Pierre Abbat is curious about using Hammer on an SSD.  The discussion that came from that has some useful points, including notes that a straightforward SSD as disk works for most anything with Hammer other than very intensive database use, due to the history retention.  If space is an issue, swapcache on the SSD and attaching a normal HDD is a fine alternative.  A SSD with Hammer can leave some features off, though I’d argue that dedup is totally worth is.  Also, SSD speed is directly correlated with size.