What is the physical disk reserve on a V-Filer supposed to be? And is it tune-able?

I have a v-filer setup and it looks like it's holding back 25% between the backend luns (on an IBM DS8300) and the front-side aggregates. This seems like a huge amount-- more than it would hold back for a physical disk and excessive in light of the existing physical protection on the DS8300.

TIA, Fred