I think it would be very useful to see a RRDTool graphic that displays the ZIL/Cache usage into Reporting area.
I have currently a SSD disk used for cache and I would like to know its usage.
There is this: http://doc.freenas.org/index.php/Arcstat. "For now, these scripts can be manually run from Shell. Future FreeNAS™ versions will automatically integrate their results into a System -> Reporting graph. "