Using a 'Fusion' setup consisting of an Apple 1TB hard drive and an OWC 240GB Mercury Extreme Pro 6G, I have confirmed the following:
- The Fusion setup attempts to keep ~4GB of SSD space available for writes.
- 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).
- There is no intelligent migration, or at least it is so slow to learn/adapt that as a practical matter is is pretty much worthless for anything I would do.
In short, writes are fast, at least if not too much is written in a short period of time and not too large a burst.
No smart migration
What I have NOT been able to find is any intelligent migration activity: repeated viewing of images which are known to be on the hard drive does NOT cause migration to the SSD. 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.
Better solutions exist
The Fusion technology has some limited benefits, but I maintain that Apple Fusion is a mediocre (at best) solution for anyone looking for workstation performance; all my real-world usage scenarios show it to be an abject performance failure as compared to having two separate volumes and using each appropriately. The Apple SSD is not particularly fast either.
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: see Dual Drives Are Better than 'Fusion'.
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
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).
The idea of wanting to store iTunes and the like on an SSD are misguided— data rates required for playing HD ideo and music are snail-pace slow, so storing music and videos on an SSD is pointless; there is no performance gain to be had. Any hard drive will do for such stuff.