I have this exact same problem - very high iowaits and 0% idle. When I shutdown logger it goes back to normal (99% idle).
However, there are no errors in the logs - just the normal mysql startup and shutdown messages
BTW - I DID have errors before, but I purged the logger db as here:
Cleaning Zimbra Logger logs
Also, I noticed a script running that seems to take up all available cpu for 2-3 minutes at a time: /tmp/.swatch_script.2339
I am running 4.5.9 as well
Any ideas?