If you've recently installed a new system with FAST Cache and you're trying to determine how much work that it's helping out with it can be confusing to determine if it's been worthwhile.
You can do a couple of things
1) collect nar files over a few days and have your EMC SE review the outputs and provide a report
2) Leverage a tool such as VNX Monitoring and Reporting and view the ongoing reports that can be generated.
VNX monitoring and reporting is a software license which is based on the frame type you have installed. It supports NAS and SAN workloads for CX4, NS and VNX systems so even if you have older arrays it can provide some value to your environment. You can license up to 5 storage arrays onto a singe M&R server.
Don't have the license but want to try it? You can download a trial version @ You need an EMC Support login id.