wishlist: dynamic log volume control

We extract great value from logging every SQL statement, its duration, etc.,
in service to debugging, troubleshooting, monitoring, etc.  But we have a
number of clusters that seem to always be bumping into disk I/O
bottlenecks.  With a system running at 100 QPS, we're inclined to quiet the
logging and only turn it up as needed (which will, by definition, be while
it was quieted).  But many of these production clusters are not easily
restarted; the customer has to be notified, etc.  "It'd be nice" if we
could adjust the volume of logging dynamically (i.e., turn-off of statement
logging, etc) without have to restart the server...