Here is my experience so far. I turned on FAST Cache for a single-plex aggregate hosting an NFS-mounted VMware datastore, and the write hit ratio is hanging out at about .125, and the read hit ratio is hanging out around .65 (both estimates are very rough based on a quick eyeball of the performance chart), with variances down to .5 and up to .89 for reads and .05 and .36 for writes. What's interesting (to me, anyway), is that the ratios are almost precisely reversed for the SP Cache: read hits are down in the same area as FAST Cache write hits, while write hits are actually even higher than FAST Cache read hits. Looking at the hits per second, FAST Cache read hits are significant, at >40, while everything else pales in comparison.
Overall, though, it seems as though roughly 80% of reads are coming out of either SP Cache or FAST Cache (mostly the latter), while almost all of the writes are hitting cache at some point. I know that conventional wisdom is that WAFL doesn't really benefit from write caching (or so I have read), but given that the LUN service time is relatively miniscule (topping out at 4 ms and generally staying between 1-2 ms), while the response time is >10x that amount generally, most of the data must be coming from cache, so FAST caching does seem to be boosting read performance. That's my conclusion, anyway, speaking as very much a novice when it comes to storage performance tuning.
Anyone have any feedback, critique, etc.?