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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  • bharatwaja - Tuesday, September 09, 2008 - link

    This is by far the best article I have ever read about SSDs... Anand you ROCK!!! and EVERYONE at ANANDTECH ROCKS!....

    Great article... Thank you........ you just saved me from spending $99 on a pretty much useless SSD.
    [Takes a printout of Anand to worship]
    Reply
  • hyperdoggy - Tuesday, September 09, 2008 - link

    I noticed that ocz have released core v2 series of their ssd. It seems to come in a different size configurations than its 1st gen product, can you confirm if this is based on the same controller as its 1st gen product Anand?

    Also, i would love to see some ssd raid action since ssd does not lose its data when the array fails. I'm more interested to see if there is other limiting factor when you raid with such fast ssds, such as the motherboard chipset itself. If raiding two of those cheap $99 core 32gig ssd helps to reduce the write bug, it might be worthwhile buy, which was something i was thinking of doing till i read the review here.

    And might i say, great review indeed Aanad.
    Reply
  • bytekeeper - Tuesday, September 09, 2008 - link

    Hi there. There is a discussion in the OCZ support forum about
    the test results published in this article and the OCZ support
    staff is thinking that you've got a faulty drive. Check here:

    http://www.ocztechnologyforum.com/forum/showthread...">http://www.ocztechnologyforum.com/forum/showthread...

    Is there any chance you can re run the test with another
    Core drive or Supertalent/Patriot JMicron JMF602 victims?

    Thanks.
    Reply
  • Gary Key - Wednesday, September 10, 2008 - link

    I posted results with a newly arrived Core drive straight off the shelf from Newegg in that thread. No changes to our initial results, although I will try a different controller later this week. Reply
  • Anand Lal Shimpi - Tuesday, September 09, 2008 - link

    In total I have four drives based on the JMF602 controller: one from OCZ, SuperTalent, Patriot and Silicon Power.

    Of the drives, the OCZ Core is the oldest. If you look at page 9 in the article I've got a table that has the SuperTalent, Silicon Power and OCZ Core drives in it. The Silicon Power and SuperTalent drive both perform worse than the OCZ.

    The situation gets more interesting if you look at the Iometer results for the drives, which I didn't publish in great depth but I did run internally. Average write latency goes down tremendously on the SuperTalent drive, but max latency more than doubled. I suspect that the SuperTalent drive may use a newer version of the JMF602, but the result is a reduction in real world performance - at least in the test cases I ran.

    The inability to complete an OS install also applies to other drives. As I mentioned in the article I had the same problem with the SuperTalent MLC drive in OS X.

    While there are undoubtedly ways of attempting to lessen the impact of the JMF602's poor design, I believe only a new controller will actually completely address these problems.

    Take care,
    Anand
    Reply
  • yottabit - Tuesday, September 09, 2008 - link

    This is why I love your website. Lots of your articles are so in-depth they make me think I'm reading some grad school journal that I should be paying money for.

    I actually first saw the Intel SSD article on TomsHardware, but didn't even bother to read it because I imagined it was going to say something like "OMG INTEL SSD ROX LOLZ SO MUCH FASTER THAN OTHER HDS"

    I come here expecting to get a detailed analaysis, and that's always what you provide! Thank you so much.

    My only gripe is you seem to have skipped a very basic test- dragging and dropping large files! You combine extracting files with other tests but you don't have any basic drag + drop tests I saw.

    This rocks for the computer industry though. I personally couldn't believe so many people were buying the existing SSDs. This new Intel stuff looks like something that would actually make an improvement without compromises. I could certainly live with the capacity, I'm still running a 36GB raptor for my boot drive. Hah.


    Reply
  • Dariusbird - Tuesday, September 09, 2008 - link

    I'm proud to be working for the company producing these IC's. Nice work on the article. Reply
  • therealnickdanger - Tuesday, September 09, 2008 - link

    It would be very nice to see where on all these charts the best 5400RPM 2.5" SATA drive would sit. Most laptop users have 5400RPM drives, so this would be MOST EXCELLENT to see the benefits of all these drives in comparison - especially the now "crappy" JM-based MLC drives. Reply
  • johncl - Tuesday, September 09, 2008 - link

    I must congratulate Intel for adressing the issues with MLC and creating a brilliant product. Also thanks to the author of this article to point to the problem of MLC that seems to go by unnoticed by so many benchmark review sites.

    The initial price from intel is suggested retail price so I guess in a short while this should come down a bit, as its a bit over the top. As the author says, at around $400 this is a no-brainer for an enthusiast considering the improvement you will experience on your system. This is for me the most important advancement in computer technology in the last years and we are just starting to see the advantages of it. Besides the person sitting behind the keyboard, the slowest thing in your computer is the harddisk. You can swap CPUs and GPUs to your hearts desire, but eventually, percieved performance of a computer often boils down to how fast you get data in and out of system memory. SSDs will fix this technological void we have had for all these years with mechanical disks.

    In general its about time we get rid of all mechanical parts in a computer, including fans. We need good cooling solutions built into our systems that rely on other means than rotating motors. If it means to stop the CPU speed race, then by all means do that, lets try to get todays performance but passively cooled. The SSD will walk hand in hand with this new trend in computing so we can get truly silent workstations. Amen to SSDs!
    Reply
  • Gastrian - Tuesday, September 09, 2008 - link

    I'm looking into creating a small multimedia PC where by I'll be archiving all my DVDs to DivX and running them through the PC instead of a DVD Player.

    Would I see any benefits in performance and acoustics (I don't want any noise from the PC interfering with my viewing) by having the OS on the Intel MLC SSD while storing the movies on a mechanical HDD?

    Essentially I want to be able to sit down and browse my movies then pick one and watch it without having to worry about loading/transfer times and noise levels.
    Reply

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