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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  • abrar - Friday, February 18, 2011 - link

    "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."

    how did you calculate this data ?
    have you used any special software?
    Reply
  • douglaswilliams - Friday, February 18, 2011 - link

    Anand,

    Will these very fast read rates speed up virus scans?

    Thank you for being technical and thorough,

    Douglas
    Reply
  • Qapa - Saturday, February 19, 2011 - link

    ALL disk reads are this fast, so of course this will have an impact in virus scanning.

    But mostly, it will have an impact that your system is faster, although those resource hogs (virus scanning programs) are running - and yes, their bottleneck were reading the files from the disk.
    Reply
  • semo - Saturday, February 19, 2011 - link

    I've noticed that MSE is mostly limited by CPU and it isn't multithreaded so even worse notebooks where the CPU freq. might be lower Reply
  • joeld - Friday, February 18, 2011 - link

    I don't believe any of the SF-1200 makers actually supported encryption on the drive, so the password was basically blank. Are the folks bringing the SF-2500 to market actually going to support disk passwords so that full-disk encryption requirements for laptops will be met? Software based full disk encryption is just too slow and flawed when paired with SSD drives... Reply
  • faster - Saturday, February 19, 2011 - link

    Props to OCZ for leading the performance market on multiple levels.

    Looking at the prices of these new high performance SSDs, including the upcoming offerings from Intel, the OCZ Revodrive X2 is looking like a value. With Revodrive's read/write of 740/720, even this next generation of drives doesn't even come close. Now that newegg sells the 240GB x2, on sale, for $540 ($679 not on sale), it is a better bang for the buck than the new drives. The revodrive is a product that has been out for a while and is available right now. If you have the open PCIe slot, it seems like a no-brainer to pick the X2.
    Reply
  • croc - Sunday, February 20, 2011 - link

    Sadly, (well maybe not so sadly) we Aussies are not allowed gun ownership. There are exceptions, but generally not... So the final conclusion, "2011 may be the year to finally pull the trigger." does not really apply. However, maybe the right year to finally purchase an SSD, time (and further testing) will tell. Reply
  • MamiyaOtaru - Monday, February 21, 2011 - link

    time doesn't "tell" anything. It's an abstract concept. I mean if you're going to take issue with idioms.. Reply
  • Qapa - Sunday, February 20, 2011 - link

    It would also be interesting to have in the benchs:
    - 1 "normal" 7200 HDD
    - 1 "normal" 5400 laptop HDD

    This would be interesting to compare since, most people do not own Velociraptors and this way we would be able to better know the system improvement on buying an SSD, and convincing other people - like our bosses - to upgrade our computers at work as well :)
    Reply
  • compvter - Monday, February 21, 2011 - link

    @faster Thats because they have two (old) sf controllers in the card. Think about the speeds they get with two of these (sf-2000 family) i would guess it will be around 1GB/s (read&write). At least current revodrives lack trim support so, that might be deal breaker for some ppl. Reply

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