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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  • proviewIT - Thursday, March 19, 2009 - link

    I bought a Vertex 120GB and it is NOT working on my Nvidia chipsets motherboard. Anyone met the same problem? I tried intel chipsets motherboard and seems ok.
    I used HDtach to test the read/write performance 4 days ago, wow, it was amazing. 160MB/s in write. But today I felt it slower and used HDtach to test again, it downs to single digit MB per second. Can I recover it or I need to return it?
    Reply
  • kmmatney - Thursday, March 19, 2009 - link

    Based on the results and price, I would say that the OCZ Vertex deserves a Editor's choice of some sort (Gold, Silver)... Reply
  • Tattered87 - Thursday, March 19, 2009 - link

    While I must admit I skipped over some of the more technical bits where SSD was explained in detail, I read the summaries and I've gotta admit this article was extremely helpful. I've been wanting to get one of these for a long time now but they've seemed too infantile in technological terms to put such a hefty investment in, until now.

    After reading about OCZ's response to you and how they've stepped it up and are willing to cut unimportant statistics in favor of lower latencies, I actually decided to purchase one myself. Figured I might as well show my appreciation to OCZ by grabbing up a 60GB SSD, not to mention it looks like it's by far the best purchase I can make SSD-wise for $200.

    Thanks for the awesome article, was a fun read, that's for sure.
    Reply
  • bsoft16384 - Thursday, March 19, 2009 - link

    Anand, I don't want to sound too negative in my comments. While I wouldn't call them unusable, there's no doubt that the random write performance of the JMicron SSDs sucks. I'm glad that you're actually running random I/O tests when so many other websites just run HDTune and call it a day.

    That X25-M for $340 is looking mighty tempting, though.
    Reply
  • MrSpadge - Thursday, March 19, 2009 - link

    Hi,

    first: great article, thanks to Anand and OCZ!

    Something crossed my mind when I saw the firmware-based trade-off between random writes and sequential transfer rates: couldn't that be adjusted dynamically to get the best of both worlds? Default to the current behaviour but switch into something resembling te old one when extensive sequential transfers are detected?

    Of course this neccesiates that the processor would be able to handle additional load and that the firmware changes don't involve permanent changes in the organization of the data.

    Maybe the OCZ-Team already thought about this and maybe nobody's going to read this post, buried deep within the comments..

    MrS
    Reply
  • Per Hansson - Thursday, March 19, 2009 - link

    Great work on the review Anand
    I really enjoyed reading it and learning from it
    Will there be any tests of the old timers like Mtron etc?
    Reply
  • tomoyo - Thursday, March 19, 2009 - link

    That was kind of strange to me too. But I assume Anand really means the desktop market, not the server storage/business market. Since it's highly doubtful that the general consumer will spend many times as much money for 15k SAS drives. Reply
  • Gary Key - Thursday, March 19, 2009 - link

    The intent was based it being the fastest for a consumer based desktop drive, the text has been updated to reflect that fact. Reply
  • tomoyo - Thursday, March 19, 2009 - link

    I've always been someone who wants real clarify and truth to the information on the internet. That's a problem because probably 90% of things are not. But Anand is one man I feel a lot of trust for because of great and complete articles such as this. This is truly the first time that I feel like I really understand what goes into ssd performance and why it can be good or bad. Thank you so much for being the most inciteful voice in the hardware community. And keep fighting those damn manufacturers who are scared of the facts getting in the way of their 200MB/s marketing bs. Reply
  • ryedizzel - Thursday, March 19, 2009 - link

    Anand,

    I just wanted to thank you for an amazing article. I am a very picky buyer and technology critic, so I always come to your site for the ‘real’ story on things. In fact for the amount of time, research, and (useful) testing methodologies you invest, I almost feel guilty receiving this information for free. Especially since your findings benefit the industry as a whole since it causes manufacturer’s to fix/improve their products (well at least the smart ones do). The i7 motherboard roundup was another great example of this. Seriously, if you have a place for donations I would send you $50 in a heartbeat. I know it’s not much but if others did the same it would add up to a decent token of appreciation.

    Oh and please don’t take people’s grammar or nitpicky corrections the wrong way. Yes it can be annoying, but in the end it does help the article become closer to perfect since others catch little things overlooked by human error. In the end we are all grateful for these articles, otherwise we wouldn’t be here reading them!
    Reply

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