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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  • aeternitas - Thursday, September 11, 2008 - link

    Wow.

    #1 Meaning; Great read.
    #2 Meaning; Why do you not use World of Warcraft in your testings more? This game still has huge system requirements on absolute full settings in crouded areas. No one is getting 60fps in that game in Shatt on a saterday night 16xAA/AF full distance view. Everything maxed running a high resolution. Also, the disk access of this game may not be as much as Crysis, but its far FAR more prolonged as people play this game for a matter of years, instead of a handfull of months tops.
    Reply
  • hoohoo - Wednesday, September 10, 2008 - link

    What?! Google Chrome Launch Time? Is this a new benchmark? The world was waiting for that test I can tell ya! Did Google slip a little something in your pocket? Reply
  • Ph0b0s - Wednesday, September 10, 2008 - link

    Sorry if this repeats what anyone else has said, but I think this is the best harddisk review I have seen. All the other hdd reviews I have read though, all go into detail about how many mb/s the drive can transfer and what the access time is, I/O performance etc. This is all very nice, but all we really care about is how does the hdd perform in real world apps and games in comparision to other drives. How much faster is my browser going to load.

    And that is why I was so impressed with this review. I realise it probably took more time to create this review, but it was well worth it. You can see how much the drives in the comparision compare to the amount of money you will spend instead of having to extrapolate from throughput benchmarks.

    I would feel a lot more confident in putting down money on the X25 after this review. Because I can see what the benefit I would get after in comparsion to other drives. Though the X25 needs to be another $100 dollars cheaper...

    Anyway, please more hdd reviews like this...
    Reply
  • bruto - Wednesday, September 10, 2008 - link

    Anand, could you please post map load times for R6 Vegas 2 using
    these drives?
    That game was killing me, and I'd like to know just how much
    happiness is around the corner :)
    Reply
  • FFFFFF - Tuesday, September 09, 2008 - link

    I am very familiar with the tech works of SSDs and it is interesting to know of the security risk that involves the technology. http://www.techworld.com/SECURITY/NEWS/index.cfm?n...">http://www.techworld.com/SECURITY/NEWS/...cfm?news...
    This is one reason why I am not going to upgrade to SSD just yet till they figure out a more secure encryption.
    Reply
  • cokelight - Tuesday, September 09, 2008 - link

    RE: PLEASE LEARN HOW TO USE COMMAS! by ggordonliddy, 19 hours ago

    You are just making it clear that you do not have a firm grasp of English. You are afraid of those who criticize poor grammar, because you know that you yourself will be exposed for the fraudulent pustule that lies beneath your slimy veneer.

    I have a life. I'm just sick of illiterate authors. Just because it is a tech site is no excuse for extremely poor writing skills. I'm talking about skills that should be completely mastered before being allowed to graduate from elementary school.

    ^
    Take your elitism elsewhere. If you actually practiced what you preached then you'd note your own comma abuse:

    "You are afraid of those who criticize poor grammar, because you know that you yourself will be exposed for the fraudulent pustule that lies beneath your slimy veneer."

    The second clause is dependent. Therefore, it does not necessitate a comma.
    Reply
  • aeternitas - Thursday, September 11, 2008 - link

    Use the reply link. Reply
  • xenon83 - Tuesday, September 09, 2008 - link

    "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."
    But when it comes to random writes ssd's in general - this intel ssd included - fails miserably.
    http://www.alternativerecursion.info/?p=106">http://www.alternativerecursion.info/?p=106
    Reply
  • johncl - Wednesday, September 10, 2008 - link

    Yes as the other poster say, the Intel drive have solved the issues with small random writes that the OZC and other MLC drives so far have been suffering from. If you read the whole article thoroughly you will see that clearly.

    Lets hope OCZ can come up with a better controller in their next generation MLC SSDs. We really need the competition here to bring those prices down, the Intel drive is a tad too expensive atm imo.
    Reply
  • balotz - Tuesday, September 09, 2008 - link

    Random writes are exactly as fast as sequential writes on this Intel SSD (10,000 random 4k writes per second).

    The article you referenced uses data from an SSD which appears to suffer from issues relating to the JMicron controller.

    The Intel SSD is obviously not affected, this is clearly shown by the numbers in this very review!
    Reply

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