Take clear notes of what ideas you had, which tests you ran, and the results you saw.70 Particularly when you are dealing with more complicated and drawn-out cases, this documentation may be crucial in helping you remember exactly what happened and prevent having to repeat these steps.71 If you performed active testing by changing a system—for instance by giving more resources to a process—making changes in a systematic and documented fashion will help you return the system to its pre-test setup, rather than running in an unknown hodge-podge configuration.
—— Google SRE Book, Chapter 12