If one of the drives died in a fusion setup, how would you boot it back up?

Discussion in 'iMac' started by fathergll, Nov 14, 2014.

  1. fathergll macrumors 65816

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    #1
    Curious, if you had a fusion drive on a iMac and lets say one of the drives failed, how would you go about reinstalling the OS on one of the drives and starting from scratch. From what I understand once one the fusion drives go your data is shot but how would you 'limp' by on one of the drives by reinstalling OSX?
     
  2. octothorpe8 macrumors 6502

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    #2
    You wouldn't. The computer sees them as one "logical volume" (I believe that's the right term) and a failure of one physical component -- either the HDD or the SSD -- renders the whole thing inoperable.

    What you'd do then is get the drive(s) replaced and back up from your Time Machine backup. Or if you've been creating bootable backup clones (with, say, Carbon Copy Cloner), you would boot off of that to minimize your downtime.

    All that said, I've been running a Fusion drive (homebrewed) for about 2 years and have had zero problems myself.
     
  3. /V\acpower macrumors 6502a

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    #3
    It'spossible to "de-fuse" the drive using terminal command. So in theory it would be possible to simply use the SSD or the HD alone.

    However in practice, i honestly don't know if you can de fuse the drive when one doesn't work. Also, I don't think it would be a good idea to keep a defect drive inside your computer, for whatever reason, since it could potentially affect performance and even hardware (depending on how exactly the drive is broken.)
     
  4. nrubenstein macrumors 6502

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    #4
    You'd presumably have to boot from USB to do it. In the typical drive failure mode, all you should need to to is not mount it to avoid issues.
     
  5. Cape Dave macrumors 68000

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    #5
    Yet another reason why I stay very far away from fusion drives. SSD all the way baby!
     
  6. fathergll thread starter macrumors 65816

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    #6
    So here's a question.....has anyone actually ever done it that we know of? This might be more of a reason to go all SSD being one less point of failure. (Though you are paying out the nose for SSD)
     
  7. ddarko macrumors 6502

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    Why do you assume SSD is more reliable than mechanical hard drives? Unlike performance where SSD is clearly superior, the case for their superior reliability is pretty complicated and not at all clear cut. It's not simply a matter of no moving parts makes SSD inherently more reliable. Reliability is more than that. For example, SSDs are more susceptible to corruption and failure caused by sudden power outages than hard drives:

    http://www.infoworld.com/article/26...risk-massive-data-loss--researchers-warn.html

    I'm not saying a single research study proves hard drives are more reliable but just pointing out there are lots of factors to consider when considering reliability. Unfortunately, manufacturers of both mechanical and flash drives are pretty opaque with actual data about how often their devices fail. The reliability ratings they give the devices are notoriously riddled with loopholes and asterisks and really can't be taken at face value.
     
  8. TechZeke, Nov 15, 2014
    Last edited: Nov 15, 2014

    TechZeke macrumors 68020

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    #8
    I think it just depends on the use-case. For example, for laptops, SSDs I think would be clearly superior in reliability since they are more prone to shock and shaking, the biggest weakness of HDDs. Laptops, having a battery, are also far less prone to sudden power outages, if it all since the computer shuts down when it runs out of battery.

    In a desktop situation where you need lots of storage, and where you are writing large amounts of data on a daily basis, I could certainly see HDDs having a more level playing field since SSDs eventually wear-out.

    Personally, having a 256 or 512 Internal SSDs in the machine itself then having an external HDD on USB 3 or thunderbolt is the way to go.
     
  9. Weaselboy Moderator

    Weaselboy

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    #9
    Presuming you knew which drive dies, you could turn off Fusion and format the good drive as a stand alone disk then restore backup data/OS to the good drive, then boot to the good drive and carry on. Assuming here all the data would fit on the one remaining drive.

    You could either use Internet recovery or a Time Machine backup to get the single disk back up and running.

    ----------

    I don't see that as the issue here either way. The issue IMO is with a Fusion setup you now have two possible points of failure rather than one. So even if SSDs and HDDs are of equivalent reliability, by using two drives to make a Fusion setup your odds of failure have just gone up 2X.
     
  10. fathergll thread starter macrumors 65816

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    #10
    At a high level how would you do that? If you turn on your Mac and the drive one of the drives is dead what are you seeing?
     
  11. Weaselboy Moderator

    Weaselboy

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    #11
    You would need to boot from an external disk or Internet recovery (command-option-r), then from there in Terminal follow this to break up the Fusion drive. Then you would be back to two, stand alone disks to troubleshoot and install the OS on the good disk.

    Most of this involves using the diskutil command in Terminal to manipulate the disks/volumes.
     
  12. fathergll thread starter macrumors 65816

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    #12


    Thanks! That's not too bad. My main concern is if I one of the drives died say 4 years down the road outside of Apple Care what would be my options for using the iMac. Thats not bad as you could technically get by on either drive especially with external storage.
     
  13. ddarko, Nov 16, 2014
    Last edited: Nov 16, 2014

    ddarko macrumors 6502

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    That definition of risk is arbitrary and unscientific. An object composed of two parts is not inherently 2x more prone to failure than an single unitary object. And why are we talking about an SSD drive like it's a single thing - a 256gb Samsung 840 Pro drive is made up of 8 x 32gb NAND modules. Doesn't that mean it has 8 points of failure we should be concerned about? On the other hand, most big capacity hard drives have 4 or 5 platters - does that mean people should be advised to stick with hard drives because they have fewer points of failure? Your yardstick "more parts = more opportunities of failure" is not a good rule of thumb. When you look at all the parts - and the parts within parts - that make up a modern computer, you see there are tens of thousands of potential points of failure. If there was evidence that a hybrid SSD/hard drive solution was more prone to failure than an all-SSD or all-hard drive solution AND that higher rate of failure is causally linked to its hybrid nature, then that would be grounds for considering a hybrid drive more risky. But absent such evidence, avoiding it because "2 is more risky than 1" is questionable advice.
     
  14. Weaselboy Moderator

    Weaselboy

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    #14
    Look I took a college stats class too and I get what you are trying to say.

    The bottom line is when you have something like a drive mechanism that is already the most failure prone part of the machine, then setup a system where two of those drives must work in tandem, the odds of that failing are going to be substantially higher than a simple, single drive system. It is just common sense.

    I have zero interest in arguing about this with you, so we can just agree to disagree.
     

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