How SSDs Work

The building block of NAND flash is the N-channel MOSFET:

Each "cell" is made up of one of these transistors. In a single-level cell (SLC) flash device, one of these transistors can hold 1-bit of data. You write data to the cell by electron tunneling; apply a high enough voltage to the gate, create a powerful enough electric field, and electrons will tunnel through the oxide and into the floating gate. Remove the voltage and the electrons will remain in the floating gate. Apply the voltage across the channel instead of the gate, reverse the bias and the electrons will go in the other direction. Simply put, that's how flash works - you've got two states, 0 and 1, and the state is preserved even if the cell has no power, making it ideal for a storage device.

Programming flash is an iterative process. The controller will apply voltage to the gate (or the channel), allow some electrons to tunnel and check the threshold voltage of the cell. When the threshold voltage has reached some predetermined value, it’s now programmed and your data is stored.

MLC vs. SLC

There are two forms of NAND flash used in SSDs today: Single-Level Cell (SLC) and Multi-Level Cell (MLC). The difference between the two is the amount of data stored per cell, with SLC it's 1-bit per cell and with MLC it's 2-bits per cell. The key here is that both SLC and MLC take up the same amount of die area, so MLC effectively doubles your capacity at the same price.

Intel actually uses the same transistors for its SLC and MLC flash, the difference is how you read/write the two. With SLC there are only two voltages to worry about, since there are two states (0 or 1). With MLC, there are four states (00, 01, 10, 11) and thus it takes longer to access since you don't want to accidentally write the wrong bit of data; you've got the same min and max voltage, you simply have more graduations in between the two now:


SLC (left) vs. MLC (right)

Below is a table of some basic stats on SLC vs. MLC performance:

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

 

Erasing performance is the same between the two, read performance takes twice as long on MLC flash and write performance can take almost four times as long. If you've ever heard people complain about MLC write speed before, this partly why. Do keep in mind though, the numbers we're talking about here are ridiculously low - even 900 µs to write to MLC flash is much faster than writing to a mechanical hard disk.

The biggest advantage of SLC ends up not being performance, but lifespan. To understand how flash wears, we first need to look at how it's organized in a storage device.

Index The Flash Hierarchy & Data Loss
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  • Googer - Monday, September 08, 2008 - link

    Today, you can pick up a 160GB HDD for $50 and a 320GB HDD for around $90-100. This make the 80GB SSD 20x more expensive than a HDD of the same size.


    Reply
  • aeternitas - Thursday, September 11, 2008 - link

    I dont know where you shop, but 100$ can get you 750GB.


    It doesnt make much of a point anyway, as flash based and typical hard drives will coexist for at least the next ten years. One for preformance, the other for storage.

    Maybe some of you are too young to remember, but it was only about 10 years ago that a 20GB hard drive cost 200$ at costco. We are a huge step in terms of the new technology from where we were before. 32 GB for 100$? Yes please. In ten years i expect to see 3+TB flash drives running over 1GBps throughput for around 200$.
    Reply
  • strikeback03 - Thursday, September 11, 2008 - link

    I assume they were referring to 2.5 inch drives, as those prices are more in line with what was stated.

    Also don't forget that with the drop in $/GB of drives has come an increase in demand for storage. Ten years ago digital photography was almost non-existent, with file sizes topping out under a megabyte. Images on the web were maybe 640x480. Now we have digital cameras that average 4-6MB as JPEGs, and some can turn out images over 30MB, not counting the medium-format backs (over 100MB IIRC for the 65Megapixel Phase 1). When I was in college any movie you saw on the network was sized to fit a 700MB CD, Now HD movies can range well over that. MP3s are still the same, or you can store in a lossless format if you choose.

    Will 3TB be small in 10 years? Possibly. The key for flash completely replacing mechanical will be getting the price low enough to hold large amounts of the current data.
    Reply
  • aeternitas - Saturday, September 13, 2008 - link

    Even if they were useing 2.5" drives in the example, it doesnt matter. The fact that these drives will be replaceing 3.5" ones makes their point just as misguided in the point they were trying to make.

    Laptop and desktop drives were differant due to size, now they arnt, so you cant comapair them (HDD vs SSD) in such a illogical manner.
    Reply
  • Blimeynext - Friday, April 23, 2010 - link

    "Intel went one step further and delivered 5x what the OEMs requested. Thus Intel will guarantee that you can write 100GB of data to one of its MLC SSDs every day, for the next five years, and your data will remain intact."

    Any tool out there which tells me how much I write to my HDD each day? It would be nice to find out my present usage and compare it with the 100 GB/day limit....

    If I write less than 20GB a day does that mean the SSD will last at least 25 years?
    Reply
  • Smokerz - Sunday, January 02, 2011 - link

    http://www.bit-tech.net/hardware/apple/2010/07/01/...

    read it all
    Reply
  • Smokerz - Sunday, January 02, 2011 - link

    sorry for the misspell Reply

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