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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  • hellcats - Saturday, March 21, 2009 - link

    Anand,

    What a great article. I usually have to skip forwards when things bog down, but they never did with this long, but very informative article. Your focus on what matters to users is why I always check anandtech first thing every morning.
    Reply
  • juraj - Saturday, March 21, 2009 - link

    I'm curious what capacity is the OCZ Vertex drive reviewed. Is it an 120 / 250g drive or supposedly slower 30 / 60g one? Reply
  • Symbolics - Friday, March 20, 2009 - link

    The method for generating "used" drives is flawed. For creating a true used drive, the spare blocks must be filled as well. Since this was not done, the results are biased towards the Intel drives with their generous amount of spare blocks that were *not* exhausted when producing the used state. An additional bias is introduced by the reduction of the IOmeter write test to 8 GB only. Perhaps there are enough spare blocks on the Intel drives so that these 8 GB can be written to "fresh" blocks without the need for (time-consuming) erase operations.
    Apart from these concerns, I enjoyed reading the article.
    Reply
  • unknownError - Saturday, March 21, 2009 - link

    I also just created an account to post, very nice article!
    Lots of good well thought out information, I'm so tired of synthetic benchmarks glad someone goes through the trouble to bench these things right (and appears to have the education to really understand them). Whats with the grammar police though? geez...
    Reply
  • Erickffd - Friday, March 20, 2009 - link

    Also created an account just to post this comment.

    Really impressive and well done article ! Will stay tune for further developments and reviews. Thank you so much :)

    Also... very impressed by OCZ's respond and commitment upon end users needs and product quality assurance (unfortunately not so commun by large this days among other companies). Certanly will buy from them my next SSDs to reward and support their healty policy.

    Be well ! ;)
    Reply
  • Gasaraki88 - Friday, March 20, 2009 - link

    This truly was a GREAT article. I enjoyed reading it and was very informative. Thank you so much. That's why Anandtech is the best site out there. Reply
  • davidlants - Friday, March 20, 2009 - link

    This is one of the best tech articles I have ever read, I created an account just to post this comment. I've been a fan of Anandtech for years and articles like this (and the RV700 article from a while back) show the truly unique perspective and access that Anand has that simply no other tech site can match. GREAT WORK!!! Reply
  • Zak - Friday, March 20, 2009 - link

    I just got the Apex. I'd probably cough up more dough for the Vertex after reading this. However, I've run it for two days as my system disk in MacPro and haven't noticed any issues, it's really fast. But I guess I'll get Vertex for my Windows 7 build.

    Z.
    Reply
  • Nemokrad - Friday, March 20, 2009 - link

    What I find intriguing about this article is that these smaller manufacturers do not do real world internal testing for these things. They should not need 3rd parties like you to figure this shit out for them. Maybe now OCZ will learn what they need to do for the future. Reply
  • JonasR - Friday, March 20, 2009 - link


    Thanks for an excellent article. I have one question does anyone know which controller is beeing used in the new Patriot 256GB V.3 SSD?
    Reply

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