How does a Fusion drive appear to the system

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

  1. macrumors 6502

    Jan 24, 2012
    A lot of hardware RAID implementations appear under Disk Utility as nothing more than regular hard drives. If you use the command line version of Disk Utility using the command line version "diskutil" it will often do as follows:

    $diskutil list

    0: GUID_partition_scheme *160.0 GB disk0
    1: EFI 209.7 MB disk0s1
    2: Apple_HFS Mountain Lion 75.9 GB disk0s2
    3: Apple_Boot Recovery HD 650.0 MB disk0s3
    0: GUID_partition_scheme *320.1 GB disk1
    1: EFI 209.7 MB disk1s1
    2: Apple_HFS Snow Leopard 320.0 GB disk1s2

    (that's a fake list I made up, FWIW but in the above disk1 could be the way a lot of hardware RAID units appear to the system - just as a normal drive even though the RAID unit may have 4 drives in it.

    There's a lot of post on the web with titles like "build your own Fusion drive." It uses CoreStorage to set up some drives/volumes into a combined unit.

    I'm curious. If someone does a "diskutil list" on a REAL fusion drive, will all the CoreStorage elements show up, or does it, like a RAID unit, simply appear to the OS as just a regular drive.

    FWIW, if any of you guys have tried spanning drives with CoreStorage, "diskutil list" will output an entry for each CoreStorage entry and then a final one for the complete, logical volume. In other words, if you used CoreStorage to span 3 disks and make it appear like one big drive, you end up with 3 CoreStorage components and a final usable volume, thus it looks like you have 4 drives on your system.

    I'm just curious, because if a Fusion drive appears as a single drive under disk utility, it implies that there's hardware doing a little more than what the guys promoting the "build your own Fusion drive" are saying....or so I think. On the other hand, if each drive inside a real Fusion drive show up as individual drives and volumes under diskutil, then it would kind of imply the actual Fusion drive is nothing more than a glorified SATA extender with an SSD and hard drive installed in it.

    This is nothing more than a curiosity question, by the way.

  2. macrumors 603

    Oct 14, 2008
  3. thread starter macrumors 6502

    Jan 24, 2012
    Thanks for the great post!

    I find the following interesting:

    1. The protocol, which I would have thought would have been identified as "Thunderbolt" is identified as SATA.
    2. It reports the SMART status on an external drive housing.

    Remarkable really.

    Once again, thanks for the post.

    Since multi drive or volume CoreStorage uses the "primary" drive as the one that handles the load, I'd have to wonder how long an SSD will last before it ends up suffering from write-cycle depletion.

    We're running into some MacBook Air's where the SSD drives are becoming problematic after 2 years.
  4. macrumors 603

    Oct 14, 2008
    I don't find this very surprising - Thunderbolt is just PCI-Express with a transport layer. There are people who got a standard PCI-E GPUs working over TB without too much hassle.

    When were these MBAs released? The write-cycle depletion is really one of the least concerns with a modern SSD - you could write dozens of GBs per day for many years until it becomes a problem.
  5. thread starter macrumors 6502

    Jan 24, 2012
    It may not be surprising but from a support standpoint the lack of the Thunderbolt identifier could be a PIA.

    Consider the following: Someone has bought a Thunderbolt external drive (not fusion) and something has gone wrong. He used to have 2 disks, one internal and one external, and one of them has blown. Unless that user is aware of what the sizes of the drives are or what designation they had, I doubt it would be immediately obvious to them which one has failed. If I asked them to open up a terminal and type "diskutil list" only one drive will show up and, regardless of whether or not it's internal of Thunderbolt, it will appear as disk0.

    If I ask them to open up System Information, will the drive be listed under "Thunderbolt" or will it be listed under "Serial-ATA?" (I don't know the answer to that question). If the external drive shows up under "Serial-ATA" then there's really no way to help someone from a remote location with this problem.

    Apple clearly could have done a better job designating these things. They could have ID'd the drive as something like "SATA under Thunderbolt" or "FireWire under Thunderbolt" (for a FireWire drive).

    I have to wonder if this isn't why some of the guys developing drive testing software seem to be getting to the game a little late. Just during the last week, Scannerz (finally) added full support for apparently all forms of CoreStorage. The TechTool Pro site STILL says that to test an encrypted drive you need to do it with e-Drive, which is telling me it's seen as an unidentified drive type, and apparently they're still testing Fusion Drives (probably because CoreStorage disk management might mess up their ability to do index checking - a guess for sure on my part!)

    ...of course I realize you have no control over any of this.;)

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