A TLC Refresher

Back in February, we published an article called Understanding TLC NAND, where we went in-depth about how NAND works and the differences between various kinds of NAND (SLC, MLC, and TLC). Back then we didn't know when TLC SSDs would be publicly available or who would be the first manufacturer. Supposedly, OCZ had interest in releasing TLC based SSDs but the supply of TLC NAND wasn't good enough for their needs. Samsung has the benefit of being a tier one manufacturer that makes its own NAND, which gives it an advantage when dealing with new technologies as it can control the output of NAND. In this case, Samsung was able to ramp up the production of TLC NAND when it wanted to, whereas OCZ must live with whatever the NAND manufacturers are ready to sell them.

While we have covered TLC in detail already, we have some new details to add:

  SLC MLC TLC
Bits per Cell 1 2 3
P/E Cycles 100,000 3,000 1,000
Read Time 25us 50us ~75us
Program Time 200-300us 600-900us ~900-1350us
Erase Time 1.5-2ms 3ms ~4.5ms

Samsung would not tell us the exact read, program, and erase latencies but they told us that their TLC is around 50% slower than their MLC NAND. We don't know the latencies for Samsung's MLC NAND either, hence we have to go by general MLC NAND latencies, which varies a lot depending on process. However, we were able to get the P/E cycle count for TLC, which is 1,000. Samsung did not specify the process node but given that they listed MLC at 3,000 cycles, we are most likely talking about 27nm or 21nm. I wouldn't find it unlikely that Samsung is rating their 21nm MLC NAND at 3,000 P/E cycles as well because IMFT was able to keep the endurance at the same level with their 20nm MLC NAND.

Physically, TLC is similar to SLC and MLC. All three consist of similar transistors, the only difference is that they store a different amount of bits per cell. SLC only stores one, whereas MLC stores two and TLC stores three. This actually creates a minor problem, as there is no multiple of three that is a power of two. Unlike with hard drives, SSD capacities typically go in powers of two, such as 64GB, 128GB, and 256GB.

NAND is actually built based on binary prefixes (Mebi, Gibi...) but is almost always referred to using metric prefixes (Mega, Giga...). For example a 128GB SSD has ~137.4GB of storage (128GiB) due to Gibi to Giga translation, but the remaining space is used as spare area.

If the raw NAND array has 17.2 billion transistors, you would get 16Gibibits (17.2Gbits) of storage with SLC NAND because each cell can store one bit of data. MLC yields 32Gib, which is still a nice power of two because all you're doing is adding one level. However, with TLC you get 48Gib, which is not a power of two. Technically nothing is stopping manufacturers from making a 48Gib die, but from the marketing and engineering standpoint it's much easier to stick with powers of two. A TLC die in this case should be 32Gib just like MLC. To achieve that, the die is simply reduced in size to around 11.5 billion transistors. 32Gib isn't exactly divisible by three, but thanks to spare bits it doesn't have to be. The trick here is that the same capacity TLC die is smaller than an MLC die, which results in more dies per wafer and hence lower production costs.

Introduction Lower Endurance - Why?
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  • mapesdhs - Friday, October 12, 2012 - link


    I have SCSI disks that are more than 20 years old which still work fine. :D

    Ian.
    Reply
  • MarkLuvsCS - Monday, October 08, 2012 - link

    Considering Write Amplification has been significantly reduced compared to the initial SSD tech, I don't believe it's going to be a problem for the consumer market. Google xtremesystems Write Endurance to see a Samsung 830 256gb with 3000 P/E still running at 4.77 PETABYTES.That page also shows you other brands and how they fare. I would trust Samsung wouldn't put this tech to use without truly understanding how it would pan out.

    That is why the worry of the 1000 P/E 840 vs 3000 P/E 830 is overblown. Either way you have little to worry about with Samsung's controllers causing any fuss unlike Other CompanieZ.
    Reply
  • Kjella - Monday, October 08, 2012 - link

    Not giving one fsck about wearing out the SSD I burned through a 10k-rated SSD in 1.5 years. Now with fairly normal SSD usage - a standard Win7 desktop with torrents etc. on other drives - I'm down to 57% health and looking at 3 years 10 months on a 5K-rated drive. I don't know exactly what is eating it but I'm guessing every log file, every time MSN or IRC logs a line of chat, every time something is cached or whatever it burns write cycles. I feel the official numbers are vastly *overstating* the actual lifespan, not understating it. TLC with 1K writes? Not in my machine, no sir. Reply
  • madmilk - Monday, October 08, 2012 - link

    There's no way MSN/IRC can burn through an SSD in 1.5 years since they're all text. You must be doing something unusual, or at least your computer is without you knowing it. A good idea would be to open up Task Manager, and select the columns that count the number of bytes written by various programs. Maybe then you can find the source of your problem. Also make sure you have defragmentation off, and sufficient RAM so you're not constantly hitting the pagefile. Reply
  • piiman - Tuesday, February 19, 2013 - link

    Better yet put the page file on a different drive and also move your temp folders to a different drive. Reply
  • Notmyusualid - Tuesday, October 09, 2012 - link

    Absolutely hilarious ending there pal... I wonder how many people got it!

    I got burned by them on a couple of drives, and promptly dumped them on some well-known auction site, sold as-is.
    Reply
  • creed3020 - Tuesday, October 09, 2012 - link

    I see what you did there ;-)

    Great review Kristian! I'll be looking at this drive as option for a new office PC I am building.
    Reply
  • B3an - Monday, October 08, 2012 - link

    Did you people even bother to read?? Because you're conveniently missing out the important fact in this article that you'd have to write 36.5TiB (almost 40TB) a year for it to last 3.5 years. I know for a fact that the average consumer does not write anywhere near that much a year, or even in 3 years. If anyone even comes close to 40TB a year they would be using a higher-end MLC SSD anyway as they would surely be using a workstation.

    Most consumers don't even write 10GB a day, so at that rate the drive would easily last OVER 20 years. But of course it's highly likely something else would fail before that happens.

    You're also forgetting out DSP which is explained in this article as well. That can also near double the life.

    I think Kristian should have made this all more clear because too many people don't bother to actually read stuff and just look at charts.
    Reply
  • futrtrubl - Monday, October 08, 2012 - link

    Granted the usual use cases won't have so much data throughput. However those same usual use cases have the user filling 3/4 of the drive with static data (program/OS/photo archive etc) reducing the drive area it's able to wear level over. So that 20 years again becomes 5 years.

    Also the 1000PE cycle stat means that there is a 50% chance for that sector to have become unusable by that time (ignoring DSP).

    I'm not saying that TLC is bad, and I am certainly not saying this drive doesn't have great value. I'm just saying that we shouldn't understate the PE cycle issue.
    Reply
  • xdrol - Monday, October 08, 2012 - link

    You sir need to learn how SSDs work. Static data is not static on the flash chip - the controller shuffles it around, exactly because of wear levelling. Reply

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