Fusion works well for many small tasks, but as testing shows, performance falls off a cliff once capacity usage approaches 120GB.
Why this happens is shown clearly in a DiskTester fill-volume analysis of performance over the drive capacity.
Once ~112GB is used, the Fusion drive plays games with where Your Stuff goes. Will it be on the SSD or the hard drive? Newly written stuff will go into the 4GB write buffer and might stick around on the SSD for a while— or might not.
UPDATE: now includes performance for the SSD and HDD portions tested separately.
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