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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  • Slimline - Thursday, February 17, 2011 - link

    This sounds interesting Reply
  • Trefugl - Thursday, February 17, 2011 - link

    Indeed. I'm particularly liking the conclusion:

    'We're still a couple months away from knowing exactly what to buy, but if you've been putting off that move to an SSD - 2011 may be the year to finally pull the trigger"

    That pretty much describes me perfectly. I do have an SSD in my work's workstation, but for home, but I've been holding out for 2011 (IMFT 25nm NAND) and I'm thinking I might not be disappointed by the wait.
    Reply
  • Drag0nFire - Friday, February 18, 2011 - link

    Indeed. I'm particularly excited to see what will happen when Intel shows up to the fight. Reply
  • MeanBruce - Friday, February 18, 2011 - link

    Intel announced their 510 SSD G3 series today, it will come in 120Gb and 250Gb capacities, SATA3 6Gb/s, read/writes of 470Mb/s and 315Mb/s respect, and will be priced at $280 and $580! It's not using an Intel controller word is Intel doesn't have an in house controller with any real speed! SandForce is really shakin' things up! ;) Reply
  • MrBrownSound - Friday, February 18, 2011 - link

    woah, no kidding. I'm looking forward to any reviews of it. Reply
  • MeanBruce - Saturday, February 19, 2011 - link

    I knew when Intel pulled their G3 SSD lineup by rescheduling the release it had nothing to do with time constraints and everything to do with the numbers released shortly after by OCZ about their new SandForce controllers, 500/500 read/writes had Intel drawing up an entirely new gameplan for the new G3 lineup! But honestly I thought they would just let a little more magic out of the bag, I had no idea their bag was empty! Now I found out the new 510 series that becomes available March 1st is just going to use a Marvell controller just like the new Crucial and Corsair drives. I still love my X-25M but it's sad when a company with that many resources kicks back on their laurels. Oh well the good news is SandForce is here and with their new client Seagate we will have lots of choices and overall it's just great for the industry at large! Just sucks to watch the one time leader down so low! Reply
  • Out of Box Experience - Saturday, February 19, 2011 - link

    Over 50% of the boxes on the Planet still run XP

    So, the big question is...

    Which one is XP compatible Out of the Box?

    I'd love to buy a Sata 3 SSD that can saturate my XP Sata 2 ports but should we stick with Older/Slower Intel Sata 2 SSD's for compatibility??????

    Any comment on this issue?
    Reply
  • anactoraaron - Monday, February 21, 2011 - link

    I know everyone has their own reason for keeping XP... but if you want to buy a $200+ SSD how can you not pony up the 100 for windows 7?

    Besides, 2 more years until XP is officially obsolete...
    Reply
  • Out of Box Experience - Monday, February 21, 2011 - link

    Bla bla bla

    Having Windows 7 does not mean we all need to throw out our XP Licences and all our software that does not run on 7

    Why can't somebody just answer my question instead of changing the subject

    We get it! You love all the spyware and DRM built into Windows 7 but others don't

    So lets just stick to the question I asked shall we?

    Which Sata 3 SSDs will be Alignment agnostic at the very least so they can be used on ANY O.S. besides Spyware 7?
    Reply
  • bennyg - Monday, February 21, 2011 - link

    Bla bla bla spyware bla drm bla bla

    You forgot to mention how locking DX10 to Vista/7 was a deliberate ploy to force gameplayers to upgrade.

    And how Win7 is just Vista done right.

    Far out some people hold grudges. I was ambivalent about Win7 when it was forced upon me - but for multicore + SSDs you just can't consider an old OS that wasn't designed when they were on the radar.
    Reply

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