It would be useful to have following per file based flush time:

• In non-verbose mode, log for flush latency beyond threshold for each file
• In verbose mode, log for flush latency irrespective of threshold for each file
• In verbose mode, log for overall flush latency irrespective of threshold.

I believe the overall threshold currently is set to 10000ms, I would ideally assume if individual files are beyond the weighted average of their respective size w.r.t global threshold, it should log it.

Since, this is only every 60 seconds and should have minimal impact, first higher verbosity level above the default logging level should be good for logging such information or if you have any specific suggestion that should be fine.

This level of detail might be useful depending on if files are spread across mount/symlinks or on other devices. If it is taking a long time to flush files it might help to know which dbs/files are having issues.