The Anatomy of an SSD

Let’s meet Mr. N-channel MOSFET again:

Say Hello

This is the building block of NAND-flash; one transistor is required per cell. A single NAND-flash cell can either store one or two bits of data. If it stores one, then it’s called a Single Level Cell (SLC) flash and if it stores two then it’s a Multi Level Cell (MLC) flash. Both are physically made the same way; in fact there’s nothing that separates MLC from SLC flash, it’s just a matter of how the data is stored in and read from the cell.


SLC flash (left) vs. MLC flash (right)

Flash is read from and written to in a guess-and-test fashion. You apply a voltage to the cell and check to see how it responds. You keep increasing the voltage until you get a result.

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

 

With four voltage levels to check, MLC flash takes around 3x longer to write to as SLC. On the flip side you get twice the capacity at the same cost. Because of this distinction, and the fact that even MLC flash is more than fast enough for a SSD, you’ll only see MLC used for desktop SSDs while SLC is used for enterprise level server SSDs.


Cells are strung together in arrays as depicted in the image to the right

So a single cell stores either one or two bits of data, but where do we go from there? Groups of cells are organized into pages, the smallest structure that’s readable/writable in a SSD. Today 4KB pages are standard on SSDs.

Pages are grouped together into blocks; today it’s common to have 128 pages in a block (512KB in a block). A block is the smallest structure that can be erased in a NAND-flash device. So while you can read from and write to a page, you can only erase a block (128 pages at a time). This is where many of the SSD’s problems stem from, I’ll repeat this again later because it’s one of the most important parts of understanding SSDs.


Arrays of cells are grouped into a page, arrays of pages are grouped into blocks

Blocks are then grouped into planes, and you’ll find multiple planes on a single NAND-flash die.

The combining doesn’t stop there; you can usually find either one, two or four die per package. While you’ll see a single NAND-flash IC, there may actually be two or four die in that package. You can also stack multiple ICs on top of each other to minimize board real estate usage.

Hey, There’s an Elephant in the Room Strength in Numbers, What makes SSDs Fast
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  • siberx - Thursday, March 19, 2009 - link

    This is, very likely, the best article I have ever read, period. Online, in magazines, about any subject... this was an absolutely fantastic read. Suddenly, all smoke surrounding SSDs has cleared and the truth shines through in editorial brilliance. It's great to see that at least some computer news sites out there can still cut through the crap and get to the heart of the issue. My already high opinion of AnandTech has risen even further.

    Thank you for taking the immense time it must have taken to compile and assemble all this information - this article is now a must-read for *anybody* considering purchasing an SSD, and it's just about all the background you could need in one place.

    In addition to all the extremely useful general SSD information contained within, the detailing of the issues with the JMicron controllers as well as OCZ's efforts to address the concerns to produce the best product possible (despite the reduced marketability to the uninformed) is reassuring and comforting in a world where tech companies seem more concerned with how much they can deceive their customers instead of producing quality products.

    In short, the article is a win on all fronts, thank you greatly for posting it. When I purchase my first SSD (which I'm considering doing reasonably soon) this article, its information and suggestions, and OCZs actions to resolve the issues with its drives will definitely be at the forefront of my mind.
    Reply
  • jkua - Thursday, March 19, 2009 - link

    I have to say, I really appreciate the effort and throughness with which you have covered the state of the SSD market today. As an engineer and scientist, I applaud your methods in tracking down and reporting the major issues with SSDs. As a consumer, I really appreciate the timeliness of this article as I was just thinking of putting an SSD in a netbook for a robotics application where mechanical drives are not ideal.

    Cheers!
    Reply
  • jkua - Thursday, March 19, 2009 - link

    That said, one thing I would have like to have seen is some numbers on power consumption for these drives compared to average mechanical desktop and laptop drives. Reply
  • aamsel - Thursday, March 19, 2009 - link

    Anyone have a link to the Intel HDD ERASE program that Anand referred to? Reply
  • HolyFire - Thursday, March 19, 2009 - link

    http://www.ultimatebootcd.com/download.html">http://www.ultimatebootcd.com/download.html (includes HDD erase 3.1)

    http://cmrr.ucsd.edu/people/Hughes/SecureErase.sht...">http://cmrr.ucsd.edu/people/Hughes/SecureErase.sht... (version 4.0)
    Reply
  • AnnonymousCoward - Thursday, March 19, 2009 - link

    AWESOME ARTICLE.

    The huge difference in read/write flash performance looks a lot like this article: http://www.anandtech.com/memory/showdoc.aspx?i=257...">http://www.anandtech.com/memory/showdoc.aspx?i=257...
    Reply
  • wind glider - Thursday, March 19, 2009 - link

    Thanks for the orgasmic review. Reply
  • wicko - Wednesday, March 18, 2009 - link

    Had a really good read here, thanks for the history and info, Anand. The only thing I don't understand is what the importance of random write is? What kind of task would benefit from high random write speeds (maybe copying many files at once)? I'm tempted to pick up a vertex drive but it depends on whether or not random write will be important for me. But the price... whoa, pretty damn expensive here in Canada.. http://www.ncix.com/products/index.php?sku=36023&a...">http://www.ncix.com/products/index.php?...X120G&am... - $625 for a 120GB!!! I kind of want 2, for RAID0, I have a lot of games installed (steam folder alone is 100GB lol). Might even have to raid 3 of em.. but not for $1800 lol. Reply
  • strikeback03 - Thursday, March 19, 2009 - link

    As mentioned in the article, the OS in general makes lots of random writes. Send an IM, it writes to a log. Load a website, it caches some images. Reply
  • AnnonymousCoward - Thursday, March 19, 2009 - link

    >I'm tempted to pick up a vertex drive but it depends on
    >whether or not random write will be important for me.
    Keep in mind, its random write is twice as fast as mechanical HDs.

    >But the price...
    It's only $110USD/32GB.
    Reply

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