I had switched to doing per_unit (every sample) logging recently in order to work around some issues I was hitting with using --log_avg_msec. After looking at my results, I could see a significant decrease in maximum and average bandwidth during runs. So I started playing with disabling other aspects of logging and was really surprised by the effect, running a 60 second random write test after an nvme format and 10 minutes of idle. I repeated the experiment many times to make sure it wasn't just noise. With a 970 Evo 1TB, I saw the following results: --log_avg_msec=100, all latency/latency percentile options disabled: max bandwidth 1.04GB/s, avg for run 0.99GB/s --log_avg_msec=100, all latency/latency percentile options enabled: max bandwidth 1.00GB/s, avg for run 0.94GB/s no log_avg_msec, all latency/latency percentile options enabled: max bandwidth 0.95GB/s, avg for run 0.91GB/s Between the best and worst performance is on the order of 100MB/s of bandwidth. Is this surprising at all, or expected with fio?
