Hi Brett, Cache cleaning in general is a very I/O intensive activity, so I agree that it is odd that your system appeared to be CPU busy during that time.
It's interesting that whatever it was, persisted for such a long time. In the past, I have developed small operator scripts which collect system information, which can be run by a simple monitoring tool and can keep a history about the observations they make. If you were to build and install such a monitoring system, you might be able to develop more clues about the "larger picture" of activity on your machine during that time period. Sorry to be so vague and non-specific, but I find that retrospective analysis of problems like this is VERY challenging. Often, the best you can do is: 1) Try to figure out some way to make the problem happen on demand, so you can cause it and observe it. 2) Instrument everything, and ensure you are preserving a history of your instrumentation recordings, so that you can have a mountain of detail when those rare events occur. It strikes me as being like high-energy physics, where your experiment generates volumes of data, and it takes weeks or months of analysis afterward to figure out what actually occurred. Not that I'm much good at high-energy physics, either, I'm afraid. :) bryan
