I am going to be adding a SSD to my base model 2011 Mac Mini using the OWC kit. I am trying to decide whether to set this up as a DIY Fusion drive or whether to just keep the SSD and HDD managed separately. One of the factors that will come into play is how the Fusion system works in managing how full it will allow the SSD to become. An empty SSD performs at much higher levels than a half full SSD... and a full SSD can slow to a crawl. For reference look at the OCZ Vertex 4 where it will run in a "Performance Mode" as long as the drive is 50% full or less. Over 50% Vertex 4 performance is dropped. Also, when benchmarking my many SSD's I can never seem to achieve the same levels as the reviews which is due to the fact that I am benching a drive that is already in use and therefore not 100% empty like most reviews. So, if the Fusion system manages the SSD such that it can fill it to capacity then you would expect much poorer performance than if the SSD was, say, half full. Of course the amount of degradation will depend on the aggressiveness of the garbage collection on the particular SSD model as well as whether TRIM has been enabled (for non Apple SSD's). One of the things hampering a clear understanding of this is the rarity of freely available benchmarking tools for SSD's in OS X. Black Magic is nothing more than raw sequential performance which is only one piece of the puzzle. It is really the small 4K performance that will be most noticeable in day to day use. The only other tool freely available that I know of is Xbench but it is an old tool and the 4K numbers don't make much sense. In Windows I have access to all sorts of free benchmarking tools that give pretty consistent, comparable results like ATTO, Crystal Diskmark, AS SSD Bench, Anand Storage Bench, etc... So the questions that this raises are: 1) What sort of algorithm does Fusion use to determine how full to allow the SSD to become? 2) How well does Garbage Collection alone maintain the performance of the Fusion Drive? 3) Is TRIM absolutely required for Fusion to maintain performance over time? Any other thoughts or discussion are very welcome. Just trying to get my head around this.