After reinstalling OS X from recovery partition, Fusion Drive disk numbering reversed

Discussion in 'iMac' started by TrimmTrabb, Jan 26, 2013.

  1. TrimmTrabb, Jan 26, 2013
    Last edited: Jan 26, 2013

    TrimmTrabb macrumors member

    Joined:
    Apr 10, 2012
    #1
    I performed a clean install on a late 2012 iMac with Fusion Drive using Internet recovery. Everything seems OK and the computer appears to boot and run OS X off the SSD, but when I run "diskutil list" in the terminal, it shows the SSD listed as disk1, and the HDD as disk0. This is in the reverse order that the computer came from the factory. An Ars Technica article shows the SSD as disk0 as well. http://arstechnica.com/apple/2012/11/fusion-drive-quick-look-our-predictions-confirmed/

    Can anyone explain why this reversal happened and whether it's anything to be worried about? Should the boot drive always be disk0? I didn't mess with any of this manually, I just let the installer do its thing. Is there anyone else with a genuine Fusion Drive that can run diskutil list in the terminal and let me know what they get?

    Thanks!

    UPDATE: After shutting down and rebooting the computer, the SSD is now reported as disk0. After googling I found a bunch of evidence that disk ordering can change quite randomly - this is completely independent of Fusion Drive. It appears to just depend on which drive is initialised first on boot. So it literally can change from one day to the next. I wonder if anyone here is knowledgable about this stuff and can back this explanation.
     
  2. itsamacthing macrumors 6502a

    Joined:
    Sep 26, 2011
    Location:
    Bangkok
    #2
    I would not be concerned .. Wipe it all and do it again if you are concerned
     
  3. drambuie macrumors 6502a

    Joined:
    Feb 16, 2010
    #3
    System info on my new 2012 iMac shows the fusion SSD as Disk 0, and the HDD as disk 1.
     
  4. TrimmTrabb thread starter macrumors member

    Joined:
    Apr 10, 2012
    #4
    Thanks for your responses.

    UPDATE: After shutting down and rebooting the computer, the SSD is now reported as disk0. After googling I found a bunch of evidence that disk ordering can change quite randomly - this is completely independent of Fusion Drive. It appears to just depend on which drive is initialised first on boot. So it literally can change from one day to the next. I wonder if anyone here is knowledgable about this stuff and can back this explanation.
     
  5. jediDev macrumors regular

    Joined:
    Jun 13, 2012
    #5
    The only thing that matters is that writes initially go to the SSD and not the HD—in other words that the system knows the SSD is the faster drive. This is almost certainly the case, but if you're interested in making sure and are comfortable with terminal you can use iostat command. Just make sure you know which drive number is ssd and which is hd, and then make sure the SSD is the drive onto which data gets written first.

    Read here for more examples of using iostat to monitor fusion drives:

    http://jollyjinx.tumblr.com

    BTW the reason iostat works to monitor what's going on is that iostat is a unix-level tool sitting underneath the Core Storage layer which is where the Fusion stuff happens. IOW, iostat has no idea that fusion drive exists. For the purposes of this, it just sees drives and data transfer.
     
  6. TrimmTrabb thread starter macrumors member

    Joined:
    Apr 10, 2012
    #6
    Thanks. I've followed your instructions and can confirm that data is being written to the SSD, even when assigned to disk1.
     
  7. leman macrumors 604

    Joined:
    Oct 14, 2008
    #7
    Every volume has its unique UUID (universally unique identifier) number (you can run diskutil cs list from terminal - these are the long hexadecimal numbers). This is what the system uses to recognise the drives. The disk identifier (disk0,disk1 etc.) might change on subsequent restarts but the UUID of its volumes is always the same. I also suspect that you could actually take the drives out and plug them into different ports and the system would still manage them in the correct way (I never tried it though).
     

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