SSD over-write question

Discussion in 'Mac Pro' started by trankdart, Aug 12, 2010.

  1. trankdart macrumors member

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    Jul 28, 2010
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    Los Angeles, CA, USA
    #1
    Maybe someone can be kind enough answer this somewhat technical question I have about SSD's.

    My understanding is that SSD's, performance aside, have limits on the number of times that each page/sector/whatever can be written to, and, like all flash memory, the limit is rather low, i.e. in the tens or hundreds of thousands of times. Once the limit is reached, you can't write to that sector reliably any more and have to replace the drive.

    Using a mortal SSD as a system drive would seem to be fine, since what you mostly do from a system/application drive is read, right? Except there's one problem.

    If you look at a get info of any file, one of the pieces of information you get, inherited from UNIX, is the date and time of the last time the file was opened. So that info has to be updated with a disk write every time the file is opened, even if it's opened for read-only as when you boot your system or launch an executable. (Actually when you boot your system all kinds of other files are written like mount tables and other Unix-type arcana on the system drive).

    My question is now obvious: if your read-only SSD gets written to a lot in reality, and has only a limited number of writes to each given spot before it goes bad, why is it OK to use it as a system drive? How long does it take to do 100,000 writes to let's say a file system catalog or index sector that holds last-open times?

    Any corrections to my world-view appreciated.

    Thanks.

    TD
     
  2. Transporteur macrumors 68030

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    UK
    #2
    Intel gives an estimate of 5 years considering writings of 20GB per day!

    Should be fine for at least 10 to 15 years and I doubt that anyone today will use drives from 1995. ;)

    Seriously, just check your activity monitor how much data is written, 20GB per day is way more than the average user will ever write to their drive.
     
  3. VirtualRain macrumors 603

    VirtualRain

    Joined:
    Aug 1, 2008
    Location:
    Vancouver, BC
    #3
    There are a lot of different opinions out there. However the facts are...

    MLC NAND cells are good for 10,000 write cycles. That's for each cell.

    How you interpret this or translate this into real-world life expectancy is the subject of some opinion and debate. In my mind, if you rewrote over your entire SSD (say 80-120GB) every day, after about 10,000 days, it would be dead. 10,000 days is 27 years. So even if you wrote to every NAND cell on your drive 10 times a day (that's writing 800GB-1TB of data a day), it would last for 2.7 years.

    Incidentally, SSD's also do wear leveling, to ensure that the same cells don't get rewritten continuously and wear out fast while others never get touched. This further improves the life span of an SSD for those not writing 800GB/day. :p

    Another fact is that Intel rates their MLC drives for up to 100GB/day of write activity for 5 years... and AnandTech has gone on record as stating this is probably a conservative estimate given Intel's track record in quoting these kinds of specs on other products. It's not a hard thing to test... Intel could probably burn out a drive in a week if they really stressed it. So I assume their specs are accurate.

    Finally, when a cell is no longer writable, I believe it is marked as bad. Most SSD's include additional NAND blocks to swap in when bad blocks need to be swaped out... maintaining your capacity even as blocks start to die. What I don't know is if there is a utility that allows you to determine how many bad NAND blocks you have so you can start to see signs of drive failure.

    It's also worth pointing out that like anything, SSD's represent a set of trade-offs... you are potentially trading longevity for performance... or stated another way, TCO for performance. However, I suspect most owners of SSD's will end up upgrading their drives to gain larger capacities and better performance long before the old ones are worn out.

    I would highly recommend reading this page... http://www.anandtech.com/show/2614/4
     
  4. VirtualRain macrumors 603

    VirtualRain

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  5. trankdart thread starter macrumors member

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    Jul 28, 2010
    Location:
    Los Angeles, CA, USA
    #5

    Thanks, extremely helpful post. :)

    TD
     
  6. Transporteur macrumors 68030

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    Nov 30, 2008
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    UK
    #6

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