The Unmentionables: NAND Mortality Rate

When Intel introduced its X25-M based on 50nm NAND technology we presented this slide:

A 50nm MLC NAND cell can be programmed/erased 10,000 times before it's dead. The reality is good MLC NAND will probably last longer than that, but 10,000 program/erase cycles was the spec. Update: Just to clarify, once you exceed the program/erase cycles you don't lose your data, you just stop being able to write to the NAND. On standard MLC NAND your data should be intact for a full year after you hit the maximum number of p/e cycles.

When we transitioned to 34nm, the NAND makers forgot to mention one key fact. MLC NAND no longer lasts 10,000 cycles at 34nm - the number is now down to 5,000 program/erase cycles. The smaller you make these NAND structures, the harder it is to maintain their integrity over thousands of program/erase cycles. While I haven't seen datasheets for the new 25nm IMFT NAND, I've heard the consumer SSD grade stuff is expected to last somewhere between 3000 - 5000 cycles. This sounds like a very big problem.

Thankfully, it's not.

My personal desktop sees about 7GB of writes per day. That can be pretty typical for a power user and a bit high for a mainstream user but it's nothing insane.

Here's some math I did not too long ago:

  My SSD
NAND Flash Capacity 256 GB
Formatted Capacity in the OS 238.15 GB
Available Space After OS and Apps 185.55 GB
Spare Area 17.85 GB

If I never install another application and just go about my business, my drive has 203.4GB of space to spread out those 7GB of writes per day. That means in roughly 29 days my SSD, if it wear levels perfectly, I will have written to every single available flash block on my drive. Tack on another 7 days if the drive is smart enough to move my static data around to wear level even more properly. So we're at approximately 36 days before I exhaust one out of my ~10,000 write cycles. Multiply that out and it would take 360,000 days of using my machine for all of my NAND to wear out; once again, assuming perfect wear leveling. That's 986 years. Your NAND flash cells will actually lose their charge well before that time comes, in about 10 years.

Now that calculation is based on 50nm 10,000 p/e cycle NAND. What about 34nm NAND with only 5,000 program/erase cycles? Cut the time in half - 180,000 days. If we're talking about 25nm with only 3,000 p/e cycles the number drops to 108,000 days.

Now this assumes perfect wear leveling and no write amplification. Now the best SSDs don't average more than 10x for write amplification, in fact they're considerably less. But even if you are writing 10x to the NAND what you're writing to the host, even the worst 25nm compute NAND will last you well throughout your drive's warranty.

For a desktop user running a desktop (non-server) workload, the chances of your drive dying within its warranty period due to you wearing out all of the NAND are basically nothing. Note that this doesn't mean that your drive won't die for other reasons before then (e.g. poor manufacturing, controller/firmware issues, etc...), but you don't really have to worry about your NAND wearing out.

This is all in theory, but what about in practice?

Thankfully one of the unwritten policies at AnandTech is to actually use anything we recommend. If we're going to suggest you spend your money on something, we're going to use it ourselves. Not in testbeds, but in primary systems. Within the company we have 5 SandForce drives deployed in real, every day systems. The longest of which has been running, without TRIM, for the past eight months at between 90 and 100% of its capacity.

SandForce, like some other vendors, expose a method of actually measuring write amplification and remaining p/e cycles on their drives. Unfortunately the method of doing so for SandForce is undocumented and under strict NDA. I wish I could share how it's done, but all I'm allowed to share are the results.

Remember that write amplification is the ratio of NAND writes to host writes. On all non-SF architectures that number should be greater than 1 (e.g. you go to write 4KB but you end up writing 128KB). Due to SF's real time compression/dedupe engine, it's possible for SF drives to have write amplification below 1.

So how did our drives fare?

The worst write amplification we saw was around 0.6x. Actually, most of the drives we've deployed in house came in at 0.6x. In this particular drive the user (who happened to be me) wrote 1900GB to the drive (roughly 7.7GB per day over 8 months) and the SF-1200 controller in turn threw away 800GB and only wrote 1100GB to the flash. This includes garbage collection and all of the internal management stuff the controller does.

Over this period of time I used only 10 cycles of flash (it was a 120GB drive) out of a minimum of 3000 available p/e cycles. In eight months I only used 1/300th of the lifespan of the drive.

The other drives we had deployed internally are even healthier. It turns out I'm a bit of a write hog.

Paired with a decent SSD controller, write lifespan is a non-issue. Note that I only fold Intel, Crucial/Micron/Marvell and SandForce into this category. Write amplification goes up by up to an order of magnitude with the cheaper controllers. Characterizing this is what I've been spending much of the past six months doing. I'm still not ready to present my findings but as long as you stick with one of these aforementioned controllers you'll be safe, at least as far as NAND wear is concerned.

 

Architecture & What's New Today: Toshiba 32nm Toggle NAND, Tomorrow: IMFT 25nm
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  • slickr - Thursday, February 17, 2011 - link

    Now this is what I'm talking about about reviews/previews. Tons of benchmarks at various settings and loads. You can really make a difference now and see how the drives perform.

    I would also like a good old fashion test with Starcraft 2, how long it takes to load a 5-6mb custom map.

    I would also like another test where you select 30 files and open them at the same time and see how much time it takes to open all. I'm talking about selecting few 3-5mb images, few MP4 360p videos, few H.264 720p videos, dozen office documents from 500kb up to 3mb, several applications like GPU-z, skype, Live, Xfire, firefox etc... and opening few highly compressed script files.
    Reply
  • MamiyaOtaru - Thursday, February 17, 2011 - link

    smaller process, less reliability, and higher price? We've been waiting for years fr prices to become reasonable next to magnetic storage but there's been barely a drop at all, and the drops that do come (smaller processes, supposedly) reduce reliability. At this point I don't see myself ever getting one for my desktop.

    Laptops sure, hard drives die there all the time, and I don't use them as my primary machine. Smaller storage requirements + hard drives dying far more often in laptops makes SSDs the better choice for me there.
    Reply
  • Chloiber - Thursday, February 17, 2011 - link

    I do generally agree. I don't want faster drives, I want cheaper drives. They are already very fast. Of course, faster is always better but at the moment I prefer low price and reliabilty over speed. Reply
  • seapeople - Friday, February 18, 2011 - link

    Seriously... I don't need a brand new controller that might or might not be reliable and is so fast that it would still seem fast if I taped myself using the computer and replayed it in slow motion. What I want is an x25m-like drive at 160 GB for under $200. Still extremely fast, legendary (for SSD's) reliability, and make it affordable.

    The reason I don't buy Ferrari's right now is not because I don't think they're fast enough, it's because they COST too much.
    Reply
  • RU482 - Thursday, February 17, 2011 - link

    this might be the problem with OCZ. They are an SSD marketing company with a manufacturing division Reply
  • TimK - Thursday, February 17, 2011 - link

    Damn, Anand, so this is what an engineering degree will get you, not to mention some heavy duty skill at writing. Comprehensive and comprehensible. Thanks very much. At your recommendation I bought an early Vertex 30GB SSD for my unibody MacBook. From time to time I take it out, thinking to have everything in one place on a bigger drive, but I just can't let go of the speed. It's still working great. Reply
  • dlang1234 - Thursday, February 17, 2011 - link

    The Samsung 470 seems to be in a lot of the benchmarks but not all, and seems to do well in every one that it is listed in.

    I can't seem to find a review of it here, but would be interested in it possibly.
    Reply
  • markjx1 - Thursday, February 17, 2011 - link

    No mention of the fact this thing was originally slated with the SF-2000 controller, which proved to be plagued with problems in the lab and the dirty little secret no vendor would discuss at CES which was why no one had anything SF-2000 based up and running. And now OCZ had to resort to slapping Sandforce's enterprise class SF-2500 controller on it.

    Great except its going to be hella expensive and not cost competitive with the Crucial C400 unless OCZ bleeds margin, and given they took a $25 million bank loan recently, well let's just say OCZ isn't a company I'd rely on to fulfill a warranty replacement a couple years down the road when your drive dies.

    Lastly, notice the "hardware isn't final" disclaimers all over the article. This is nothing more than OCZ trying to get some buzz, and have painted themselves into a corner now if they go switching back to the SF-2000 since they've already set expectations high.
    Reply
  • jwilliams4200 - Thursday, February 17, 2011 - link

    I thought the article was fairly well done. The only problem I have with it is a passing mention to the SSD being unusuable on a Macbook Pro, and yet not a single benchmark shows any problems with the SSD. It seems the benchmark suite Anand is using needs to have some more components added. Perhaps a latency test? Reply
  • Anand Lal Shimpi - Sunday, February 20, 2011 - link

    Check back on the site by the end of the week ;)

    Take care,
    Anand
    Reply

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