2TB Fusion Drive is Larger Than Advertised

Discussion in 'iMac' started by ghsNick, May 21, 2016.

  1. ghsNick macrumors 68030


    May 25, 2010
    Hey Everyone -

    My 27inch 5K iMac came with a 2TB Fusion drive but when I opened my computer it says 2.12TB are available...am I missing something or did Apple give us more than we expected?

    And yes, I know about 124gb of SSD is on the Fusion Drive but I thought it would 2TB total.
  2. chrfr macrumors 604

    Jul 11, 2009
    I guess it's more than you expected, but that's how the Fusion Drive works. The spinning disk is 2TB and the SSD is 128GB, thus getting you 2.12 TB.
  3. varian55zx macrumors 6502a


    May 10, 2012
    San Francisco
    Well, I'll break it down for you.

    You thought it would be 2 tb total, and yet you realized it's 2.12 tb. You're aware there's a .12 tb ssd component with it.

    So, that would be where the extra .12 tb comes from. It's the SSD portion added to the HDD portion but it's much easier to market a clean "2 tb" than it is "2.12 tb of storage space". Nobody will care about the .12 when looking at an ad.
  4. ApfelKuchen macrumors 68040

    Aug 28, 2012
    Between the coasts
    Right. Apple has chosen to not advertise the capacity of the SSD/Flash storage when they sell Fusion drives. When you consider that a 1TB HDD doesn't deliver exactly 1TB of storage to begin with, then adding the nominal capacity of the SSD/Flash to the nominal capacity of the HDD would further distort matters. Consider it under-selling/over-delivering.

    The System Report for my 3TB Fusion drive reads as follows:

    Logical Volume Group:
    Name: Macintosh HD
    Size: 3.12 TB (3,120,721,960,960 bytes)

    Device Name: APPLE SSD SD0128F [nominally, a 128GB SSD - but some space is always reserved to allow for wear]
    Size: 120.99 GB (120,988,852,224 bytes)

    Device Name: APPLE HDD ST3000DM001
    Size: 3 TB (2,999,733,108,736 bytes)
  5. grahamperrin macrumors 601


    Jun 8, 2007
    That would be the total if the solid state drive was used for caching (non-permanent storage).

    With both the hard disk drive and the SSD used for permanent storage, with the combination of Apple Core Storage (storage system) and HFS Plus (file system): you get the sum of the two.

    An alternative to that combination – ZFS, and the SSD used for Level 2 adaptive replacement cache (L2ARC) – would give you 2 TB permanent storage with superior performance. But OS X can not boot from ZFS, so you would need to either:
    • give part of the SSD to L2ARC, part to an HFS Plus startup volume
    • more simply, add a drive (not necessarily an SSD) for the startup volume.
  6. maflynn Moderator


    Staff Member

    May 3, 2009
    You mean your 2 terabyte hard drive coupled with the 128GB SSD? ;)

    Yeah its the combined total of both drives
  7. ApfelKuchen macrumors 68040

    Aug 28, 2012
    Between the coasts
    Considering the difficulty most folks seem to have with Fusion, I can't begin to imagine the kind of threads we'd have under ZFS with L2ARC. ;) Personally, I see ZFS and L2ARC to be high performance systems. What benefit would they have for the average Fusion user, who's satisfied with nearly the performance of an SSD, at HDD prices? (I'm one of them - I don't require redundancy in my primary storage array - up-time is not mission-critical, a Time Machine restore is fine, if/when it's needed.)

    It's possible we may see something like L2ARC in future Macs, especially Mac Pro, for one reason in particular - SSD/Flash caching of attached HDDs and networked storage (extending the benefits of Fusion beyond the box). Volume spanning between internal and locally attached storage is a bit too risky, and totally inappropriate for shared data stores. Allocating a fair chunk of Flash to the caching of slower external storage seems a natural to me.
  8. grahamperrin macrumors 601


    Jun 8, 2007
    Good questions.

    Here, for example - iMac 2012 Booting to OS X Utilities & Macintosh HD missing - it's reasonable to assume that the SSD has suddenly failed. No redundancy of data or metadata with the average Fusion Drive hybrid, so - unless there's a backup - the end user may treat the loss of the file system as catastrophic.

    With ZFS: if the SSD is given to L2ARC then the file system should be unharmed by failure of the SSD part of the hybrid. And so on (we should probably take this to another topic if you'd like to know more).
  9. ApfelKuchen macrumors 68040

    Aug 28, 2012
    Between the coasts
    Yes, just as catastrophic as the failure of the HDD in a single-HDD iMac, or of the SATA SSD or PCIe Flash in a single-drive MacBook, or of any drive in a multi-drive, non-redundant configuration (like a split SSD/HDD with OS and apps on SSD, data on HDD). How is the failure of a Fusion drive any more catastrophic than any other storage failure in today's lineup of Macs and iOS devices, none of which can be configured for internal RAID? How many thousands of MacRumors threads have similar titles, with no Fusion drive in sight?

    To revisit/rephrase Murphy's Law for a moment... Failure is not an option, it's an expectation.

    For the most part, redundancy doesn't find its way into consumer products (or even most business products), because the principle purpose of redundancy is sustained up-time, and sustained up-time is not a priority for the vast majority of users. Do you seriously think that the world's PC makers wouldn't be delighted to add $100-$200 to the price of each machine they sell, if there were significant numbers of customers willing to pay for storage redundancy?

    I absolutely agree, there are cases where L2ARC (or nearly any other caching scheme but Fusion) will result in a beneficial outcome - should the SSD fail, the system could fail-over to the HDD and keep working (new thread: "Why is my system's performance so much slower all of a sudden?"). There's a more likely failure mode, however... The SSD survives, but the HDD fails. We're back to catastrophic, aren't we?

    In the case of Fusion, L2ARC, a self-contained hybrid drive, or even RAID, a backup is still essential - drive failure is simply one of many possible hazards, and following a drive failure, unless you have full redundancy, a restore from backup is inevitable.

    Anyway, we're talking Apples and Oracles. Note that in that 2008 Oracle L2ARC article, the HDD storage pool consisted of 44 mirrored drives, and the L2ARC cache consisted of 6 SSDs in a RAID-like configuration (read "Isn't flash memory unreliable? What have you done about that?" in that article, especially the part about successfully yanking out busy L2ARC drives). In that system, a total failure of the L2ARC was highly unlikely. Since nearly every caching scheme but Fusion is redundant, I doubt they had the kind of discussion we're having here.

    Apple approached Fusion with very different design goals. The "competing" design wasn't a cached RAID, it was "OS and apps on SSD, data on HDD." If they were going to charge customers for a 128GB SSD plus 1TB HDD, their customers would expect to get additional, usable storage out of the arrangement. Otherwise, they'd just break the Fusion configuration and use them as separate drives (which some do anyway). The point of Fusion has always been near-SSD performance on a large-capacity volume, at a low, low price. Seriously, if tomorrow, Apple was to offer a machine with a mirrored Fusion drive, how many would actually run it as configured, and how many would break the configuration in order to have additional, independent internal drives?

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8 May 21, 2016