The code that currently maps Server log levels to WiredTiger log levels currently skips DEBUG_1, since it produces too much output information. Upon running a mongod instance, in particular it appears that a journal flushing log message (see screenshot below) is printed about 10 times per second. Increasing the verbosity level of this message will potentially make mapping Server DEBUG_1 messages back to DEBUG_1 WiredTiger messages possible without overwhelming the log with noise.