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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  • PubicTheHare - Thursday, February 17, 2011 - link

    Anand,

    Would this be the drive to pair with a Sandy Bridge Macbook Pro, or do we still need to wait for native TRIM support in OSX?

    I'm really eager to buy a Sandy Bridge MBP and do this if the V3P is the one to get. I can't recall which controller you recommended for Macs.
    Reply
  • markjx1 - Thursday, February 17, 2011 - link

    Why no mention of the fact this was originally slated for the SF-2000, and now its got the enterprise class SF-2500 with disclaimers of "hardware not final" all over the article?

    The truth is they had major problems with SF-2000 and nobody wants to talk about it - of all people I'd think you would have. Now go ahead and delete my post again.
    Reply
  • markjx1 - Thursday, February 17, 2011 - link

    Resorting to using the enterprise-class SF-2500 means I worry about cost competitiveness against other upcoming Gen3 offerings from Intel and Crucial/Micron.

    OCZ took a $25 mil bank loan recently so they need a winner. I worry about the "hardware is not final" disclaimers in the article. Sending these samples out for some buzz smells like buying time while they work out production dilemmas since originally they were going to use a different SF controller.
    Reply
  • bplewis24 - Friday, February 18, 2011 - link

    Why are you trolling? You have 3 of essentially the same posts on this page. We get it, you think he "sold out" for not mentioning that it's using a different controller.

    Brandon
    Reply
  • vol7ron - Thursday, February 17, 2011 - link

    Does this mean a higher QD, might improve performance.

    It looks like QD of 16 might be the sweet spot in terms of latency vs IOPS
    Reply
  • DJMiggy - Thursday, February 17, 2011 - link

    Thanks for the great article and thank you OCZ for making my Vertex 1 even more obsolete. That's ok though! Not a complaint! lol Just looking forward to my next upgrade! Reply
  • Rasterman - Thursday, February 17, 2011 - link

    Since the drive is 400MB/s read, if you use two in RAID-0 are you going to get 800MB/s or the realistic max of 6gpbs of ~700MB/s?

    If you use RAID-0 X4 are you going to get 1600MB/s or ~700MB/s?
    Reply
  • jwilliams4200 - Thursday, February 17, 2011 - link

    With a decent RAID card, then RAID 0 of N drives will give you slightly less than N times the performance of a single drive.

    For on-motherboard RAID, you will also get approximately N times the performance of a single drive, but there is usually a ceiling to total performance on motherboard RAID. It is around 600 MB/s for ICHR-10.
    Reply
  • myterrybear - Thursday, February 17, 2011 - link

    I've been a loyal reader of this sight, but what realy myths me if you's use EVERY other controler but those found on the AMD platform .... especialy more so with a Sata 6.0 SSD like this it would match up well for those of us amd users whom have paid money to upgrade the motherboard to have that sb8xx controler onboard to utilize hard drives & other devices that run on the now becoming Sata 6.0 standard. Reply
  • Per Hansson - Thursday, February 17, 2011 - link

    Hi, has the much talked about hibernation problem with the Sandforce drives been solved in the 2xxx series?
    Loosing Hibernation is pretty much a deal killer for me in a laptop :(

    http://www.google.se/search?hl=sv&safe=off&...
    Reply

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