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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  • VaultDweller - Wednesday, March 18, 2009 - link

    I love it when people critique someone's critique of grammar... and get it wrong.

    It's an SSD, not a SSD.
    Reply
  • gwolfman - Wednesday, March 18, 2009 - link

    lolz Reply
  • sidex - Wednesday, March 18, 2009 - link

    I would like to know the firmware version of Vertex used in your review. To me sounds the old 0112 Reply
  • kensiko - Wednesday, March 18, 2009 - link

    Yes that is important to know.

    I'm sure this is not done with the latest firmware available which is 1199. This version got better performance.

    Firmware 1275 is coming also.

    Anand, will you update your benchmarks with the latest firmware?

    If not, then the benchmarks are obsolete.
    Reply
  • Anand Lal Shimpi - Wednesday, March 18, 2009 - link

    I tested with the shipping firmware for this article (0122). I've been playing around with 1199 in the lab and will most likely have an update in a couple of weeks once I've done a thorough evaluation of it. By then I should also have the final version of the new Samsung drive and maybe even some other interesting things.

    For now, I've got to get to work on the new Mac Pro and the updated Ion article :) I need a small break from SSDs por favor :)

    Take care,
    Anand
    Reply
  • VaultDweller - Wednesday, March 18, 2009 - link

    Awww, don't you have some underlings to do your SSD-related will?

    Would love to see an update, and would love to see Corsair's SSD drive tested as well (it's based on Samsung's last generation MLC controller, and doesn't seem to emphasize sequential like the Summit does).
    Reply
  • Anand Lal Shimpi - Wednesday, March 18, 2009 - link

    I will do an update on the new firmware, I just want to do it right so it'll take some time :)

    I'll put in a request for the Corsair drive as well :)

    -A
    Reply
  • Slash3 - Wednesday, March 18, 2009 - link

    Page 29: "Not all applications will launch faster than a VelociRaptor on a SSD, but let's not forget that the VelociRaptor is the world's fastest hard drive."

    Really? What about the nice and speedy enterprise-level 15k SAS/SCSI drives everyone neglects to acknowledge? :)
    Reply
  • George Powell - Wednesday, March 18, 2009 - link

    I believe it refers to consumer drives. While SAS drives are beginning to be a possibility on the desktop with newer motherboards supporting them natively, the drives themselves are too expensive and too noisy for most consumers to actually want them.


    Reply
  • FishTankX - Wednesday, March 18, 2009 - link

    Good info. However, I noticed one mistake.

    Second page
    Samsung had a MLC controller at the time but it was too expensive than what SuperTalent was shooting for.
    Reply

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