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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  • Abjuk - Wednesday, September 2, 2009 - link

    Agreed CM, my current project at work takes about six minutes to build from scratch and CPU usage never gets above about 35%. The process is totally IO bound.

    It really depends on whether you have several large source files or several hundred small ones.
    Reply
  • Weyzer - Tuesday, September 1, 2009 - link

    Good article and testing, but why was the Crucial M225 not mentioned at all? It's performance is similar to the vertex drives, I know, but I think it could have been mentioned somewhere, if it is in the good or bad range. Reply
  • jasperjones - Tuesday, September 1, 2009 - link

    javascript:link('frmText') $997 @ Newegg omgomgomg

    Needless to say, that price will come down quickly. So more seriously, after reading the article I really feel I understand better what to look for in an SSD. Thanks!
    Reply
  • paesan - Tuesday, September 1, 2009 - link

    Wow, does NE really think that anyone will buy the Intel drive at that price. OMG!!! Funny thing, it is in stock and it says limit 1 per customer. Lol Reply
  • CList - Tuesday, September 1, 2009 - link

    Obviously someone is buying them at that price or they'd lower it. The people who can't wait two or three weeks and are willing to be gouged for these drives are the ones that allow NewEgg to give us low margins on other products while not going out of business :D

    Reply
  • ravaneli - Tuesday, September 1, 2009 - link

    I just decided to buy one and when I opened newegg i couldn't believe my eyes. I hope that is only because they have a few drives left, and once Intel pumps up some stock in the retailers the prices will go back to Intel's retail.

    Does anyone know what are the production capabilities of Intel's SSD factories? I don't want to wait a whole year until the market saturates.
    Reply
  • LazierSaid - Tuesday, September 1, 2009 - link

    This article was so good that Newegg doubled their X25M G2 prices overnight.

    Reply
  • medi01 - Tuesday, September 1, 2009 - link

    Yep, very impressive advertisement indeed. Reply
  • HVAC - Tuesday, September 1, 2009 - link

    I'd rather have ewoks in the sequels than Jar-jar ... Reply
  • Naccah - Tuesday, September 1, 2009 - link

    Newegg's prices on all the Intel SSDs skyrocketed. The X-25 G2s are $499 now. Is this price a reflection of the high demand or did Intel change the price again? Reply

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