The simplicity and compehensiveness of Brendan Gregg’s USE method blows my mind every time I revisit it as part of my Solaris skills refresher.
From the USE method page:
For every resource: check utilization, saturation and errors.
The reason I’m always impressed is because in addition to the method itself, Brendan provised a complete checklist for Solaris performance investigations. So every time there’s an issue, you can use this list to zoom in on one of the physical server components: CPU, memory, storage I/O, storage capacity, network and interconnects (CPU, memory and I/O).
If you’re running any SmartOS environments on Joyent, there’s a separate performance checklist for SmartOS virtualised environments – also very useful.