Solid State Drive Buyer's Guide

SLC vs. MLC, Continued

Without getting too deep or technical, think of a transistor as a two-story building. The ground on which the building is erected is the substrate, the first storey is the floating gate, and the top storey above it is the control gate. Between the substrate and the floating gate is a thin layer of oxide material. Think of the oxide as the stairs people have to go up to get from the ground to the first storey. Lastly, think of electrons as the people wanting to travel from the ground level up to the first storey. If there are no people in the first storey, then the building has been evacuated, or “erased.” If there are people up there, then you have data resident and the cell has been “programmed.”

As said earlier, SLC only requires one electron per cell; MLC requires more than one. It takes more energy to move more people from place to place. The metaphor frays a bit here, but essentially it takes more voltage to move and hold electrons in that floating gate. More voltage acts like more people stomping on those stairs, wearing them out more quickly. Given enough time, the stairs will break. The electrons effectively saw a hole in the oxide insulation layer and the floating gate can no longer reliably hold its electron(s). When this happens, the SSD controller flags the problem spot as a bad block and passes its data to another block.

 

Because MLC uses more electrons and requires more voltage (more sawing) to move those electrons, MLC cells will wear out faster than SLC—about 10 times faster. You might see SLC drives noted with 100,000 cycle ratings and MLC with only 10,000 cycles. If you erased and reprogrammed an MLC block once per minute continuously, it would exceed its life expectancy in about one week.

Fortunately, SSDs don’t write to the same block over and over. The drive controllers use “wear leveling” to distribute the programming cycle load evenly across the drive’s million’s of NAND memory cells. An 8Gb NAND die with 4,096 blocks, if perfectly leveled, wouldn’t start exceeding its 10,000 cycle mark for 75 years. This is why having a controller with good wear leveling algorithms is so important. Moreover, the more cells are in a drive, the more real estate there is for wear leveling. This is why there’s a nearly linear relationship between SSD capacity and drive endurance. Wear is also why enterprises have historically gravitated to SLC drives. The extra performance is good, but large corporations are positively phobic about the possibility of premature failure.

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  • Shadow703793
    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.
  • grimjester
    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.
  • nonxcarbonx
    This is an even better ssd article than anandtech's ssd anthology. Nice work.
  • Eggrenade
    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.
  • Tomsguiderachel
    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 :)
  • Tomsguiderachel
    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.
  • Anonymous
    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).
  • Tomsguiderachel
    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.
  • Tomsguiderachel
    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.
  • tommysch
    I think Ill stick to my 4x1TB RAID 0 array for now. BTW they are ghosted each week. o_0
  • nebun
    love my 4x ssd raid 0 500MB+ read and 300MB+ write
  • Luscious
    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!
  • extremepcs
    "For comparison, we might look at a middle-of-the-roach HDD"

    :)
  • Shadow703793
    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)
  • Tomsguiderachel
    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!
  • JackNaylorPE
    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.
  • pps
    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.
    ???
  • williamvw
    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.
  • williamvw
    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.