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
POST A COMMENT

97 Comments

View All Comments

  • npp - Monday, September 08, 2008 - link

    I first sought the review of the drive on techreport today, and it was jawdropping - 230 Mb/s sustained read, 70 Mb/s write, 0,08 s access time... And all those unbelievable IOPS figures in the iometer test. The review here confirms all I've read, and it's amazing. Now I can see why SATA 3 is on the way - saturating a SATA 2 channel may become a real issue soon.

    The only field where the drive "fails" is write performance - and now I can imagine what the SLC version will be able to deliver. I guess it will be the fastest single drive around.

    I really liked the comment about Nehalem - sure, one of those SSD beasts will make much more of a difference compared to a $1k Bloomfield. Nice!
    Reply
  • vijay333 - Monday, September 08, 2008 - link

    lots of good info...thanks.

    in for one as soon as they bump up capacity and reduce price...not asking for much i think :)
    Reply
  • wien - Monday, September 08, 2008 - link

    Excellent review, and a good read throughout. I especially enjoyed the way you guided us through your thought-process when looking into the latency issue. I love fiddling around trying to figure stuff out, so that part made me envious of your job. :) Reply
  • darckhart - Monday, September 08, 2008 - link

    i don't know the technical differences, but i've run into so many problems with the jmicron controllers on the recent motherboards these days that i can't understand why anyone would choose to use jmicron for *any* of their products. surely the cost isn't *that* much lower than the competition? Reply
  • leexgx - Monday, September 08, 2008 - link

    i thought there an problem with SSD + intel chip sets makeing poor performace wish SSD,
    as an intel chip set was used have you tryed doing some tests on an nvidia board or AMD
    Reply
  • Gary Key - Monday, September 08, 2008 - link

    There was until the March 2008 driver updates from Intel. Performance is basically on-par between the three platforms now with Standard IDE and AHCI configurations, still testing RAID. Reply
  • michal1980 - Monday, September 08, 2008 - link

    IMHO, the price drop will be even more brutal then you think.

    in a year, prices should be, 1/2 and capacity double. so about 300 dollars for a 160gb. Flash memories growth rate right now is amazing.
    Reply
  • rree - Wednesday, January 06, 2010 - link

    http://ecartshopping.biz">http://ecartshopping.biz

    Air jordan(1-24)shoes $33

    Nike shox(R4,NZ,OZ,TL1,TL2,TL3) $35
    Handbags(Coach lv fendi d&g) $35
    Tshirts (Polo ,ed hardy,lacoste) $16

    Jean(True Religion,ed hardy,coogi) $30
    Sunglasses(Oakey,coach,gucci,Armaini) $16
    New era cap $15

    Bikini (Ed hardy,polo) $25

    FREE sHIPPING
    http://ecartshopping.biz">http://ecartshopping.biz
    Reply
  • leexgx - Thursday, January 22, 2009 - link

    we need the review of the new V2

    http://www.dailytech.com/Exclusive+Interview+With+...">http://www.dailytech.com/Exclusive+Inte...on+on+SS...
    Reply
  • ksherman - Monday, September 08, 2008 - link

    And then if they can keep that price, but double capacity again two years from now, a $300 320GB SSD would be exactly what I am looking forward to for my next laptop! Reply

Log in

Don't have an account? Sign up now