Fusion drive's permanent OS/App storage?

Discussion in 'iMac' started by mrbrycel, Nov 8, 2012.

  1. mrbrycel macrumors member

    May 18, 2010
    I'm still a little confused on how the fusion drive works. I know the two are as one volume, and that the software manages everything itself, but what does that really mean?

    Are the OS and the applications permanantly stored on the SSD? Or is it really moving things back and forth from the hard drive everytime you use something more than another?

    I can't see the OS/apps exceeding 128GB, so it would make sense for them to all stay permanantly stored on the ssd, but is that the case?
  2. timcullis macrumors member

    Oct 9, 2012
    One of the many other threads about the new Fusion drive had a link to someone who had put together his own Fusion setup using the hooks that are already in the operating system, and he describes how it works.

    There seems to be a lot of background rewriting going on with his setup and I think we won't know for certain until some people get hold of actual machines.

    It might be that "as delivered" everything is sitting on the SSD on the basis, 'why not?' And then infrequently accessed files (whether apps or data) get 'archived' to the HDD when space starts running out. Parts of the OS are undoubtedly going to reside on the SSD all the time, many frequently used apps are likely to be placed there as well.

    It's important to realise that some datasets that are presented to you as one file (e.g. iPhoto libraries) are actually many hundreds of files under the surface and the system might well decide to put some on the HDD, some on the SSD.

    But I'm not bothered finding out too much about how it works, so long as it produces the promised performance boosts!
  3. dearlaserworks macrumors regular

    Apr 28, 2012
    Eastern Shore, USA
    Fusion will keep as much as it can on SSD, so since the fresh iMac will be less than 128GB, everything would sit on SSD until you add your own files. Fusion will keep 4GB free on the SSD to allow for new files to write to SSD initially.
  4. flynz4 macrumors 68040

    Aug 9, 2009
    Portland, OR
    Re: boldest section above: this is why I am planning to go full SSD (along with my external Pegasus R4). My Aperture 3 (A3) library is hundreds of thousands of files... and it is significantly larger than the 128GB SSD portion of the Fusion drive. Having it thrash between the SSD/HDD will not be great.

    My preferences in priority order:
    1- Complete A3 library on the internal SSD
    2- Complete A3 library on a dedicated external TB attached SSD
    3- Complete A3 library on the Pegasus R4

    Assuming the BTO prices for the SSD are reasonable (~$1k)... Then I'll go full SSD. I am hoping not to revert to #3... Although the 8TB Pegasus R4 is actually quite a strong performer. I have it configured as 4TB RAID 10. I am very ready to move my library to full SSD.

  5. mrbrycel thread starter macrumors member

    May 18, 2010
    Thanks for all the explanations.

    So if in total, you only had less than 100GB stored on the computers internal volume, this would all be stored on the SSD, and it wouldn't run anything off the HDD right?

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