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Apple’s Fusion: Not Very Smart

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MacMini buyers should see Dual Drives Are Better than 'Fusion' before buying.

Testing an Apple Fusion drive is fraught with perils of erroneous conclusions.

It turns out that a special procedure is needed to actually make a Fusion volume, not just something that looks like a Fusion volume.

Using a 'Fusion' setup consisting of an Apple 1TB hard drive and an OWC 240GB Mercury Extreme Pro 6G, the following became clear with repeated efforts:

  • The Fusion setup attempts to keep ~4GB of SSD space available for writes. In the background, space is cleared out after writing is done, in anticipation of future need.
  • Fusion apparently uses a “displacement” algorithm: newly written material displaces older material. This is a good technique (which ought to be configurable perhaps), but far less sophisticated and useful than promoting frequently-used files (“intelligent migration”).
  • No evidence of intelligent migration (“most frequently used”), or at least Fusion is so slow to learn/adapt that as a practical matter is effectively worthless for any kind of workflow. But it appears that absolutely no migration is done for frequently used files— not observed at all in operation, even trying to force it by repeatedly accessing files.
  • Saving a large (2GB) Photoshop file (uncompressed for save speed) benefits from SSD speed.
  • Saving a 2nd 2GB file right away after the first 2GB save shows performance falling off a cliff as the writes are forced to the hard drive (not enough SSD space).
Aura SSD for 2013 Mac Pro

No smart migration

No intelligent migration activity (moving frequently accessed items to the SSD): repeated viewing of images which are known to be on the hard drive does not cause !tw

migration to the SSD, even after many attempts and days.

Hence performance remains poor for reads on files that I’ve used for real-world viewing over and over.

Hence storing a Lightroom catalog or similar on a Fusion drive is massively inferior to storing it on an explicit SSD volume. Where people get fooled is starting to use a Fusion drive which has an SSD still with ample space. That’s not Fusion at work, that’s just the SSD not yet full.

As such, one might “get lucky” depending on when the Lightroom catalog and its previews are created, in terms of capacity used and hence whether the files go onto the SSD or HDD.

Better solutions exist

The technology has benefits for write speed, but Apple Fusion is a mediocre (at best) solution for anyone looking for workstation performance; all real-world usage scenarios tested show it to be a performance failure as compared to having two separate volumes and using each appropriately. The Apple SSD is not particularly fast either, as SSD speed goes.

But the Apple reality distortion field remains strong.

Casual usage— a bump up in performance over just a hard drive is fine— why not if one accepts the limitations and risks.

But an explicit SSD boot drive of ~240GB along with a 1TB hard drive remains superior for most everything, at least if one is willing to apply a small amount of brain power to one’s usage. If not, then it’s a “toaster” for routine tasks, and as such Fusion is fine. But to get excited about Fusion for serious work is laughable.

Two volumes are better

See Dual Drives Are Better than 'Fusion'.

Two volumes consisting of a 240GB OWC 6G SSD and the Apple 1TB HDD delivers guaranteed faster performance for important tasks, because one can use the SSD specifically for anything where fast I/O speed is needed (e.g. saving big files, Lightroom catalogs, current projects, the system/apps, etc).

Fusion is no help for iTunes

The idea of wanting to store iTunes and the like on an SSD are misguided— data rates for playing HD video and playing music are snail-pace slow, so storing music and videos on an SSD is of no value; there is no performance gain to be had. Any hard drive will do for such stuff.


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