A Wear Leveling Refresher: How Long Will My SSD Last?

As if everything I’ve talked about thus far wasn’t enough to deal with, there’s one more major issue that directly impacts the performance of these drives: wear leveling.

Each MLC NAND cell can be erased ~10,000 times before it stops reliably holding charge. You can switch to SLC flash and up that figure to 100,000, but your cost just went up 2x. For these drives to succeed in the consumer space and do it quickly, it must be using MLC flash.

SLC (left) vs. MLC (right) flash

Ten thousand erase/write cycles isn’t much, yet SSD makers are guaranteeing their drives for anywhere from 1 - 10 years. On top of that, SSD makers across the board are calling their drives more reliable than conventional hard drives.

The only way any of this is possible is by some clever algorithms and banking on the fact that desktop users don’t do a whole lot of writing to their drives.

Think about your primary hard drive. How often do you fill it to capacity, erase and start over again? Intel estimates that even if you wrote 20GB of data to your drive per day, its X25-M would be able to last you at least 5 years. Realistically, that’s a value far higher than you’ll use consistently.

My personal desktop saw about 100GB worth of writes (whether from the OS or elsewhere) to my SSD and my data drive over the past 14 days. That’s a bit over 7GB per day of writes. Let’s do some basic math:

  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 the way I have been for the past two weeks 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.

This assumes a perfectly wear leveled drive, but as you can already guess - that’s not exactly possible.

Write amplification ensures that while my OS may be writing 7GB per day to my drive, the drive itself is writing more than 7GB to its flash. Remember, writing to a full block will require a read-modify-write. Worst case scenario, I go to write 4KB and my SSD controller has to read 512KB, modify 4KB, write 512KB and erase a whole block. While I should’ve only taken up one write cycle for 2048 MLC NAND flash cells, I will have instead knocked off a single write cycle for 262,144 cells.

You can optimize strictly for wear leveling, but that comes at the expense of performance.

Why SSDs Care About What You Write: Fragmentation & Write Combining Why Does My 80GB Drive Appear as 74.5GB? Understanding Spare Area
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  • shabby - Monday, August 31, 2009 - link

    The 80gig g2 is $399 now!
  • gfody - Tuesday, September 1, 2009 - link

    The gen2 80gb is at $499 as of 12:00AM PST
  • maxfisher05 - Monday, August 31, 2009 - link

    As of right now (8/31) newegg has the 160GB Intel G2 listed at $899!!!!!!!!!!!!!!!!!!! To quote Anand "lolqtfbbq!"
  • siliq - Monday, August 31, 2009 - link

    Great article! Love reading this. Thanks Anand.

    We gather from this article that all the pain-in-@$$ about SSDs come from the inconsistency between the size of the read-write page and the erase block. When SSDs are reading/writing a page it's 4K, but the minimum size of erasing operation is 512K. Just wondering is there any possibility that manufacturers can come up with NAND chips that allows controllers to directly erase a 4K page without all the extra hassles. What are the obstacles that prevent manufacturers from achieving this today?
  • bji - Tuesday, September 1, 2009 - link

    It is my understanding that flash memory has already been pushed to its limit of efficiency in terms of silicon usage in order to allow for the lowest possible per-GB price. It is much cheaper to implement sophisticated controllers that hide the erase penalty as much as possible than it is to "fix" the issue in the flash memory itself.

    It is absolutely possible to make flash memory that has the characteristics you describe - 4K erase blocks - but it would require a very large number of extra gates in silicon and this would push the cost up per GB quite a bit. Just pulling numbers out of the air, let's say it would cost 2x as much per GB for flash with 4K erase blocks. People already complain about the high cost per GB of SSD drives (well I don't - because I don't steal software/music/movies so I have trouble filling even a 60 GB drive), I can't imagine that it would make market sense for any company to release an SSD based on flash memory that costs $7 per GB, especially when incredible performance can be achieved using standard flash, which is already highly optimized for price/performance/size as much as possible, as long as a sufficiently smart controller is used.

    Also - you should read up on NOR flash. This is a different technology that already exists, that has small erase blocks and is probably just what you're asking for. However, it uses 66% more silicon area than equivalent NAND flash (the flash used in SSD drives), so it is at least 66% more expensive. And no one uses it in SSDs (or other types of flash drives AFAIK) for this reason.
  • bji - Tuesday, September 1, 2009 - link

    Oh I just noticed in the Wikipedia article about NOR flash, that typical NOR flash erase block sizes are also 64, 128, or 256 KB. So the eraseblocks are just as problematic there as in NAND flash. However, NOR flash is more easily bit-addressable so would avoid some of the other penalties associated with NAND that the smart contollers have to work around.

    So to make a NAND or NOR flash with 4K eraseblocks would probably make them both 2X - 4X more expensive. No one is going to do that - it would push the price back out to where SSDs were not viable, just as they were a few years ago.
  • siliq - Tuesday, September 1, 2009 - link

    Amazing answers! Thank you very much
  • morrie - Monday, August 31, 2009 - link

    My laptop is limited to 4 GB swap. While that's enough for 99% of Linux users, I don't shut down my laptop, it's used as a desktop with dozens of apps running and hundreds of browser tabs. Therefore, after a few months of uptime, memory usage climbs above 4 GB. I have two hard drives in the laptop, and set up a software raid0 1GB swap partition, but I went with software raid1 for the other swap partition. So once the ram is used up for swap, the laptop slows noticeably, but after the raid0 swap partition fills up, the raid1 partition really slows it down. Once that fills up, it hits swap files (non raid) which slow it down more. But thanks to the kernel and the way swappiness works, once about 4 GB of Ram plus about 3 GB of physical swap is used, it really slows. I can gain a bit of speed by adding some physical swap files to increase the ratio of physical swap to ram swap (thus changing swappiness through other means), but this only works for another 1 GB of ram.

    No lectures or advice please, on how I'm using up memory or about how 4GB is more than sufficient, my uptimes are in the hundreds of days on this laptop and thanks to ADD/limited attention span, intermittent printer availability for printing out saved browser tabs and other reasons (old habits dying hard being one), my memory usage is what it is.

    So, the big question is, since the laptop has an eSATA port, can I install one of these ssd drives in an externel SATA tray, connected via eSATA to the laptop and move physical swap partitions to the ssd? I believe that swap on the ssd would be a lot faster even on the eSATA wire, than swap on the drives in the laptop (they're 7200 rpm drives btw). I'm aware that using the ssd for swap would shorten it's life, but if it lasts a year till faster laptops with more memory are available (and I get used to virtual machines and saving state so I can limit open browser windows), I'll be happy.

    Buying two of the drives and using them raided in the laptop is too costly right now, when prices drop that'll be a solution for this current laptop.

    Externel SSD over eSATA for Linux swap on a laptop? Faster than my current setup?
  • hpr - Monday, August 31, 2009 - link

    Sounds like you have some very small memory leak going on there.

    Have you tried that Firefox plugin that enables you to have your tabs but it doesn't really have a tab open in memory.


    Have fun filling up thousands of tabs and having low memory usage.
  • gstrickler - Monday, August 31, 2009 - link

    You should be able to use an SSD in an eSATA case, and yes, it should be faster than using your internal 7200 RPM drives. You probably want to use an Intel SSD for that (see page 19 of the article and note that the Intel drives don't drop off dramatically with usage).

    If you don't need to storage of your two internal 7200 RPM drives (or if you can get a sufficiently large SSD), you might be better off replacing one of them with an SSD and reconsider how you're allocating all your storage.

    As for printer availability, seems to me it would make more sense to use a CUPS based setup to create PDFs rather than having jobs sit in a print queue indefinitely. Then, print the PDFs at your convenience when you have a printer available. I don't know how your printing setup currently works, but it sounds like doing so would reduce your swap space usage.

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