System Memory and Fusion Drive questions

Discussion in 'iMac' started by BIG-b, Apr 19, 2013.

  1. BIG-b macrumors newbie

    Apr 1, 2013
    Hello all,

    I am along long time Windows user and IT network/systems Admin by trade (Mainly Windows Server/Unix Environment..

    I am a newbie when it comes to the MacOS though.. I went all out Apple a month ago ! I bought the entire family iPhone 5's, iPad 4 (VZW) for the family to share, and a shiny new iMac i7, 32GB Ram, 1TB Fusion Drive, and the GTX 680MX 2GB video card... ( I l
    ove ie by the way !! )

    I have a few questions in regards to the way the Mac OS handles memory allocation.. (Ram and the Hard Drive.)

    I purchased istat menus and noticed today it said my memory is at 100% used and since the machine is still running wonderfully I believe it is just seeing the memory that is inactive and the MacOS is not really low on resources..

    - Why in the world would the "VM Size" show 469GB ? Is this similar to the Windows Page File ?
    - Does inactive mean not in used but has been allocated and available ?
    - Is there anyway to see the actual space used on the 128GB SSD portion of the Fusion setup or to actually see the data that is on it ?

    I am really curious about the Fusion and I think it would be interesting to see it in action as it moves files back and forth ...

    I searched for the answers on this and found some answers but I figured I would receive more personable answers by posting :D

    Thanks !!

    Attached Files:

  2. simsaladimbamba

    Nov 28, 2010
  3. Nuke61 macrumors 6502

    Jan 18, 2013
    Columbia, SC
    You'll need to use Terminal, which is the OS X command line. Open Finder, click on Applications, open Utilities and lastly, open Terminal. In Terminal, type the following: diskutil cs list
    What this does is tells diskutil to use Core Storage and list any drives. In other words, show me any Fusion drives, and show me their actual physical attributes. You should see a 1.1 TB Macintosh HD, followed by a 121 GB drive and a 999 GB drive. These two drives are the SSD and the HDD that make up the Fusion drive. The reason we needed to do this is to find out their disk numbers. In my case the SSD is disk2 and the HDD is disk0.

    Now we're going to enter the Terminal command to see activity for the two drives that make up the Fusion drive. Do this by entering the following in Terminal: iostat disk0 disk2 1 <or your disk numbers, if different>
    What this does is tell iostat to show I/O activity for disk0 and for disk2 every 1 second.

    When you're done watching the disk activity, use Ctrl-Z.
  4. marzer macrumors 65816


    Nov 14, 2009
    VM Size is the maximum potential of addressable memory by the system. A combination of physical RAM and paged file memory. Yes, the same as Windows Page File. It's unlikely you'd ever come close to achieving that figure as your machine would probably be ground to a halt long before that :)

    Inactive is memory that was released by a previous running program(s). If the program or programs are ran again BEFORE another program claims that space, they can re-use the dormant code/data that was not yet overwritten. Essentially cached if nothing else preempts it.

    From a user point of view, your total free chip memory = Free memory + Inactive memory.

    As for data allocation across the Fusion, you'd probably need some way to observe the file directory which associates block allocations at the device level. Not sure how to go about that. Maybe some digging around in Core Storage perhaps.

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