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Apple’s Fusion Drive

With the new MacMini and new iMac, Apple offers its new “Fusion” drive, which front-ends a hard drive with a 128GB SSD (probably 120GB in reality, given Apple’s checkered history in mis-stating capacity).

Write operations hit the SSD drive first and thus are relatively fast compard to the hard drive, and some files are also stored on the SSD for fast access (OS files, applications). In the background, files migrate to and fro.

Apple’s description suggests that the SSD uses MFU (most frequently used) or perhaps MRU (most recently used) to store useful stuff on the SSD. No doubt it will prove much more responsive than a hard drive alone, and in this sense it is a great technology for mainstream users.

Reliability concerns

Apple has published a knowledge base note: Mac mini (Late 2012) and iMac (Late 2012): About Fusion Drive, on which I comment here.

Presented as a single volume on your Mac, Fusion Drive automatically and dynamically moves frequently used files to Flash storage for quicker access, while infrequently used items move to the hard disk.

As a result you'll enjoy shorter startup times, and as the system learns how you work you'll see faster application launches and quicker file access. Fusion Drive manages all this automatically in the background.

Let it be clear that more complexity, especially with data storage, implicitly carries a negative implication in terms of reliability and bugs (unless one is talking about systems engineered for fault-tolerance).

It is for the reliability aspects that I would rather have one separate SSD and one separate hard drive on which I selectively store my data (e.g. system and apps and a few things like scratch and Lightroom catalogs and big working files on the SSD, photo files on the hard drive). In practice, this works great, though it does entail using one’s brains to a minor degree. Apple prefers to require zero brain power for users of its products, and that is an admirable goal.

Reliability: with the Fusion drive, if either drive goes south then the system dies. Two drives will be less reliable than one drive, end of story. That’s because the Fusion drive approach apparently is either/or: a file is either on the SSD or it’s on the hard drive; the SSD drive is not a write-through caching solution but a sort of extra-smart JBOD. A caching solution would have had fault-tolerant aspects and supported more than one hard drive, but that involves its own complexities also. Bottom line is that SSD prices are steadily dropping and that simpler is better (but at a higher price for a larger SSD, for those who want guaranteed faster performance without the complexity).

However, the target market involves a lot of customers who just store casual data (email, contacts, etc) and who are not too worred about a week’s downtime at the Apple Store to get a failure fixed. For that type of use, the Fusion drive sounds like a very useful technology. I am not one of those people, and I doubt that very many professionals are either. But for my family members who do basic stuff (email, web, calendar, etc), the Fusion drive drive sounds nifty, offering value in terms of low price an (promised) relatively high performance.

Complexity

There are other implications however, as pointed out in the Apple tech note:

  • “ Earlier versions of Disk Utility can't be used with a Fusion Drive.”
  • Limit of one partition, and the Fusion drive functionality is not available for the 2nd partition.
  • Unclear whether one can swap in a larger or faster drive to replace the Apple OEM hard drive.
  • “Third party disk utilities may or may not work with a Fusion Drive”.
  • “ the system attempting to mount the Fusion Drive in Target Disk Mode must have OS X Mountain Lion version 10.8.2 or later”.
  • Etcetera

Probably not a big deal for casual users, but the point is, complexity breeds more complexity. I deem this a marginal solution for that reason.

Alternatives

Barring a bracket change, the prior MacMini and this new one can both take two internal drives. OWC offers both the bracket and drives.

A higher performance and simpler and more flexible solution for those seeking very high performance in a MacMini is a 480GB or 960GB SSD (single drive) with an optional 2nd SSD or hard drive (or two hard drives). A 480GB SSD is more expensive than Apple’s Fusion offering, but one 480GB SSD is likely beyond adequate for most users, and it is very, very fast (and simple).


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