What is "% Cache WP"

Hello all,

We are doing some load testing in a new environment we are building out and I noticed that as we were loading up the system out "% Cache WP" eventually hit 100% and Avg Write Response time shot up. I can only assume that "% Cache WP" is something with write caching, but google has failed to turn up any meaningful definitions for me.

Attached is a screen shot from our load testing.