Comment 1 for bug 1414039

unity8 has a big log file, because qtmir has all debug logging info turned on by default.
This can be changed just for unity8 by setting adding this env var to its upstart job:
   QT_LOGGING_RULES="*.debug=false"

Alternatively can apply this policy for all Qt apps in the system by creating ~/.config/QtProject/qtlogging.ini with these contents:

[Rules]
*.debug=false

Could use "*=false" to hide nearly all app output. For more info, see http://doc.qt.io/qt-5/qloggingcategory.html

I think the latter suggestion is easiest to implement should we want to turn the warnings back on for developer mode.