have wondered at length why we’re spending so much (virtual) ink covering Fusion Drive. Isn’t it just a plain caching solution? Isn’t it the same as Intel SRT? Hasn’t Linux been doing this since 1937?
No, no, and no. Intel’s Smart Response Technology is a feature available on its newer Ivy Bridge chipsets, and it allows the use of a SSD (up to 64GB in size) as a write-back or write-through cache for the computer’s hard drive. One significant difference between FD and a caching technology like Intel SRT is that Fusion Drive alters the canonical location of the data it tiers, moving it (copying it, really, because we don’t see a “delete” file system call during Fusion migrations,
.. FD keeps a minimum of 4GB free on the SSD.
.. The actual impact of the tiering on user operations appears negligible—actually using the Mini as a regular computer feels nice and quick, as if it were a regular SSD-equipped Mac. In fact, for a casual user with less than about 110 GB of stuff, FD shouldn’t ever actually touch the hard drive.”