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Does Fusion Actually Migrate Files?
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.
The question is, does Fusion actually migrate “frequently used” files to the SSD? All my testing suggesed that the answer ws “no”; it turns out that a special procedure is needed to actually make a Fusion volume, not just something that looks like a Fusion volume.
In general, read speeds are far more important that write speeds. With some obvious exceptions, most all files are written once and read over and over. So the key questions are:
- Do frequently used files stick to the SSD?
- Do frequently used files on the hard drive get moved to the SSD?
Testing to try to force migration (repeated reads of the same files, repeated real world batch processing of image files, etc) did not result in any migration of those files even after many reads and even after giving the system hours to think about it.
It is possible that under some circumstances migration happens (this cannot be ruled out from my testing). But since the real-world scenarios I tested (the actual work I tend to do!) did not cause this benefit to accrue, it is eminently reasonable to conclude that counting on Fusion for performance gains is a misplaced idea.
The Fusion 4GB buffer on the SSD
Fusion maintains a 4GB buffer on the SSD. This buffer is used to soak up any disk writes that occur; if too many writes (4GB or more) occur in a short time, this buffer overflows and writes are then forced to the hard drive. An example of this might be downloading 5GB of image files, a large video file, etc.
So what does Fusion actually do?
All observations point to a very simple approach: writes initially go to the 4GB buffer. To maintain that 4GB buffer space, some items are “pushed down” to the hard drive. That is all.
As far as my testing shows, no actual migration back to the SSD occurs. This has implications for photographers working with Lightroom, Photoshop, etc: the key files that could benefit from speed might be pushed down to the hard drive, and stay there.