There is a Solaris “bug” (if you can call it that) that limits ARC cache to 7/8 of total system memory. This was intentional so that the ARC cache couldn’t cause contention for swapfile space in RAM. That “bug” still exists in Nexenta 3.1.2. This is why I have 30GB of RAM free on a 192GB system. 1/8 system memory made sense when monster systems were 8GB. That left 1GB of RAM that wouldn’t be touched for ARC caching to ensure proper system operation. Today the amount of RAM in systems dwarfs what most people would have used 10 years ago, and as such we need to make modifications. Fortunately, we can tune this variable.
http://www.zfsbuild.com/2012/04/18/let-zfs-use-all-of-your-ram/