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SLC vs. MLC

Solid State Drive Buyer's Guide
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Now we have to take one more technical step and take you right into the heart of what makes SSDs tick. (OK, they don’t tick per se. With no moving parts, SSDs are actually completely silent—another advantage over hard drives.) We said previously that SSDs use NAND flash chips. Within each of these chips are millions of cells. There are only two types of NAND cells today: single-layer (SLC) or multi-layer (MLC).

An SLC cell can hold one data bit, yielding a value of either 0 or 1. An MLC cell can hold more than one, with today’s technologies generally yielding two bits per cell, yielding values of 00, 01, 10, or 11. Because you can fit four times as many possible values in a cell, the data density in MLC chips is higher. This is why MLC drives inevitably have higher capacities than their SLC cousins.

 

There are some downsides with MLC, though. The more values you have to read from, the harder it can be to get a positive fix on a value. Consider a glass of water. If you have only two possible values, empty or full, there’s no ambiguity about the state of the glass’s contents. But if you have four possible water levels, you might need to look a little harder to see if the glass is three-quarters full or only half-full (or half-empty, depending on your disposition). MLC technology will increase the rate of erroneous value readings, which is why MLC drive controllers use more powerful error correction algorithms. Going through error correction cleans up the mistakes, but it also takes more time to guarantee accuracy. This is one reason why MLC drives generally feature slower throughput than SLC.

Beyond error correction, there are different ways to build NAND memory dies that can affect performance. For example, every memory device uses a fixed-size chunk of memory called a page to transfer data to and from an external device. The size of the page can vary from one NAND product to the next, but whereas SLC allows you to program four or more parts of a page independently, MLC requires you to program the entire page before moving to the next page. SLC is using each page more effectively, so overall performance is improved. SLC is also far more efficient at writing. An MLC array will take almost twice as long to start write programming compared to SLC.

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  • 4 Hide
    Shadow703793 , November 29, 2009 6:17 PM
    Imo, I'd keep away with any SSD drive using a JMicron. Dosen't matter if the stutering,etc issues were fixed. Indylinx and Intel controllers are the best right now.

    Anyways, I got an X25-M G2 (OEM) for $220 @Newegg during Black Friday.
  • 0 Hide
    grimjester , November 29, 2009 7:44 PM
    The point about getting a small SSD for software and a larger disk for data can't be stressed enough. The price per GB looks completely different if you only need 64G. There's little difference in price between the cheapest hard drive you can get and the cheapest 500G one.

    An SSD is just an extra cost of $150-300. It has no practical effect on the storage space your computer has.
  • 1 Hide
    nonxcarbonx , November 29, 2009 9:03 PM
    This is an even better ssd article than anandtech's ssd anthology. Nice work.
  • 0 Hide
    Eggrenade , November 29, 2009 10:38 PM
    I wouldn't say it's better than Anandtech's; there's no mention of random reads or writes, which is why performance just after startup is so good. It's also a lot less technical, which is probably better for most Tom's Guide readers.
  • 1 Hide
    Tomsguiderachel , November 30, 2009 3:06 AM
    EggrenadeI wouldn't say it's better than Anandtech's; there's no mention of random reads or writes, which is why performance just after startup is so good. It's also a lot less technical, which is probably better for most Tom's Guide readers.

    Exactly :) 
  • 1 Hide
    Tomsguiderachel , November 30, 2009 3:07 AM
    nonxcarbonxThis is an even better ssd article than anandtech's ssd anthology. Nice work.

    Thank you. I hope it was a good fit for Tom's Guide readers' needs.
  • 0 Hide
    Anonymous , November 30, 2009 3:51 AM
    Next page broken http://www.tomsguide.com/us/ssd-value-performance,review-1455-11.html, sorry couldnt find anywhere to submit feedback. Page not working on firefox 3.5.5 (does not scroll).
  • 0 Hide
    Tomsguiderachel , November 30, 2009 4:05 AM
    none007Next page broken http://www.tomsguide.com/us/ssd-va [...] 55-11.html, sorry couldnt find anywhere to submit feedback. Page not working on firefox 3.5.5 (does not scroll).

    I'm using the same browser and that page works for me. I will report the bug, thanks.
  • 0 Hide
    Tomsguiderachel , November 30, 2009 4:07 AM
    TomsguiderachelI'm using the same browser and that page works for me. I will report the bug, thanks.

    Oh--I see that you mean the final page of the article not the penultimate page. FYI There is no content on that last page so you didn't miss part of the article.
  • 0 Hide
    tommysch , November 30, 2009 1:59 PM
    I think Ill stick to my 4x1TB RAID 0 array for now. BTW they are ghosted each week. o_0
  • 0 Hide
    nebun , November 30, 2009 4:10 PM
    love my 4x ssd raid 0 500MB+ read and 300MB+ write
  • 0 Hide
    Luscious , November 30, 2009 6:25 PM
    Another wonderful Swiss-cheese article from TG!

    Your author fails to mention in his "pros-and-cons" anything about the importance of the file system when choosing an SSD. It is a known issue with Windows XP that formatting a solid state drive as NTFS can cause the hard drive to randomly lock up. Netbook owners looking to replace their drives need to keep this in mind, especially on systems running XP.

    This is the same guy who wrote an article about ergonomics yet failed to mention anything about trackballs in his work.

    Thumbs down for you Tom's!
  • 2 Hide
    extremepcs , November 30, 2009 7:04 PM
    "For comparison, we might look at a middle-of-the-roach HDD"

    :) 
  • 0 Hide
    Shadow703793 , December 1, 2009 11:36 PM
    LusciousAnother wonderful Swiss-cheese article from TG!Your author fails to mention in his "pros-and-cons" anything about the importance of the file system when choosing an SSD. It is a known issue with Windows XP that formatting a solid state drive as NTFS can cause the hard drive to randomly lock up. Netbook owners looking to replace their drives need to keep this in mind, especially on systems running XP.This is the same guy who wrote an article about ergonomics yet failed to mention anything about trackballs in his work.Thumbs down for you Tom's!

    You do bring up a good point. What I would like to see are benchmarks for the different file systems available (ie FAT,NTFS, ext3,etc)
  • 0 Hide
    Tomsguiderachel , December 2, 2009 4:29 PM
    Shadow703793You do bring up a good point. What I would like to see are benchmarks for the different file systems available (ie FAT,NTFS, ext3,etc)

    That sounds like a great article idea for Tom's Hardware!
  • 0 Hide
    JackNaylorPE , December 2, 2009 11:31 PM
    Nothing to say against SSD performance but the exaggerated yearly time savings just don't take into account a little thing called reality. For example, how does Joe office worker get thru his day ?

    1.a Arrives at desk, hits PC on button and times how long it takes to boot into Windoze ?

    1.b Arrives at desk, hits PC on button grabs coffee cup and heads over to coffee machine ?

    2.a Needing to edit a Word proc document, opens word processor and stares at screen waiting for program to load ?

    2.2 Needing to edit a Word proc document, opens word processor and while program loads, scans over the boss's red pencil marks edits to get a handle on what he's gotta correct ?

    SSD technology is great and increased adoption will continue to drive down costs but these ROI arguments about increased productivity are not reflective of how humans (not robots) work.
  • 0 Hide
    pps , December 4, 2009 2:20 PM
    I don't get it. This article first discribes how SLC is superior in every technical category, then it says that they are making improvements to MLC and concludes that MLC is the future.
    ???
  • 0 Hide
    williamvw , December 9, 2009 5:20 PM
    nonxcarbonxThis is an even better ssd article than anandtech's ssd anthology. Nice work.

    Thanks! However, even I'll second Eggrendade's comment. I wrote this piece specifically as an introduction to SSDs for those still getting their feet wet -- not the type wanting an in-depth analysis of file system performance. Anand's series is excellent but obviously far more technical. Different strokes.
  • 0 Hide
    williamvw , December 9, 2009 5:56 PM
    ppsI don't get it. This article first discribes how SLC is superior in every technical category, then it says that they are making improvements to MLC and concludes that MLC is the future. ???

    It's a price vs. performance issue. Yes, SLC rules on performance, but its progress has stayed fairly flat while MLC continues to improve while driving down cost per gig. Thus while SLC remains the technical champion on benchmarks, MLC has caught up enough to make it more attractive on a price/performance basis in an increasing number of environments.
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