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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  • eva2000 - Thursday, February 17, 2011 - link

    Thanks for the review, curious why the 256GB Crucial RealSSD C300 only included in some benchmarks and not others ? Reply
  • Anand Lal Shimpi - Thursday, February 17, 2011 - link

    That's simply a data entry issue, I'll get it fixed :)

    Take care,
    Anand
    Reply
  • therealnickdanger - Thursday, February 17, 2011 - link

    Also, I'm curious as to why none of the REVO benchmarks are included? On the one hand, I understand how niche PCIe drives may be, but on the other, they can offer significant performance over both SATA 3Gbps and 6Gbps drives. It would be nice to see the new Heavy and Light benchmarks applied to those as well. Reply
  • ludikraut - Thursday, February 17, 2011 - link

    +1 on including REVO drive results in the drive comparisons. Reply
  • markjx1 - Thursday, February 17, 2011 - link

    because REVO drives are garbage. they're nothing more than a bunch of vertex2's slapped onto a board with a cheap Silicone Image controller that has a PCI-X to PCIe bridge. Resale value on REVO is crap and if one of the drives dies, you have a nice paperweight. Reply
  • bji - Thursday, February 17, 2011 - link

    Your own personal opinion on the drives are not a reason to disclude them from the review. Reply
  • therealnickdanger - Thursday, February 17, 2011 - link

    Garbage that can pull down 800MB/s...? Go away, troll. For users without SATA 6Gbps, it's a very practical solution to achieve huge speed. In fact, even people WITH SATA 6Gbps would get a boost. The cost/GB is even under $2 for some of the REVOs, making them a reasonable alternative. Reply
  • Anand Lal Shimpi - Thursday, February 17, 2011 - link

    All graphs should now have the 256GB C300 in them :) Reply
  • Breogan - Thursday, February 17, 2011 - link

    Is the data in the Incompressible Write Speed Test mixed for the SF2500 drive or does it perform actually worse after trim?. It seems weird to me that a dirty drive performs as if it was stock and the trim actually worsens its perfomance. Reply
  • Anand Lal Shimpi - Thursday, February 17, 2011 - link

    It's not TRIM that is making the drive slower, it's the next write pass that's actually pushing the drive into a lower performance state as there's more garbage collection/cleaning that's going on. If I hadn't TRIMed and just ran another pass of the AS-SSD test you'd see the same number.

    Take care,
    Anand
    Reply

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