How much space is it good leave in a 256 SSD?

Discussion in 'MacBook Pro' started by letsudo, Dec 19, 2014.

  1. letsudo macrumors member

    Joined:
    Jan 19, 2012
    #1
    I heard that you should leave a certain amount empty for the computer to operate smoothly.
     
  2. mrcat macrumors newbie

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    Oct 29, 2014
    Location:
    London, England
    #2
    Don’t Fill Them to Capacity
    You should leave some free space on your solid-state drive or its write performance will slow down dramatically. This may be surprising, but it’s actually fairly simple to understand.

    When an SSD has a lot of free space, it has a lot of empty blocks. When you go to write a file, it writes that file’s data into the empty blocks.

    When an SSD has little free space, it has a lot of partially filled blocks. When you go to write a file, it will have to read the partially filled block into its cache, modify the partially-filled block with the new data, and then write it back to the hard drive. This will need to happen with every block the file must be written to.

    In other words, writing to an empty block is fairly quick, but writing to a partially-filled block involves reading the partially-filled block, modifying its value, and then writing it back. Repeat this many, many times for each file you write to the drive as the file will likely consume many blocks.

    As a result of its benchmarks, Anandtech recommends that you “plan on using only about 75% of its capacity if you want a good balance between performance consistency and capacity.” In other words, set aside 25% of your drive and don’t write to it. Only use up to 75% of your drive’s free space and you should maintain ideal performance. You’ll see write performance start to slow down as you go above that mark.

    link

    I never fill up system drives, ssd or not, my mac mini has a 500gb drive, out of 499, 406 is free.
     
  3. letsudo thread starter macrumors member

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    Jan 19, 2012
  4. maflynn Moderator

    maflynn

    Staff Member

    Joined:
    May 3, 2009
    Location:
    Boston
    #4
    That seems a little excessive. The rule of thumb used to be 10%, so 2GB free. I wouldn't want to try to live in 192GB of storage just to keep 64GB free.
     
  5. leman macrumors 604

    Joined:
    Oct 14, 2008
    #5
    The SSD (at least the Apple ones) comes with plenty over-provisioned space from factory, so I don't really see much purpose in micro-managing the free space. My SSD has need practically full since at least half a year, without any noticeable detrimental effect so far.
     
  6. maflynn Moderator

    maflynn

    Staff Member

    Joined:
    May 3, 2009
    Location:
    Boston
    #6
    Back in the day, it was not for SSD longevity but OS X stability. OS X, like any OS can become unstable if all or even most of the space is consumed on the drive. That's why back in OS X 10.0 and up, people tried to keep 10% free. I think overall, as the space gets constrained OSX will warn you.

    I do think keeping a few gig free is good for the overall performance and stability of the OS. I never think about the SSD and its longevity.
     

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