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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  • kensiko - Wednesday, March 18, 2009 - link

    No, OCZ Summit = newest Samsung controller. The Corsair use the previous controller, smaller performance. Reply
  • VaultDweller - Wednesday, March 18, 2009 - link

    So what's the difference?

    The Summit is optimized for sequential performance at the cost of random I/O, as per the article. That is clearly not the case with the Corsair drive, so how does the Corsair hold up in terms of random I/O? That's what I'm interested in, since the sequential on the Corsair is "fast enough" if the random write performance is good.
    Reply
  • jatypc - Wednesday, March 18, 2009 - link

    A detailed description of how SSDs operate makes me wonder: Imagene hypothetically I have a SSD drive that is filled from more than 90% (e.g., 95%) and those 90% are read-only things (or almost read-only things such as exe and other application files). The remaining 10% is free or frequently written to (e.g., page/swap file). Then the use of drive results - from what I understood in the article - in very fast aging of those 10% of the SSD disk because the 90% are occupied by read-only stuff. If the disk in question has for instance 32GB, those 10% are 3.2 GB (e.g., a size of a usual swap file) and after writing it approx. 10000 times, the respective part of the disk would become dead. Being occupies by a swap file, this number of reads/writes can be achieved in one or two years... Am I right? Reply
  • sbuckler - Wednesday, March 18, 2009 - link

    Depends on how smart the controller is? Shuffling around the data now and again in the background would make sense. Reply
  • Frallan - Wednesday, March 18, 2009 - link


    @AT
    This is why i come to AT to read up on the developments.

    @OCZ
    Well played :0)

    The ruler of the roost are the Intels however I will be able to afford one of those when there are cows enjoying themselfs by dancing on the moon. My next upgrade will be a Vertex - not only bc its Valu for money but equally much bc. OCZ obviously takes care of thier customers and listens to reason.
    Reply
  • pmonti80 - Wednesday, March 18, 2009 - link

    This is the kind of article that makes me come back here. Reply
  • nowayout99 - Wednesday, March 18, 2009 - link

    OCZ, you should listen to Uncle Anand. ;) Hopefully Mr. Petersen understands that it's tough love.

    And the final product seems perfectly cool -- great performance at a better price than Intel. It's the first SSD I'd be able to reasonably consider.
    Reply
  • SOLIDNecro - Wednesday, March 18, 2009 - link

    Thx for this article Anand, I have been in a hotly contested debate over OCZ vs Samsung with my "Asperger Enhanced" nemisis/close friend...
    (In all fairness, I should mention I use the BiPolar SSE instruction set myself)

    He was only looking at Samsung, I said he should look into what OCZ has now.

    His reply was "I don't know them, and don't want to be disapointed"
    (Long story behind that...He's from the Server/Workstain/HPC crowd, I am from the hardcore OC/Gamer/Desktop group, so he is not familiar with OCZ)

    Looks like the Samsung (And alot of others) has "Issues" with performance degrading over time that are somewhat solved by Intel and OCZ (Plus maybe a few other companies that use the Rev B JMicron controller on there low cost SSD's)

    I agree the OCZ Vertex offers the best bang for low buck SSD today, and I am tempted to grab one. But a year from now, anyone that bought a current gen MLC SSD will be saying "I coulda had a V-8" if that TRIM technology does what it promises!!!
    Reply
  • James5mith - Wednesday, March 18, 2009 - link

    As people continue to try and push the envelope of storage performance in a variety of ways, and as 6gbps SATA becomes available, the performance of SSD's will only go up.

    As always, I wanted to say thanks for the great article and keep them coming. It's the only way the rest of us can keep pace with what's happening out there in the world of performance storage.
    Reply
  • vailr - Wednesday, March 18, 2009 - link

    Is there any benefit in using 2 SSD's in a Raid 0 configuration?
    And: any differences between motherboard Intel Raid vs. a Raid controller card from Areca, for example. Also: can the "Trim" command work while in Raid mode? Probably not, I'm guessing...
    Reply

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