't see such an in-depth exploration of SSD performance and issues in any magazine. [Now I have to prepare to have someone point me to an article in some magazine that does exactly that.]

    Cheers,
    Jarred
    Reply
  • KikassAssassin - Tuesday, September 09, 2008 - link

    Then I guess I should point you to an article in last month's issue of my favorite data storage magazine.

    http://www.solidstatedisksmonthly.com/2008/08/ever...">http://www.solidstatedisksmonthly.com/2...erforman...

    Unfortunately, their website seems to be down at the moment, but keep checking it, I'm sure it'll be back up soon (and don't be fooled by the article's title. It's actually only 23 pages without the ads).
    Reply

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