The following reports the beginning of an operation that can be very lengthy. It would be helpful for diagnosis to report the completion with a "durationMillis" attr.

{"t":{"$date":"2020-05-12T07:22:57.433Z"},"s":"I","c":"STORAGE","id":22324,"ctx":"conn108695","msg":"Downgrading WiredTiger datafiles.","source":{"sn":1,"ln":37327}}